View source for Paper - Cytological studies of Langerhans's islets, with special reference to the problem of their relation to the pancreatic acinus tissue (1920)

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=Cytological Studies Of Langerhans's Islets, with Special Reference to the Problem of their Relation to the Pancreatic Acinus Tissue=

S. Saguchi 

Anatomical Laboratory, Medical School, Kanazawa, Japan

Six Plates (Sixty-One Figures) And Four Text Figures


==Introduction==

As is well known, the pancreas of vertebrate animals is studded with groups of peculiar cells which are characterized by a transparent appearance due to the lack of zymogen granules, and which accordingly can be distinguished with ease from the neighboring acinus cells. Since Langerhans's ('69) discovery of these groups in the pancreas of the rabbit, many investigators who have devoted themselves to the study of the pancreas have concentrated their attention on these structures — designated later as Langerhans's cell-groups or islets — and there has been much discussion as to their morphologj^, development, and significance. This led to the formulation of various hypotheses, at present far from being in agreement. The most complex and difficult problem is that of the origin and ultimate fate of the islet of Langerhans, especially its genetic relation to the acinus tissue. Some believe that the islet is formed in an early embryonic stage by budding off and separating from the solid primitive cell-cord, and that once formed, it may exist as a permanent organ thi'oughout life. Others, while not denjdng its embryonic development, hold that the islet may originate from the acinus tissue of the adult pancreas. Laguesse and his supporters claim that islet cells may be formed, on the one hand, by the transformation of acinus cells and, on the other hand, by a reversion to the latter, the two tissues being thus well balanced (balancement theory). There are other observers who incline to the belief that islet cells are derived from the duct epithelium.

Decisive points in these discussions, especially of the problem of the genetic relation between the islet and the acinus tissue, have been the determination as to whether the islet is sharply marked off from the acinus tissue by a connective-tissue capsule or is in immediate contact with it, and whether or not the two behave independently under experimental and pathological conditions. Manj^ investigators seem to attach considerable importance to the question of the existence or non-existence of a capsule around the islet. We find it impossible to accept as adequate the postulation that the existence of a capsule is a sign of the independence of the islet, since it is difficult to decide whether the capsule is perfect or not. Even when the islet appears to be sharply separated from the- acinus tissue, it can hardlj" be said that the two have no genetic connection. On the other hand, it would be rash to assume a transition between the islet and the acinus tissue merely because the two are sometimes found in immediate contact, for there are many instances in which cells belonging to different tissues are in close apposition, although a transition between them is not conceivable.

Those who have tried to solve this problem experimentally have hgatured the pancreatic duct, allowed the animal to fast, or injected pilocarpi!! or secretin, and studied the changes thus produced in the size and number of the islets. Their observations have not, however, been in accord. Some (Fischer, Dale, and Vincent and Thompson) believed that they found an increase in the number of islets in starvation, while others noticed neither an increase nor a decrease of the islet tissue. On the other hand, ligature of the pancreatic duct caused, in some instances, the disappearance of only the acinus tissue, the islet remaining intact; in others, the disintegration of both tissues. That lack of uniformity in the results of these experimental investigations is perhaps due primarily to an inequality in the experimental methods employed; secondly, to the fact that the islets vary greatly in size and number not only in the different animals, but also in different portions of the same pancreas, as has been pointed out by various investigators. Under these circumstances there is always a danger of misinterpretation when one attempts to draw any general conclusion from the changes in size and number of islets in any circumscribed area of the pancreas.

It will be seen from the foregoing that the evidence brought forward to show a transition between the islet and the acinus tissue is by no means positive. The points which morphologists must solve first of all are: What are islet cells? What is their internal structure or specific property? The characteristics of islet cells pointed out by various investigators are not an accurate index. It has been claimed that these cells are distinguished from acinus cells in the following details: 1) they contain no zymogen granules and therefore appear more transparent; 2) they form groups showing a definite disposition; 3) a lumen in continuity with the pancreatic duct cannot be discovered within the islet; 4) the form, structure, and staining reactions of the nuclei of islet cells are different from those of acinus cells; 5) some islet cells contain a certain type of granules. Of the above, 1 and 3 are not characteristic of islet cells alone; there are, on the one hand, acinus cells from which almost all of the zymogen granules have been extruded; on the other, acini in which no lumen is evident. The specific disposition of cells is better seen in the larger islets. The character of the cytoplasm is of greater importance in distinguishing islet cells from acinus cells. Not all of the islet cells contain the minute granules which have been regarded as specific of them. From these considerations it is evident that the large islet is easily defined by the above characteristics, but the smaller one, consisting of only a few cells or even of a solitary cell, can be identified only with the greatest difficulty if it contains no specific granules. In other words, it is a difficult problem to decide whether the cells which are interspersed among acinus cells and which are devoid of zymogen granules belong to the acinus cells proper or not. The specific granules mentioned above are the only bodies of which we should take account in characterizing any islet cell, other properties being of a rather negative nature, as Benslej^ ('11-' 12) pointed out. We must have more positive identifying characteristics in order to define the islet cell accurately ; the more numerous these are, the more accurately can the cell be defined, so that even a solitary islet cell among acinous cells may be detected with ease. It is then only that the solution of the problem as to a possible transition between the acinus and islet tissue will become an easy one; for the process in question, if there be such, must be accompanied not only by definite changes in the shape, position, and arrangement of the cells, but also by changes in their internal structure, by which we should be able to trace the gradual disappearance of intracellular structures in the acinus cell and the appearance of new cell-constituents which are characteristic of the islet cell, or vice versa. This is the reason why a cytological study of the islet is of great importance in the definition of the islet cell and, consequently, to the question of a transition between the islet and acinus tissue.

The pancreas of Rana temporaria was selected as the material for this study as well as for the cytological study of the acinus in a preceding paper (Saguchi, '20). The latter investigation has greatly facilitated not only the study of the minute structure of the islet cells, but also the solution of the question of a transition. The methods employed for fixing and staining were the same for both, and this has enabled me to compare the structures thereby brought to light, both in acinus cells and in islet cells, in one and the same preparation.

==Minute Structures of Islet Cells==

===Historical===

The islet cells, though they have long been a problem for observers, have not yet been satisfactorily investigated in regard to their minute structures, at least not by the newer cytological methods. In looking over the hterature, one finds several references to the cytoplasmic structure. These briefly may be summarized as follows:

Since Langerhans's time, it has been frequently stated that the islet, either in whole or in part, consists of clear, transparent cells which are devoid of zj^mogen granules. Lewaschew ('86) describes two kinds of islet cells, both of which have the transparent cytoplasm, but which can be distinguished from each other by the shape and mode of delimitation of the latter.

Massari ('98), Diamare ('99), Schulze ('00), Levi ('04), Vincent and Thompson ('07), and others distinguish between two kinds of islets cells, one of which stains more heavily than the other. Vincent and Thompson, among others, pointed out that this difference is observed in the pancreas of various vertebrates; they designated the faintly staining islet as the leptochrome tissue and the heavier staining one as the bathychrome. According to these authors, the former constitutes the well-known islet of Langerhans; the latter, in mammals, consists of small groups of cells or even of solitary elements scattered throughout the secreting alveoli, while in birds, reptiles, and amphibia, it forms a solid mass of cells. The bathychrome tissue shows a remarkable tendency to form syncytium and stains deeply with ordinary dyes; this is especially true of the Flemming fixation.

Some investigators, on the other hand, have found special granules in some of the islet cells. Laguesse ('93) stated that there are present in the developing islet cells of the sheep embryo minute safranophile granules. These have since been observed by Laguesse ('95-96;' '01, '09-10) and by many other investigators (Carlier, '96; Gianneh and Giacomini, '96; Diamare, '99; Mankowski, '02; Ssobolew, '02; Pearce, '02-'03; Levi, '04; Marchioni, '04; Tschassownikow, '06; Lane, '07-'08; Gelle, '11; Piazza, '11; Bensley, '11-12, etc.). According to Laguesse, these granules correspond in a general way to zymogen granules in their behavior toward fixing and staining, differing from the latter, however, in their smaller size and greater resistance to acetic acid. Mankowski, Ssobolew, and Gelle call special attention to their affinity for safranin, while Carlier and Pearce designate them as eosinophile. Mankowski and Piazza have found that the granules in question have the property of reducing silver nitrate, so that in silver preparations they take a brown or black color.

According to the above authors, the granules are not evenly distributed throughout the islet cells, being abundant in some, while other cells are almost devoid of them. On the contrary, Lane and Bensley hold that there are two kinds of granules which are present in different kinds of cells, both of which stain with neutral gentian, although they differ from each other in their behavior toward certain fixing reagents.

That the protoplasm of islet cells assumes an alveolar appearance in fixed preparations, caused by the presence of small vacuoles, has been shown by Laguesse ('9o-'96, '01, '09-'10), Diamare ('99), Bensley ('11-' 12), Retterer and Lelievre ('13), and others. Laguesse is of the opinion that these vacuoles are filled with clear liquid which, when stained, presents under certain conditions a gummy aspect.

Fat droplets may also occur in islet cells. Dogiel ('93) noticed certain corpuscles which stained black in osmic-acid solution; he claimed that they were fat globules produced in the protoplasm in consequence of the regressive metamorphosis of the cell. Laguesse ('09-'10), Stangl ('01), and Saleni ('05) have confirmed the presence of fat corpuscles in islet cells, without, however, attributing any special importance to it.

Trophospongia are also found in islet cells. Holmgren ('04) was the first to describe this network as being connected with the processes of interstitial cells and which can often be canalized. Tschassownikow ('06) stated that the trophospongium of the islet cell was not constant, while Bensley ('11-12) claimed to have observed it in both fresh and fixed preparations.

The occm-rence of mitochondria in the islet cells was described by Bensley ('11-'12), Herwerden ('12), and Arnold ('12). According to these authors, the mitochondrial substance is present in the form of either minute granules or delicate filaments, the thick, coarse chondrioconts, such as are seen in the acinus cell, being entirely lacking.

It will be seen from these observations that the protoplasm of the islet cell presents either a homogeneous or a reticular appearance, and that the islet cells are of two kinds, distinguishable from each other by the tingibility of the plasma and by difference in granular content. It was also reported that fat droplets, mitochondria, and the intracellular network may occur in the islet cell, to which, however, no special importance has yet been ascribed.

Concerning the nuclei of these cells, the observations of vari.ous investigators are not in accord. The nuclei are described as being either smaller or larger than those of acinus cells, and the occasional occurrence of even giant-nuclei has been pointed out. They may be spherical, elliptical, or even rod-shaped. Many investigators believe that the nucleus has a dense reticulum which is rich in chromatin corpuscles, while others say that it is poor in these granules. Such diversity of opinion may be due to the technical methods employed. The only point of coincidence is that the nucleoli are either very small or altogether absent from the nucleus.


==Observations==

Specific granules: Cell types: the nucleus. We designate as specific granules of the islet cell those minute granules which are seen in the preparations fixed in sublimate-osmic-chromic acid, formalin, Maximow's, Regaud's, Meves's or Zenker's fluid, and stained with acid fuchsin, iron-hematoxylin, etc. These elements are best brought into view by fixation with Zenker's fluid and staining with Heidenhaiii's iron-hematoxylin. From these reactions, it is obvious that osmic acid is not essential to the preservation of the granules. Furthermore, acetic acid does not dissolve them, but appears rather to favor their fixation. From these properties, it is most probable that the granules are similar with those which have been described by Laguesse and many others; contrary to the view of Laguesse, Mankowski ('02), and Tachassownikow ('06), who maintain that they are stained with safranin, as is the case with chromatin, I have found that these granules do not stain with alum-hematoxylin, but are, rather, acidophilic and show a tingibility analogous to that of the nucleolar and mitochondrial substance, differing only from the latter in that the specific granules offer greater resistance to the action of acetic acid. These granules, in part at least, seem to have a certain affinity for silver salts, for in Cajal's preparations they are stained a pale brown color, as has been pointed out by Mankowski ('02) and Piazza ('11).

The specific granules are of very minute size (figs. 1 to 3, b; 14 to 17) and they are so closely packed that it is almost impossible to distinguish the individual granules; thus the protoplasm exhibits, at first glance, a granular or powdery appearance, as was first pointed out by Laguesse. Under these circumstances, it is exceedingly difficult to decide whether or not this granulation may be due to the interlacement of fine, tortuous filaments. The granules, in the majority of cases, are distributed throughout the whole cell-body, although not infrequently they are accumulated in large numbers at one end of the cell.

Most of the islets consist of cells which contain the specific granules, and others which are either entirely devoid of them or seem to contain only a very small number so that the protoplasm is characterized by a hyaline, transparent appearance. These two types of cells may be designated, respectively, as granular and non-granular islet cells. The granular cells can further be classified under three types according to the tingibility of the granules, the shape and structure of the cell-body and of the nuclei, etc. These types are designated as a, b, and c. The descriptions are based mainly upon preparations fixed in osmic acid mixtures and stained with acid fuchsin or iron-hematoxylin.

The specific granules of the a cells (fig. 14) stain heavily, thus giving to the protoplasm a distinctly granular appearance. The cells of this type are of various shapes: cylindrical, pyramidal, polygonal, or even, as often occurs, more or less irregularly shaped, due apparently to pressure from neighboring cells. The nucleus, spherical or polygonal in shape, is provided, in the majority of cases with a thick nuclear membrane which is thrown into folds; the nuclear network is well marked and its meshes are closer than those of the nuclei of acinus cells.

The cells of b type (figs. 1 to 4, b; 16, 17) are characterized first of all by the fact that their specific granules stain more faintly with iron-hematoxylin than those of the a cells. Like the latter, the b cells are, for the most part, cylindrical or pyramidal in shape, but narrow, elongated cells are not infrequently met with. The shape and structure of the nucleus constitute the second criterion by which these cells may be distinguished from a cells. The nucleus is large and oval in shape, with a smooth and indistinct contour ; the nuclear network, as seen in iron-hematoxylin preparations, is represented by indistinct, faintly staining cords or threads on which one or more small nucleoli may be suspended.

The c cells (figs. 52, 53) contain specific granules which stain even more faintly than those of b cells, and which, in most cases, are not evenly distributed throughout the cell-body, being interrupted here and there by transparent, non-granular protoplasm, especially at one end of the cell. The c cells are usually smaller than the preceding two types of cells and are either long and narrow or short-cylindrical, pyramidal or even polygonal in shape. The nucleus, irregular in form, is enclosed by a less distinct membrane which may be thrown into folds. The nuclear network is as indistinct as that of the b cell, and the mainnucleolus is likewise small.

Non-granular cells (figs. 1 to 5, e, 18 to 21), which compose the greater part of the islet elements, contain no specific granules or at most, very few. Most of these cells are cylindrical or pyramidal in shape, either straight or curved, the short, cylindrical or polygonal cells being few in number. Usually the nucleus is oval, though more or less spherical nuclei are never entirely absent. Ordinaril}^ its position is in the middle of the cell, but cells in which the nucleus is situated near one end are also often met with. According to the behavior of the- protoplasm and the nucleus toward iron-hematoxyhn or acid fuchsin, the non-granular cells can be divided into two groups which will be designated as d and e cells. The d cells (figs. 1, d, 20) are characterized by the fact that their protoplasm has a greater affinity for the abovementioned dyes; either the whole or a part of the cell-body may be heavily stained. The nuclear membrane is less smooth. The intranuclear network is very apparent and the meshes are closer, as is the case with the a cells. The nucleolus is very small.

The d cells are few in number as compared with the other type of non-granular cells which constitute a large part of the islet elements. The protoplasm of the e cells (figs. 1, 3, 4, e; 18, 19, 21) is transparent when stained with iron-hematoxyhn or acid fuchsin, which, in fact, is responsible for the clear appearance of the islet in general. The nucleus has an even contour and contains one or two small nucleoli and several nucleolar corpuscles suspended in the indistinct nuclear net. Large nucleoli such as are seen in acinus cells were never observed here. The nuclei of the cells are similar to those of b cells in form, contour, and character of the intranuclear network, differing from them only in their heavily staining nucleolar corpuscles.

Although, on the whole, the elements constituting the islets of the pancreas fall into five groups mentioned above, there are some cells which do not belong to any of these types, but which possess the combined characteristics of any two of them and must therefore be regarded as intermediate or transitional forms. These forms will be described when relation between the types of islet cells comes under consideration.

As noted above, the nuclei of the islet cells are, in general, of oval form, though the more spherical type is always represented.


That the nuclei of the islet cells exceed more or less those of acinus cells in bulk is evident from the figures 1 to 5. The nucleus is mostly situated in the middle of the cell-body, but there are cases in which it is placed near one end of the cell, which seems to be dependent upon the arrangement of the cells in the islet. The nuclei of b and e cells have an even contour, while those of a and c and sometimes d cells are surrounded by a membrane which is thrown into folds. The nuclear network, as seen in the preparations fixed in osmic-acid mixtures and stained with iron-hematoxylin or acid fuchsin, is most apparent in the nuclei of a and d cells; that of b, c, and e cells stains more or less faintly. In alum-hematoxylin preparations (fig. 23) it is seen that the chromatin granules are smaller and more numerous and that the network is closer in the nuclei of islet cells than in those of acinus cells. Another and more important difference is that the main-nucleolus of the former is always smaller than that of the latter, and contains one or two nucleolini which are situated at the periphery. The nucleolini are stained black by the Cajal method (figs. 48 to 50), while in ordinary preparations they are dissolved out so that there appear one or two small, clear vacuoles (fig. 51). In addition to the main-nucleolus, there are several side-nucleoli. In contradistinction to the preparations stained with iron hematoxylin, in the aium-hematoxylin preparation no structural differences of the nuclei between the various types of islet cells can be observed. This is due to the fact that the two staining methods exhibit two essentially different constituents of the nucleus: alum-hematoxylin stains mainly the chromatin, iron-hematoxylin the nucleolar substance. The structural differences in the nuclei of the various types of islet cells are therefore observed with respect, not to the chromatin,, but to the nucleolar substance. The chromatin content of the nuclei is fairly constant. Mobile the amount of the nucleolar substance undergoes a considerable change; this seems to point to the important significance of the latter in the metabohsm of the cell (Saguchi, '20).

Mitochondria. In the study of mitochondria, the following methods were applied : fixation in Meves's, Benda's, or Altmann's fluids, sublimate-osmic-chromic acid, sublimate-osmic acid, or Zenker's fluid, and staining with iron-hematoxylin or acid fuchsin.

The specific granules present in the a, b, and c cells bear, in their staining reactions, a striking resemblance to mitochondria, except that they offer greater resistance to the action of acetic acid. Except for these granules, there are no mitochondrial filaments in these cells. If we assume that mitochondria are a constant constituent of all cells', then they should also be found in these three types. I am of the opinion that the specific granules in these cells represent mitochondria. Evidence in favor of this view is: 1) the resemblance of the staining reactions of the two as described above; 2) the development of the specific granules takes place in a manner identical to that of the mitochondria in the acinus cell (Saguchi, '20); and 3) it is probable that the ultimate fate of the specific granules is the same as that of the mitochondria in the other types of islet cells, a question which will be taken up when the lipoid corpuscles are considered. It is necessary to say here a few words with regard to certain granules of mitochondrial nature. These are scattered sparsely throughout the cytoplasm of the a and b cells (fig. l,b); they are larger than the ordinary specific granules. Some of them are certainly of nuclear origin, since it can often be clearly seen that the nucleolus constricts off pieces which pass out of the nucleus into the cytoplasm (fig. 15); some of them, however, especially the smaller ones, may be formed by the growth and eventual fusion of the specific granules.

The non-granular cells, i.e., d and e cells, contain typical mitochondria in two forms — fiJamentous and granular (figs. 1, 2, 3, 7, e; 18 to 21) Chondrioconts in these types of islet cells are tor. tuous, very delicate filaments which can be made out only with diflficulty in the stained preparations. In the preparations treated with the method of Meves or of Altmann they stain very faintly, whereas in those which were fixed in osmic-sublimate mixture and stained with iron-hematoxylin, they appear as relativelj^ thick, more heavily staining filaments (fig. 19). They are distributed evenly throughout the cytoplasm, without any local accumulation. They seem to be present more abundantly in the d cells, especially in the neighborhood of the nucleus, than in the e cells (fig. 1). The mitochondrial granules are of various sizes: from very small to several times the diameter of a filament. They are commonly scattered throughout the cytoplasm, but frequently they are gathered together in greater or less amount near one end of the cell. It is my belief that a certain proportion of these granules are produced by the disintegration of chondrioconts; in fact, it is not infrequently observed that the latter carry bulbous enlargements which may become separated from them and increase in size (fig. 20). Some of the granules are most probably of nuclear origin, as is the case with a and b cells.

Lipoid corpuscles. Many of the islet cells contain some spherical corpuscles of a fatty or lipoid nature, which may be designated as lipoid corpuscles. These stain gray with osmic-acid solution or in mixtures containing this reagent. If the piece fixed in osmic acid is treated with any reducing reagent, for example, by the Faure-Fremiet method, they appear as deeply stained corpuscles. The corpuscles are also found in the Weigl (figs. 38, 41) and Golgi preparations, although in the latter they present a shrunken, thorn-apple appearance. In the first method of Ciaccio ('11) they are stained a reddish-yellow color with sudan III (fig. 5), while in the second method, they take a gray color with a reddish tinge. If Ciaccio's assumption be true, these corpuscles must contain neutral fat in addition to the lipoid substance. They are likewise easily stained with sudan III and scarlet red as applied to frozen sections. They cannot be seen in the preparations fixed in non-osmic mixtures and stained with alum- or iron-hematoxylin, but are represented by clear vacuoles (figs. 22, 23) which are nothing more or less than the lipoid corpuscles emptied of their contents by the dissolving effect of reagents, and which, when present in abundance, give a clear, reticular, or alveolar appearance to the protoplasm. Even in osmic preparations, it often occurs that the corpuscles are dissolved out by dehydrating alcohol, and xylol, or chloroform (figs. 18, 20, 21).

The lipoid corpuscles are characteristically spherical or droplike in shape; they vary in size, showing all grades of transition between the smallest and the largest granules which often reach the diameter of a nucleus. In most cases they are evenly distributed throughout the protoplasm, but it not infrequently occurs that they are accumulated near one end of the cell where the latter is in contact with a blood-vessel.

These lipoid corpuscles are present in b, d, and e cells, the largest and greatest numbers being found in the last. They are usually lacking or, if present, are few in number and of very small size in a and c cells; the smallest can be manifested only by the Golgi method.

It should be mentioned in this connection that the lipoid corpuscles, in their physical and chemical properties, bear a strong resemblance to the lipoid granules found in the basal portion of the acinus cell, for the details of which the reader is referred to my previous paper (Saguchi, '20). It may be that the two structures have the same functional significance, although in acinus cells the production of the substance which constitutes them does not take place to such a marked extent as in the islet cells.

As regards their genesis, no definite conclusion can be reached at present. It seems probable, however, taking into account my previous observation that the lipoid granules in the acinus cell are derived from mitochondrial filaments, that this is also the case with the lipoid corpuscles in the islet cell. We have seen that the mitochondrial granules in the e cells are formed by the disintegration of the chondrioconts; it is possible to assume that these granules, after having increased in size to a certain extent, may change their physical and chemical properties and become transformed into the smallest Upoid corpuscles, which gradually increase in size, either by growth or by fusion. In the same manner, the lipoid corpuscles of the h cells may be regarded as derived from the specific granules or the granules of mitochondrial nature which are contained in this type of cell.

The problem as to the fate of the lipoid corpuscles is not a simple one. The view of Dogiel ('93) that the islet must be regarded as a dead spot, since it contains fat droplets which are produced in consequence of the metamorphosis of the cell, is not the correct one, for no degenerative process can be seen either in the cells or in the nuclei. This and the fact that the islet is composed mainly of cells of this type, rather strongly suggest that the lipoid corpuscles are a cell constituent which plays an important part in the function of the islet. It is presumable that this lipoid substance is to be extruded, either as such or after having undergone certain changes, into the blood-vessel with which the cells in question are intimately connected.

The urano-argentophile apparatus. By the term urano-argentophile apparatus, I understand those filamentous or granular corpuscles which can be made manifest by the Cajaluranic nitratesilver method. Since this method is one which is employed for exhibiting the Golgi intracellular network apparatus, the question arises whether the urano-argentophile and the Golgi apparatus are to be identified or not. In fact, I have found that the same intracellular network can be brought into view in pancreatic acinus cells not only by the Kopsch, Sjovall, and Weigi method, but also by the Cajal method. This is not, however, the case with islet cells; the Cajal method does not exhibit the net, but a different apparatus which can easily be distinguished from it by both its morphological and chemical characteristics, and which therefore must be treated of separately.

The urano-argentophile apparatus is stained brown or black with the Cajal method, as mentioned above, so that it stands out clearly from the faintly staining ground substance of the protoplasm (figs. 6, 28 to 31). These corpuscles are filamentous or granular in shape. The filaments are of varying thickness and length and are more or less tortuous in their course. They rarely ramify or anastomose with one another, and are usually distributed evenly throughout the cytoplasm, although it often occurs that they are gathered together in a given region of the cell-body. The thinner filaments have an even contour, while the thicker ones often have spherical or fusiform enlargements along their course, which may be set free and may give rise to urano-argentophile granules. The latter, spherical or oval in shape, are also of various sizes, and stain more heavily than the filaments. They are either evenly scattered throughout the cytoplasm or accumulated at one or both ends of the cell. Neither granules nor filaments pass into the vacuoles which correspond to the dissolved out lipoid corpuscles.

As regards the distribution of urano-argentophile granules and filaments in islet cells, it can be said that cells containing granules in association with filaments are rarely seen; consequently, cells containing granules are usually devoid of filaments, or at the most, contain short, thick rods. By far the largest number of islet cells are of the latter type; these cells are cylindrical in shape and are in contact with blood-vessels at one or both ends. They also contain a great number of spherical vacuoles corresponding to Hpoid corpuscles (figs. 6, e, 29, 30). On the other hand, there are cells which are provided wdth granular uranoargentophile corpuscles, but in these the vacuoles are not evident; the cells of this type usually occur in small groups between acinus cells.

The cells containing urano-argentophile filaments can be subdivided into three kinds according to size, shape, and position. In the first place, there are some cylindrical cells which occur in the peripheral part of the islet or are interspersed between acinus cells, and which are laden with a large number of either thin or thick filaments. They have, in most cases, spherical nuclei which are usually situated near one end of the cell (figs. 25 to 27) . The second type of cell is small, and often long, narrow, and cylindrical in form. It is always situated at the periphery of the islet and contains a small number of filaments (fig. 32). The third tA^pe is situated between the cells containing urano-argentophile granules; its protoplasm stains more or less deeply and possesses a few tortuous filaments. It is characteristic of this kind of cell that the vacuoles corresponding to the lipoid corpuscles appear to be lacking, at least no larger ones are visible (fig. 33).

The question now arises, how can we identify these five types of cells with the a, b, c, d, and e cells. This is an exceedingly difficult problem, since we possess at present no technical methods by which to bring into view both specific granules and uranoargentophile corpuscles in one and the same preparation. The presence of large numbers of both lipoid corpuscles and urano argentophile granules in the cells forming the chief constituent of the islet seems to point to that fact that these cells correspond to the e type. Of the cells in which vacuoles are not evident, those which are laden with delicate urano-argentophile filaments may be identified with the a cells, as the form and position of the cell-bodies and the nuclei of the two are in accord. Secondly, the non-vacuolated, granule-containing cells, which form small groups scattered throughout the acinus tissue, may be regarded as corresponding to the h cells; in fact, the lipoid corpuscles of the latter are small in size and few in number, so that it is difficult to make them out in the Cajal preparations. Whether the narrow cells (figs. 31, 32) in the periphery of the islet belong to the h type or c type is difficult to decide, although the cells which contain the deeply staining protoplasm, and which are interposed between the e cells, correspond in all likelihood to d cells, for other types of cells than they are not usually found between the e cells.

That the urano-argentophile granules are formed by the disintegration of filaments has been mentioned above; but the development of the filaments themselves and the ultimate fate of the granules are points which have not been definitely determined. It not infrequently occurs that the disintegration of the filaments into granules takes place near one end of the cell or that the granules are especially abundant at one or both ends of the cell. From these observations, it would seem that there is some intimate relationship between them and blood-vessels. I am of the opinion that these granules, after having undergone certain modifications, pass into the blood-stream.

Argentophile granules. In preparations fixed in formalin and treated with silver nitrate, according to Cajal, there can be seen in the islet, or scattered through the pancreas cells which contain brown-stained granules (figs. 46, 47). Although these may be designated argentophile granules, they can also be made manifest by fixation in formalin and by staining with iron-hematoxylin. These granules are of small size, although larger than the specific granules previously spoken of. In some cases they are so numerous as to fill up the cell-body, in others they are few in number or located only near one end of the cell.


The cells containing argentophile granules contain few or no vacuoles corresponding to the dissolved out lipoid corpuscles; from this fact and from their position in the periphery of the islet, it is conceivable that these cells belong to the type which contains specific granules. I am of the opinion that the argentophile granules are present in both a and h cells; in fact, some of the nuclei of the cells containing argentophile granules are spherical in shape and situated near one end of the cell. Unlike these granules, the specific granules are stained only a faintly brown color in the Cajal preparation. The e cell contains no argentophile granules (figs. 48, 49, 50).

The genesis of argentophile granules may easily be followed in certain cells which are met with in the periphery of the islet or between acinus cells. In shape these cells (figs. 45, 46) are similar to acinus cells and contain spherical nuclei in which numerous argentophile granules of various sizes are visible. In some cases these granules are found in the nucleus; in others, they are also met with in the cell-body, while in the nucleus they are reduced in number. From these observations, the inference that the argentophile granules produced within the nucleus pass into the cytoplasm would appear justifiable. In my preceding work on the acinus cells of the pancreas I have referred to the fact that the nucleolini are stained a brown color by the Cajal method, and that they often pass out of the nucleolus, even out of the nucleus into the cytoplasm. In a similar manner the argentophile granules, I think, must be derived from the nucleolini, the production being very active in this case. As regards the ultimate fate of these granules, no definite conclusion can be reached at present. It is, however, well within the bounds of possibility that, after having undergone certain changes, they are eliminated from the cell-body or transformed into other cell constituents.

Intracellular cord or network apparatus. A structure comparable to the Golgi network is also found in islet cells. Although it is manifested with ease by the Weigl method, it can also be seen in preparations fixed in sublimate, sublimate-osmic-chromic acid, or Rabl's mixture, and stained with iron-hematoxylin. The structure presents four principal forms (figs. 34 to 40, 42 to 44) :

1) There are found irregularly spherical or elliptical corpuscles of various sizes, which consist of a deeply staining cortex and a pale main mass (figs. 35, 36). These corpuscles are often connected with one another by threads of varying thickness which show the same staining reaction as the corpuscles (fig. 37). From the shape and position on the one hand and from the absence of lipoid corpuscles on the other, it is probable that the cells which contain these bodies belong not to the d or e type, but to the granular type of cell, especially to the a and h types. 2) There are typical nets which are formed by the anastomosis of thin or thick, often double-contoured threads (figs. 38, 39, 42). These are situated either between the nucleus and an extremity of the cell or along the side of the nucleus; in the latter case, the net is flattened and elongated (fig. 42). 3) Another form of Golgi's apparatus is represented by rings or loops (figs. 40, 43, 44). The ring (usually there is only one) is generally situated near the nucleus. There are also cases in which the ring or loop emits various prolongations which either anastomose with one another, end freely in the surrounding protoplasm, or run along the side of the nucleus toward the other end of the cell. 4) The apparatus sometimes appears as threads (fig. 21), which may be straight or curved, situated near one end of the cell, or alongside of the nucleus. The latter three forms of the Golgi's apparatus are found in cells with lipoid corpuscles. The lipoid content of these cells, however, is small; therefore they must be looked upon as belonging not to the e type, but to the h type. Most of the e cells, which contain a large amount of lipoid substance, are devoid of the apparatus in question (fig. 41). There are also other forms which may be regarded as transitional between the types mentioned above. I am of the opinion that these gradations point to a course of development and of involution of the structure. In fact, it is not difficult to see how the corpuscles become elongated and connected with one another (fig. 37) in order to form the typical net. On the other hand, the net undergoes regressive metamorphosis by a gradual thinning and adherence of the trabeculae or threads, so that, at last, it leads to the formation of a single ring or loop, or even a delicate filament w^hich gradually disappears (fig. 21).

It will be seen from the above account that the b cells contain the Golgi apparatus in the height of its development; in the a cells it is in the earlier stage of its existence, while in the e cells it is in the terminal stage.

Pigment granules. Some of the islet cells contain yellowishbrown pigment granules which are well preserved in sublimate or in Rabl's mixture (fig. 22). They stain black by the Cajal method (fig. 49), as is the case with the pigment in nerve cells. They are present in two forms — small granules and relatively large, somewhat spherical corpuscles, which when viewed in sublimate or Rabl preparations, consist of a cortex of pigment granules, and of a more faintly staining internal substance. In the Cajal preparations, on the contrary, the whole of the corpuscle takes a deeply brown color and its surface is furnished with short, thorn-like prolongations which correspond to the row of pigment granules found in ordinar}^ preparation. Although varying in number and intracj^toplasmic position, they are present in the e cells, that is to say, in those which are laden with lipoid corpuscles. In addition, the pigment content of the cells is subjected to much variation according to the islet examined.

MITOSIS AND AMITOSIS

Bizzozero and Vassale ('87), Laguesse ('95-'96), Ssobolew ('02), Lane ('07-'08), and others record that they have found mitotic figures in islet cells. Laguesse ('09-'10), in addition, describes that in man mitosis is rarely found in the islet, whereas amitosis is not infrequently met with. In the cytological study of the islet m}^ attention was repeatedly attracted to the presence of karyokinetic figures in both b and e cells (figs. 56 to 58). Cells with two nuclei are also found in the islet, the nuclei being situated either one upon another or side by side according to the shape of the cell. It is my belief that they are produced, not by mitosis, but by an amitotic fragmentation of the nucleus. Cases are occasionally observed where a connection exists between the two nuclei which may be regarded as being in the nature of a constriction (fig. 59). On the other hand, the amitosis accompanies no degenerative process of the islet cell. So far as I have been able to ascertain, the islet cell undergoes no degeneration, at least there are visible no changes in the nucleus which maj^ be looked upon as degenerative. In a word, the islet cells multiply either by mitosis or amitosis, without decreasing in number by degeneration.

THE RELATIONS BETWEEN THE DIFFERENT TYPES OF ISLET CELLS

x4.s shown in the preceding pages, the islet cell contains several cytoplasmic structures, w^hich are not found in an equally developed state in one and the same cell, but rather in such a way that a given group of cells will be rich in one structure, while another group is poor in it. This makes possible several modes of classification of islet cells according to the character and amount of the structures concerned. These classifications have already been referred to. The next important step is to determine the types of islet cells by summarizing the results thus obtained. As I pointed out in the introduction, there is always danger of misinterpretation if one tries to classify the islet cells according to one or two in distinct* properties. We must therefore take into account as many positive structural characteristics as possible. The most practical and convenient plan is to sort out the cells according to a given structure, which serves as the fundament of classification, and then assign other structures to the types thus determined.

The following descriptions of the nuclei are based upon the preparations stained with iron hematoxylin or acid fuchsin, as the alum-hematoxylin preparation does not reveal any marked differences between the nuclei in the different types of cells.

1. a cells. This type of cell (fig. 14) is heavily laden with specific granules which are deeply stained with iron-hematoxylin or acid fuchsin. While mitochondria are lacking in these cells, it is highly probable that they are represented by the specific granules, the two structures bearing a striking resemblance to each other in their microchemical reactions. On the other hand, a cells contain few or no lipoid corpuscles, while the urano-argentophile apparatus exists in the shape of dehcate tortuous filaments (fig. 25). The Golgi apparatus consists of spherical or elliptical corpuscles of irregular contour (figs. 34 to 36). Pigment granules are not seen. The cell-body of this type is cylindrical or conical in shape; its spherical nucleus is situated near the basis of the cell; it is surrounded by a thick nuclear membrane which is often thrown into folds and stains deeply with iron-hematoxyhn; the nuclear net is very distinct. The nucleolus is always smaller than that of the acinus cell.

2. b cells. The specific granules of b cells are faintly stained so that the granulation of the cytoplasm is indistinct (figs. 1 to 4, b, 16, 17). As is the case with a cells, mitochondrial filaments cannot be found. Lipoid corpuscles, however, are always present in b cells, though most of them are small in size (figs. 2 b, 16). The urano-argentophile apparatus is fairly well developed, and is either filamentous or granular in form (figs. 26 to 28). The Golgi apparatus occurs in the form of the typical network which may be regarded as being at the height of its development (figs. 38, 39, 40, 42). The 6-cell is cubical or cylindrical in form and contains an elliptical nucleus, the boundary of which, though indistinct, has generally an even contour; it is in the middle of the cell-body. The nuclear net stains very faintly. The nucleolus is small.

3. The c cells are the most ill- defined type of islet cells. The granulation of the cell-body is very indistinct and is replaced here and there, especially near one end of the cell, by a homogeneous protoplasm (figs. 52, 53). Although it is certain that these cells contain neither mitochondria nor lipoid corpuscles, it is difficult to decide in what form urano-argentophile and argentophile granules, as well as the intracellular apparatus of Golgi occur. It seems, however, that these structures are never found in a well-developed state (figs. 31, 32). The c cells are generally small in size and either cubical or narrow, long and cylindrical in shape. The nucleus is irregularly shaped and surrounded by a wrinkled boundary membrane; one or more small nucleoli, which are suspended in the ill-defined nuclear net, are visible in the nucleus.

4. The d cells are few in number and are interspersed among e cells (figs. 1, d, 20, 33). They differ from the latter in the following details: 1) the number of lipoid corpuscles is smaller than the e cells (figs. 1, d, 33); 2) the urano-argentophile apparatus seems to occur in the form of delicate filaments (fig. 33) ; 3) the protoplasm of the d cells stains more deeply and appears darker in iron-hematoxylin or uranic nitrate-silver preparations; 4) the mitochondria are more abundant than in e cells (fig. 1, d); 5) the nucleus exhibits a distinct network with closer meshes (fig. 1, d).

5. The e cells contain very few if any specific granules and thus the cell-body has a more transparent appearance. The most characteristic feature of the protoplasm of the e cells is the great abundance of mitochondrial filaments (figs. 1, e, 18, 19, 21), lipoid corpuscles (figs. 5, e, 41) and urano-argentophile granules (figs. 6, e, 29, 30), whereas argentophile granules appear to be entirely absent (fig. 48). The Golgi apparatus, in the form of either rings or threads (fig. 21), and pigment (figs. 22, 49) are also visible in them. The cells are the largest of the islet cells and are mostly of cylindrical form. The nucleus, as seen in iron-hematoxylin preparations, is similar to that of the b cell. It is oval in shape and usually situated at the middle of the cell-body, though not rarely it is placed near an end of the cell. The faintly staining nuclear network is provided with one or several small nucleoli.

It can scarcely be said that all the islet cells belong to one of the five types just spoken of, as there can be found intermediate forms. For example, the cell shown in figure 15 must be regarded as a transitional form between the a and 6 or a and e cell. The specific granules of this cell are of the same nature as those of the a cell, but the nucleus has an oval form with an even contour, and it has an indistinct nuclear network, just as is the case with the 6 or e cells. The b and e cells, on the other hand, have lipoid corpuscles in common. There are cells which, like the e cells, contain a large number of lipoid corpuscles, but in which specific granules, though few in number, are visible. These cells must be considered as intermediate between b and e cells. In fact, there are found cells which contain both specific granules and mitochondrial filaments. The relation of c cells to other types of islet cells is a problem which is difficult to solve. From the granular appearance of their protoplasm and the absence of mitochondrial filaments, it seems not unlikely . that they bear some relation to b cells. But the possibilitj^ of a transition between e and c cells is by no means to be excluded.

Summarizing the above observations upon the cytological character of the islet cells, we come to the conclusion that the five types of islet cells are connected with one another by all grades of transition: a cells pass into b cells and the latter into e cells ; on the other hand, e cells pass into b cells and perhaps into c cells. The d cells and e cells have many characteristics in common, so that a transition between them is quite conceivable. I am of the opinion that the d cell is a resting phase of the e cell; the nucleolar hyperchromasy of the nucleus and cytoplasm with abundant production of mitochondrial filaments strongly suggests this view. From these considerations it must be admitted that a cells and c cells, though connected with 6 or e cells, have no relation to any other types of islet cells; in other words, they cannot be regarded as intermediate between any two types. Are they to be regarded as a resting phase of other types of islet cells, or as an intermediate condition between the islet cell and some other cell which forms a constituent of the pancreas? To solve this complex and difficult question it is not sufficient to investigate the islet tissue only, but other elements of the pancreas, especially the acinus cells, must be subjected to a careful examination. If, in this way, a transition between the a cells or c cells and the acinus cells can be traced, there can be no doubt that the islet is not an independent organ. A discussion of this matter wdll be taken up in connection with the relation between islet and acinus cells.

==Distribution of Islet Cells in the Pancreas==

Since the pancreas of the frog is small, it is an easy matter to cut cross-sections through it and to examine the distribution of the islet cells. If search is made in such sections for cells which show the characteristics mentioned above and the cells sketched, it will be found that islet cells (for the sake of simplicity, this designation is given not only to cells which constitute the typical islet, but also to those which occur either in small groups or individually, and are to be identified with the former) are scattered throughout the pancreas either individually or in smaller or larger groups, as shown in figures 60 and 61. Whether the predominant occurrence of the islet cells is single or in groups seems to depend upon the region from which the section is taken.

That islet cells occur alone or in small groups among acinus cells has been noted and described b^^ Laguesse ('01), TscKassownikow ('00), Vincent and Thompson ('07), Bensley ('11-' 12), and others. According to Laguesse, these cells contain safranophile granules which are either scattered throughout the cellbody or confined to the basal portion of it. Tschassownikow believes these granules to be chromatophile, while Vincent and Thompson say that the protoplasm of these cells gives a bathychrome reaction. Piazza ('11), on the contrary, is of the opinion that these solitary cells belong to islets, the main part of which is not contained in the section under examination, for they are laden with argentophile granules as is the case with cells of the typical islet. On the other hand, the cells, containing minute granules, which Arnold ('12) regards as a sort of acinus cell, are to be identified with solitary islet cells. At least, from the microchemical reactions of the granules found by him, it is probable that they correspond to the a or h type above referred to.

In the pancreas of the frog the solitary islet cells are met with everjrwhere, not only in the center, but also near or even at the periphery immediately beneath the capsule. Most of them are of the h type, while cells belonging to the a type are few in number. I was unable, however, to determine whether or not other types of solitary islet cells occur. The solitary cell (fig. 2, h) is situated between the acinus cells ; at one extremity it is in contact with the basal membrane, while the other extremity ends either between the neighboring acinus cells or reaches the lumen. Under these circumstances, it often occurs that the solitary cells, especially those belonging to the a type, extend along the basal membrane, thus assuming a polj^gonal or stellate form. The fact worthy of note is that the solitary islet cells are in close relation to the blood-capillaries, just as the case with the typical islet. In fact, the blood-capillary with which they are in contact is in a distended condition.

Besides the solitary islet cells, there are found scattered through the pancreas small groups of cells. These behave, in relation to the neighboring acinus cells as well as to the basal membrane, like the solitary cells. These groups of cells may be designated as non-tj'pical islets. The cells composing them belong either to one or to several acini (figs. 3, b, e; 7). In the latter instance the islet cells belonging to different acini are usually separated by a blood-capillary to which they converge. In either case, the islet in question is composed, in most cases, of two kinds of cells — b and e. It is extremely rare to see an islet composed of e cells only. The b cells are situated at the periphery of this group, while the e cells are in the center; in other words, the e cells are separated from the neighboring acinus cells by b cells. Islets of this type are distributed throughout the pancreas; in the superficial portion of the organ, however, they are rarely met with.

The most characteristic feature of the solitary islet cells and of the cells composing the non-typical islet above referred to is that they are situated in the acinus and are arranged in the same row as the neighboring acinus cells (fig. 7) . In the third form of cell groups, which may be designated as typical islets, the cells are disposed in an entirely different manner. Figures 1, 4, 5, and 6 show parts of such a typical islet. It consists of cell cords which anastomose with one another and form a sort of network, the meshes of which are occupied by blood-capillaries. The ends of the cord may pass gradually into the pancreatic acini or may be separated from the latter by the basal membrane. In the former case islet cells are in immediate contact with acinus cells; in other words, the islet cells are enclosed by a connective-tissue membrane which is in direct continuity with the basal membrane of the acini. The cells composing the cord are mostly of elongated cylindrical form, and are arranged in a row in side-by-side apposition, the both ends of the cell being in contact with the membrane which encloses the cord. There are, however, cells which end freely before reaching the membrane of the opposite side; this is dependent, in part at least, upon the direction in which the section is carried through the cords. Cases in which the cells of the islet show no regular arrangement may also be due to the same cause. It sometimes happens that two cords run side by side, separated only by the connective-tissue membrane (fig. 6). My observations show that all typical islets, however diversified they may appear, consist of a system of cellcords which intertwine with blood-capillaries.

The typical islets usually consist of cells already mentioned. In most of the islets, especially in the larger ones, the e cells are the most abundant, although the h cells may sometimes exceed them in number, especially in the smaller islets, while a, c, and d cells are few in number or entirely lacking (fig. 1). As regards the position of these different types, it can generally be said that the h cells are situated at the periphery of the islet so as to form a sort of cortical layer. As is the case with the non-typical islet, the e cells are here, too, separated from the neighboring acinus cells by h cells. In addition, the h cells not infrequently are of such a form and arrangement as to suggest compression from the acinus tissue, d cells, if present at all, are almost always interposed between the e cells, while a and c cells are located at the periphery of the islet. The e cells lie in contact v/ith the bloodcapillaries, which is not always the case with h cells.

The typical islets are, in the majority of cases, larger than the non-typical ones, and may be found everywhere in the pancreas except in the peripheral layer, the largest ones being located generally near the center of cross-sections of the pancreas.

It follows from the above observations that the pancreas contains either solitary islet cells or groups comprised of few or many cells. These cell groups may be subdivided into nontypical and typical islets. Most of the islet cells belong to the e and h type, while a, c, and d cells are few in number. The e cells form an essential constituent of the typical islet, whereas h cells are usually isolated or occur in small groups. Solitary cells and cells in small groups are enclosed by the basal membrane, thus forming a part of the acinus. The cell cord of the typical islet is enclosed by a delicate membrane which is either in direct continuity with the basal membrane of the acinus or separates the islet cells from the neighboring acinus cells; a special boundary membrane or capsule around the islet does not exist.

==Relation Between Islet Cells and Acinus Cells==

Postembryonic development of islet cells

Concerning the development of the islet, there is wide divergence of opinion among different investigators. Gianneh ('98), Diamare ('99), Opie ('00), Pearce ('02-'03), Kuster ('04), Helly ('06), and others beheve that the islet, although derived from the primitive undifferentiated pancreatic cell cords of the acinus remains independent through life. There are many others who assume that not only primitive cell cords, but also specific differentiated pancreatic cells (i.e., acinus cells containing zymogen granules) may give rise to islet cells. Lewaschew ('86), Pischinger ('95), Laguesse ('95-'96, '01, '05,'09-'10),Mankowski ('02), Tschassownikow ('06), Dale ('04), Vincent and Thompson ('07), and Fischer ('12) are supporters of this hypothesis. The arguments on which it is based are: 1) the absence of a boundary membrane between the islet and acinus tissue and the immediate contact or connection between the two; 2) the presence of transitional forms between them. It has been assumed by some that acinus cells transform into islet cells by the gradual disappearance of zymogen granules and by the change in shape, size, and staining reactions of the cell-body and the nucleus, while the lumen is lost to view and the cell boundaries become indistinct. Laguesse ('01), among others, states that in the transformation of acinus cells into islet cells the zymogen granules diminish in number and come to stain more faintly, while the minute granules characteristic of the islet cells appear in the base of the cell. These granules gradually increase in number so that the cell in question finally assumes the appearance of a typical islet cell. Tschassownikow ('06) regards the cells which contain chromatophile granules, and which are scattered through the pancreas individually or in small groups, as transitional between the acinus and islet cells.

Other investigators have opposed the above view, and they attach considerable importance to the presence of the connectivetissue membrane which forms a sort of capsule around the islet, sharply marking it off from the acinus tissue. Piazza, who takes into consideration the fact that the behavior of the blood and nerve supply and of the connective tissue of the islet is quite different from that of the acinus tissue, and especially the fact that the chemical characters of cells of the two tissues differ widely from each other, has come to the conclusion that a transition between the two is impossible.

Experimental studies also have been made to solve this complex and difficult problem. Some investigators have found that the ligature of the pancreatic duct results in the disappearance of the acinus tissue, while the islets remain intact. Schulze ('00), Ssobolew ('02), and others have, from these observations, been led to the conclusion that the islet is an independent organ, whereas Laguesse ('05) claims that these findings are not contrary to his view that the islet is derived from the acinus tissue. Mankowski found that both tissues disappeared in the same experiment. On the other hand, there are many investigators who have observed, after the injection of pilocarpin or secretin or after a period of fasting, that the islets increased in number; according to Gelle ('11), Fischer ('12), and Retterer and Lelievre ('13), this is due to the increased transformation of acinus cells into islet ceUs.

Kyrle and Weichselbaum (Kyrle, '08; Weichselbaum and Kyrle, '09) believe that the islet cells are derived from small pancreatic ducts, while Hansemann ('02) attributes a mesenchymal origin to the islet. Their views, however, seem to have gained few adherents among histologists.

The behavior of the capsule or the size and number of the islets, in either normal or experimental conditions, is not sufficient to solve the problem of the genetic connection between the islet and acinus tissue. Transparent cells containing no zymogen granules cannot always be regarded as transitions, for the resting acinus cells and centroacinus cells may have the same appearance; a distinction between the two is often made with difficulty in cases where the islet cells are interspersed among acinus cells. Cells containing, on the one hand, minute granules which are assumed to be a characteristic constituent of the islet cell, and, on the other, zymogen granules, as has been described by Laguesse, can certainly be considered to be transitional between the acinus and islet tissue. It must be borne in mind, however, that there are islet cells in which the minute granules cannot be manifested and the protoplasm of which, in ordinary preparations exhibits a clear, transparent appearance.

That the islet cells can be characterized by the presence not only of minute granules, but also of several other protoplasmic structures, has been mentioned above, and if there be a transformation of acinus cells into islet cells, the formation of the specific cytoplasmic constituents in the latter cells and the disappearance of those in the former must take place in a visible manner; thus the general process of transformation can be followed more accurately than with any other methods. For this reason as many recent cytological methods as possible must be employed in the investigation of the islet, and, at the same time, the minute structures of the acinus cells must be taken into account.

From a careful study I have come to the conclusion that there is a transformation of the acinus cell into the islet cell, whereby not only the cell-body and the nucleus change shape and position, but the intracytoplasmic structures also undergo a series of definite alterations. This process of transformation may be divided into two stages:

First stage. The acinus cells which are about to transform into islet cells gradually decrease in volume, and some of them take a rather elongated form as if from compression between the neighboring cells (figs. 3, A', 8, 9). Concomitant with this change, a gradual reduction in the number of zymogen granules takes place. The nucleus seems to undergo no remarkable change in size, although it takes the form best adapted to that of the cell-body. The striking changes are those of the intranuclear network. The nucleolus or nucleoli, as well as nucleolar corpuscles, increase in volume so that the network becomes more pronounced; the nuclear membrane, in addition, increases in thickness, stains more deeply than before, and is often thrown into folds. In other words, the nucleus exhibits a marked nucleolar hyperchromasy (Saguchi, '15). The process is not confined to the nucleus; it extends at successive periods, to the cytoplasm so that the latter stains more or less deeply with iron-hemotoxylin or acid fuchsin.

The most striking change in the cytoplasm is that of the mitochondria. In my previous work I mentioned that the acinus cell contains thick and coarse mitochondrial filaments. In the first stage of transformation (fig. 8) there appear along the course or at the ends of these filaments, which have meanwhile become thickened and more or less shortened, spherical or oval enlargements in which one can distinguish very soon after their formation a clear inner part and a heavily staining cortical layer, due perhaps to the production of liquid in the accumulated mass of mitochondrial substance. This process of liquefaction extends over the whole length of the thickened mitochondrial filaments, while the heavily staining cortical layer disappears; at last there remain only canaliculi or spaces filled with the clear liquid (figs. 9, 10).

In preparations treated by the uranic nitrate-silver methods of Cajal, one often sees acinus cells containing 'ong or short, brown or black, spherical or oval, rod-, club-, or dumb-bell-shaped corpuscles which are scattered through the cytoplasm, often reaching the distal end of the cell (fig. 24). Concerning the significance of these corpuscles no definite conclusion could be drawn from my observations since I could not follow out the process of their formation. One thing, however, is certain — they have no definite genetic connection with the Golgi apparatus, which shows the same staining reaction, for there are found cells in which these structures are visible, independent of the network apparatus near the base of the cell. I am of the opinion that the formation of these corpuscles and the liquefaction of mitochondrial filaments are one and the same process; the mitochondria undergo a definite change and there is formed a substance which no longer exhibits the specific mitochondrial reaction, but becomes impregnated by the Cajal method. It will be seen from a comparison of the preparations that the above-mentioned bodies are in full accord not only in position, but also in shape, with the clear spaces or vacuoles produced by the liquefaction of mitochondria, so that it is conceivable that the former is only the positive figure of the latter. The corpuscles gradually disappear from the cell. From the fact that they show a tendency to proceed from the basal part of the cell toward the lumen, and that they are often gathered near the latter, it is presumable that they become in part, at least, extruded into the lumen.

Second stage. A remarkable phenomenon of this stage is the passing out of a certain intranuclear constituent. In the preparations fixed in osmic mixtures, especially in sublimate-osmicchromic acid, or in Zenker's mixture, and stained with ironhematoxyhn or acid fuchsin, there can easily be found nuclei from which a deeply stained substance passes out in the form of delicate, tortuous filaments or of minute granules (figs. 10, 11). These at first are accumulated around the nucleus, but gradually extend over the whole cytoplasm, and are finally so densely packed together that the individual filaments are no longer visible, the cytoplasm exhibiting rather a granular appearance (fig. 13). These granules or filaments are nothing else than what we have designated specific granules and the cell in question now becomes a cell (fig. 14).

The nucleus maintains its original spherical or polygonal form and the meshes of the nuclear net become closer. The most striking feature in this stage consists in the decrease in size and volume of the nucleolus and nucleolar corpuscles, which may perhaps be correlated with the passing out of the filaments or granules. I am of the opinion that the specific granules of the islet cell are derived from the nucleolar substance. The nucleolus passes out of the nucleus not only in the mode described above, but also in the form of larger spherical droplets or corpuscles formed by its constriction as is seen in figure 15.


Argentophile granules and urano-argentophile filaments seem to make their appearance in this stage; their development, so far as can be seen from my material, has already been mentioned (figs. 25, 45, 46). Certainly, the cells with nuclei containing many argentophile granules, as well as those with delicate, tortuous urano-argentophile filaments, judging from their shape and position, are nothing but acinus cells which are destined to transform into islet cells.

The Golgi apparatus of the acinus cell decreases in volume in the first stage (fig. 24), and seems to disappear finally. The irregularly spherical or oval corpuscles described above, which show the same staining reaction as the apparatus and which form a network by fusion, in all probability appear in the second stage (figs. 34 to 36). How they are formed and what relation they bear to other cell constituents it is impossible to determine definitely, although it is certain that they are neither the remains of, nor are they derived from, the Golgi apparatus of the acinus cell from which the islet cell has taken origin.

It will be seen from the foregoing that the most striking features in the transformation of acinus cells into islet cells are, on the one hand, the disappearance of zymogen granules, mitochondria, and the Golgi apparatus characteristic of the former cells and, on the other, the increase in the amount of nucleolar substance and of nucleolini, followed by their passing out of the nucleus. This nucleolar substance, after passing out, forms the specific granules of the a and h type of cell, while the nucleolini give rise to the argentophile granules. The urano-argentophile apparatus and the intracellular network of the islet cell are also newly formed cell-structures. The passing out of the nucleolar substance and nucleolini leads to the decrease in volume of the main nucleolus — a condition which is regarded as characteristic of the islet cell. It must be mentioned, however, that this process is not limited to the above case; I have also found (Saguchi, '20) that the mitochondrial filaments of the acinus cell are derived from the nucleolar substance, and that nucleolini can pass out of the nucleolus and eventually out of the nucleus. In the case of the islet cells, the process must be regarded as being accelerated to a considerable degree.


Ultimate fate of islet cells

Most of the investigators who beheve that islet cells are derived from acinus cells seem to admit that there is also a reversal of this transition. According to Lewaschew ('86), the transformation begins with the accumulation of the cytoplasm around the nucleus and ends with the appearance of zymogen granules, while the cell boundaries become gradually more pronounced and the cell-body larger. Laguesse ('95-'96, '01) likewise describes that, in this transformation, the cell-body becomes pyramidal; the nucleus, in which a large nucleolus now appears, passes to the base of the cell and the minute granules of the cj^toplasm disappear, while zymogen granules are produced. In addition, this author pointed out that it is more difficult to follow 'the period of involution' (that is to say, the stage in which the islet returns to the acinus) than 'the period of deconstitution of the acinus,' as the process of involution rapidly spreads over the islet. Laguesse ('01) regards, as transitional, areas that are occupied by somewhat larger, indistinctly bounded cells, and by more numerous centroacinus cells, and through which solitary islet cells or small groups of them are scattered.

It is, as pointed out by Laguesse, a difficult matter to follow the transformation of islet cells into acinus cells; this is perhaps due to the very indistinct nature of change which takes place in the process. Nevertheless, I have been able to detect what are to be regarded as transitional forms between the two tissues. While c cells (figs. 52, 53), as shown under the previous heading, can be considered as derived from 6 or e cells, there exists, on the other hand, no connection between these and any other type of islet cells. They seem rather to be in close relationship with the acinus cells. In the peripheral part of the islet we often find cells with a non-granular, rather transparent cytoplasm, and a more round, vesicular nuclei (figs. 54, 55). These cells may be regarded as transitional between c cells and acinus cells; in common with the former, they contain no zymogen granules and the nucleolus is small, while they have the non-granular cytoplasm with a small number of mitochondrial filaments characteristic of acinus cells. They cannot be regarded as acinus cells in the resting state, since the resting cell, besides still containing some zymogen granules, has a large nucleolus and a great number of thick mitochondrial filaments.

From these observations I have been led to the conclusion that the transformation of islet cells into acinus cells takes place in the following manner: First, the cytoplasm of the c cell becomes transparent by losing its granular contents; the nucleus then rounds off and comes to stain a darker color; finall}^, there appear mitochondrial filaments around the nucleus. The cells thus formed, yet containing neither zymogen granules nor large nucleoli, remain for some time in the peripheral part of the islet, during which period they store up, on the one hand, the nucleolar substance so that the nucleolus increases in volume, and produce, on the other, mitochondrial filaments which afterward can take part in the formation of zymogen granules, as do the typical acinus cells.

Formation and fate of the typical islet

From the above observations it follows that a cells are derived from granular cells and that c cells are transformed into acinus cells. On the other hand, there exist transitions between the various types of islet cells; that is to say, between a cells and h or e cells and between h and c cells. Considering the existence of these transitions, it is conceivable that the cells constituting the typical islet are supplied from acinus cells on the one hand, and transformed into them on the other. The formation of the typical islet (see text-figures) must be regarded as being inaugurated with the development of solitary a cells interspersed among acinus cells from which they are derived (fig. 2 and text fig. a). These solitary islet cells with the addition of a few newly formed ones constitute small islets. They consist, for the most part, of h cells, situated at the periphery, and of e cells, placed in the center, the cells belonging either to an acinus or to two or three neighboring acini (fig. 3). The latter is especially the case if the islet shows a tendency to enlarge. But it cannot be said that all of the small islets pass into the large typical ones. Most of them return, I think, after a relatively long duration, to the acinus tissue through the stage of the c cell; this seems particularly to be the case in the peripheral part of the pancreas, where large islets usually are not found. Some of them, however, especially those which are situated in the center of the pancreas, may enlarge at the expense of the whole acinus, even of the several neighboring acini. At the beginning of this transformation the initial islet cells maintain the same arrangement as is seen in an acinus; at one end, they are in contact with the basal membrane; at the other, with the cells of the opposite side so that the contact line is in continuity with that of the neighboring acinus cells, as seen in figure 7 (also text fig. b) . In the course of time, this arrangement changes; the cells become elongated so that there is interdigitation between the opposite cells (text fig. c) ; the process continues until the upper ends of the cells touch the basal membrane of the opposite side. Thus the typical cellcord of the islet is formed (text fig. d, and fig. 1). From this mode of formation, it is evident that the diameter of the cord is nearly the same as that of the acinus, most of the islet cell being double the length of the acinus cell. The typical islet thus formed seems to be in existence for a relatively long period, during which the islet cells are formed by the conversion of acinus cells, while some of them revert to the latter. This influences not only the form, but also the size of the islet, and conditions the inconstancy of the contour. From these circumstances, it is conceivable that, if the newly formed islet cells exceed in number those cells which are destined to return to the acinus tissue, the islet would increase in size; and that, if the reverse is the case, it would be reduced. In addition, the increase in volume of the islet seems, in part at least, to be effected by the mitosis and amitosis taking place within it; the decrease in volume, on the contrary, is due entirely to the return of the islet cells to the acinus tissue. Notwithstanding close inspection, I have not been able to detect any degenerative process in islet cells, the amitotic figure often found in the islet being no sign of cell degeneration as shown in the previous chapter,

Figs, a, b, c, d Schematic figures representing the development of islet cells and the change of their arrangement for the formation of the typical islet. Islet cells are shaded; 6c, blood-capillary.


I cannot say how long the typical islet exists as such. The process of increase and decrease in bulk must take place very slowly. It is also an extremely difficult matter to determine whether an islet is in the course of development or of involution. Laguesse ('01, '09-' 10) distinguishes between three periods in his evolutionary cycle of the islet: the deconstitution period of the acinus, the resting period, and the involution period of the islet. I am of a different opinion, so far as I have been able to ascertain, no such distinction can be made in the structure of various islets, but one islet may contain cells in all three periods; in other words, it may have not only b and e cells, but also a and c cells. The ratio of these types of cells in the islet is not in every case the same, which fact may be taken advantage of in determining whether an islet is in the evolutive or in the involutive period. Some difficulty is met with, however, by this determination; for example, in a case where the islet is rich in b cells, it is hard to decide whether they are derived from a cells or from e cells. On the other hand, I have not been able to perceive a figure corresponding to the involutive change mentioned by Laguesse which occurs so abruptly that it spreads rapidly over the whole islet. In fact, solitary islet cells and small groups can be found scattered throughout the pancreas of the frog, and it would be obviously absurd to connect these with the involutive change.


==Functional Significance of Islet Cells==

As to the functional significance of the islet there is considerable difference of opinion, which may be briefly summarized as follows: 1) The islets have no function worth mentioning ( Nagay o) . 2) They have something to do with the nervous system (Langerhans, '69). 3) They belong to the lymphatic structure (Kiihne and Lea, Sokoloff, Renaut, Mouret, Pischinger, Pugnamt, Schlesinger, Katz and Winkler) . 4) They are either embryonic remains or incompletely developed acini (Gibbes, Piersol, Harris and Gow, Gianelli '00). 5) They are either exhausted or temporarily modified parts of the pancreatic acini (Lewashew, '86; Mankowski,'02; Vincent and Thompson, '06). 6) The islet cells are real glandular elements and yield the secreted matter to the pancreatic duct (a view advanced by Gianelli in 1898).

7) The islets are either the parenchyma changed pathologically or a stage of its regressive metamorphosis (Kasahara, Grineff, '11; Fischer, '12); Dogiel ('93) believes them to be dead spots.

8) The islets belong to that group of glands which are assumed to take part in the so-called internal secretion. This is the view of many investigators, such as Laguesse ('93, '95-'96, '09-' 10), Diamare ('99), Ebner ('99), Hansemann ('02), Pearce ('02-'03), Heiberg ('09), Piazza ('11), and others.

It is evident, from these various views advanced by different investigators, that the islet is a tissue, the functional significance of which is extremely difficult to determine. From the topographical and cytological behavior of the islet, it is impossible to conceive that it has no function, or that there exists any connecJ:ion between the islet and the nervous or lymphatic system. The protoplasm of the cell elements composing the islet is too well differentiated to be regarded as embryonic remains. In their fully developed condition they have no connection with the lumen, which excludes the possibility that they participate in the pancreatic secretion. The islet cells show no sign of regressive metamorphosis. There can be found no degenerative change of the nucleus; the latter rather multiplies by mitosis and, in part at least, by amitosis which never leads to the death of the cell. Most of the islet cells contain fatlike corpuscles; this accumulation of fat, however, is not associated in any way with the degeneration of the cell as mentioned by Dogiel, but seems rather to have an important share in the function of the cell. Finally, islet cells cannot be identified with a resting stage of acinus cells, for they differ from the exhausted cells in their cytoplasmic structure.

The various views mentioned above being excluded as improbable, we are left with a conception that the islet is an organ for internal secretion, giving certain substances to the bloodstream. I, for my part, do not hesitate to admit this hypothesis as the most probable one. A strong argument for it is that most of the islet cells are, at one or both ends, in contact with distended capillaries. Even when there is a solitary cell or a few cells forming a group, the blood-capillary to which the islet cells are attached has a wide lumen. It is also seen that the bloodcapillary widens with the development of the islet. These facts, together with the fact that the islets have no lumina which are continuous with the pancreatic duct, strongly suggest that the cells bear a close relation to the blood-vessels. Import and export of substances are faculties with which living cells are endowed; the gland cells, for example, take on materials from the blood-stream in order to give the secreted fluid to the secretory duct. In such a tissue as the islet, where most of the cells are not in contact with any other tissue than a, blood-vessel, the elaborated products can be given only to the blood-stream.

Since the substances imported from the blood usually do not appear as formed elements, it is safe to conclude that the various cytoplasmic constituents of the islet cell are morphological expressions of substances formed by the elaboration of the imported material and are to pass into the blood-stream.

Now the question arises, which of the cytoplasmic constituents must be regarded as the specific secretion of the islet cell? As for the specific granules, they are contained in the a and h cells which are situated in the peripheral part of the typical islet and which are often of such a form and an arrangement to suggest compression against the neighboring acinus tissue, so that they do not always come into intimate relationship with the blood-capillary. These facts demonstrate sufficiently that the specific granules take no important part in the secretion of the islet cell. Like the mitochondrial substance to which they are similar in chemical and staining reactions, they are to be regarded rather as the mother-substance of secretion.

The e cells, on the contrary, form the principal elements of the islet, being in close relation to the blood-vesselsi The lipoid corpuscles and urano-argentophile apparatus, which are present in a fully developed state in the e cells, must therefore be looked upon as specific, secreted matter of the islet. In a previous paper ('20) I have shown that the acinus cell produces two sorts of secretions: one derived from zymogen granules and the other collected in the form of the Golgi intracellular apparatus. In a similar manner, the islet cell produces the lipoid and urano-argentophile substance; the lipoid corpuscles differ in some respects from zymogen granules, although the two are in accord in that they both offer little resistance to the action of acetic acid; they bear rather a strong resemblance in its chemical character to the lipoid granules found in the basal portion of the acinus cell; at least, the result of fixing and staining of the two is nearly the same. The urano-argentophile apparatus, on the other hand, corresponds to the Golgi intracellular apparatus of the acinus cell in that they can both be brought into view by the Cajal uranic nitrate-silver method. But that there is some difference between them is obvious from the fact that the Weigl and Kopsch methods do not exhibit the same thing, but another network of a different nature in a and h cells. However that may be, I am of the opinion that the above two cell constituents, after having undergone a certain chemical alteration, pass into the bloodcapillary.

I have mentioned in my previous paper on the pancreas of the frog that the acinus cell contains lipoid granules which are collected in the basal portion, and that it is highly probable that these pass into the blood-vessel and therefore may be looked upon as internal secretion matter. From this it is conceivable with some reason that the internal secretion of the pancreas is derived from islet cells, as well as from acinus cells, though to a less extent, a view held by Lepine ('05), Sirtori, ('07), and Grineff ('11). But the same idea may also be formed from another point of view; that is to say, from the fact that the cells which are considered to yield internal secretions not only form larger and smaller groups but are also scattered throughout the pancreas.


Kanazawa, Japan

January 5, 1920


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==Plates==

EXPLANATION OF FIGURES

1 A part of the typical islet, b, 6-type of cell; d, d-type; e, e-type; A, acinus cell; be, blood-capillary. Fixation: sublimate-osmic-chromic acid (saturated solution of sublimate, 4 parts; 2 per cent osmic acid, 2 parts; 1 per cent chromic acid, 4 parts); staining: iron-hematolxyin. XllOO.

2 Cross-section of an acinus containing a solitary islet cell, b. be, bloodcapillary; A, acinus cell. The same fixing as figure 1, and staining according to Altmann's acid fuchsin method. XllOO.

3 A non-typical islet consisting of a few numbers of islet cells; the latter belonging to three adjoining acini, b, 6-type; e, e-type; C, centroacinus cell; A, acinus cell; A', acinus cell showing the nucleolar hyperchromasy of the nucleus and the cytoplasm, and preparing for transformation into the islet cell. The same fixing and staining as figure 1. XllOO.


PLATE 2 EXPLANATION OF FIGURES



4 A part of the typical islet, b, b-type of cell; c, c-type; e, e-type; be, bloodcapillary; A, acinus cell. Fixed in 4 per cent formaldehyde and stained with iron-hematoxylin. X 1 100.

5 A part of the typical islet, e, c-type of cell; be, blood-capillary; A, acinus cell. Fixed and stained according to Ciaccio's method for lipoid proper ('11). XllOO.

6 A part of tj'pical islet, e, c-type of cell; be, blood-capillary; A, acinus cell; Treated according to Cajal's uranic nitrate-silver method for the Golgi's net. XllOO.

7 A non-typical islet; a group of islet cells at the end of an acinus, where they have not yet their definitive shape and arrangement, the contact line of the opposite acinus cells being in direct continuity with that of the opposite acinus cells. 7, islet cell (e-type); A, acinus cell. Fixation: sublimate-osmic acid; staining: iron-hematoxylin. XllOO.


PLATE 3

EXPLANATION OF FIGURES



Figures 9, 10, 14, 15, 17, 20, and 21 are of sections of the pancreas fixed and stained as in figure 1, plate 1. Figures 8, 11, 12, and 16 are of preparations treated as in figure 2, plate 1. Figure 13 is of a section fixed in Zenker's fluid and stained with iron-hematoxylin. Figure 18: fixing according to Meves and staining with iron-hematoxylin. Figure 19: the same fixing and staining as figure 7. Figure 22: fixing according to Rabl (sublimate-picric acid) and staining with iron-hematoxylin. Figure 23: fixing in trichloracetic acid according to Holmgren and staining with Mayer's hemalum. X2400.

In figures 8 to 59, all cells are so delineated that the cell-ends where they are in contact with blood-capillaries are always directed toward the lower side of the plate, except the case in which the islet cell comes in contact with the blood-vessel at both ends. X2400.

8 to 11 Various stages of transformation of acinus cells into islet cells. In figures 8 and 9 the nucleus and the cytoplasm show the nucleolar hyperchromasy; mitochondrial filaments being in the process of liquefaction. Figures 10 and 11 show the passing out of an intranuclear constituent in the form of filaments or of granules; diminution of the nucleolus. X2400.

12 and 13 Accumulation of the granules and filaments passed out. X2400.

14 a-type of cell. X2400.

15 A transitional form between the a-type and b- or e-types of cell. X2400.

16 /^-type of cell, containing specific granules and some lipoid corpuscles. X2400.

17 6-type of cell. X2400.

18 e-type of cell, containing delicate mitochondrial filaments, and vacuoles corresponding to the lipoid corpuscles dissolved out. X2400.

19 e-type of cell, containing mitochondrial filaments, which are somewhat thickened in consequence of the action of osmic acid. X2400.

20 d-type of cell. X2400.

21 e-type of cell. X2400.

22 e-type of cell, containing pigment corpuscles and granules. X2400.

23 e-type of cell; the nuclear network is distinctly stained with hemahmi. X2400.

PLATE 4

EXPLAXATIOX OF FIGURES

Figures 24 to 33 are treated according to the Cajal's uranic nitrate-silver method for the Golgi's network. Figures 34 to 41: according t ) Weigl's osmic acid method for the Golgi's net. Figures 42 to 44: fixing in saturated solution of sublimate, and staining with iron-hematoxylin. X2400.

24 Acinus cell preparing for transformation into the islet cell, the product of liquefaction of mitochondria being seen as droplets stained a darkly brown color. X2400.

25 to 28 Various stages of develojiment of the urano-argentophile apparatus. The cells belong to the a- and ?;-typ'3s. X2400.

29 and 30 Cells (e-type) containing the fully developed urano-argentophile apparatus, and vacuoles corresponding to the lipoid corpuscles dissolved out. X2400.

31 and 32 Cells belonging to the c-type. X2400.

33 A cell with the darkly staining protoplasm and the filamentous uranoargentophile apparatus, perhaps corresponding to the d-type. X2400.

34 to 37 Acinus cells preparing to transform into islet cells, round or oval irregularly shaped corpuscles appearing in the cytoplasm. X2400.

38 to 40, 42 to 44 Various forms of the Golgi's network. X2400.

41 A fully developed e-cell, which contains no Golgi apparatus. X2400.

PLATE 5

EXPLANATION OF FIGURES



Figures 45 to 50 are of sections fixed in 10 per cent formalin and afterw^ard treated with silver nitrate according to Cajal. Figure 51: fixing in 10 per cent formalin, and staining with Altmann's acid fuchsin. Figures 52 to 55, 58 and 59: from sections fixed in sublimate-osmic-chromic acid, and stained in iron-hematoxylin except the figures 55 and 58, which are stained according to Altmann. Figures 56 and 57: Zenker-fixing and iron-hematoxylin-staining. X2400.

45 to 47 Transitional forms of acinus cells into islet cells, the passing out of argentophile granules from the nucleus. X2400.

48 and 50 e-cells, containing no argentophile granules, but clear vacuoles corresponding to the lipoid corpuscles dissolved out. X2400.

49 Pigment corpuscles in the cytoplasm. X2400.

51 Showing a nucleus, the nucleolus of which contains a small clear vacuole corresponding to the nucleolinus. X2400.

52 and 53 c-cells, in which the transparent protoplasm has appeared at one end of the cell. X2400.

54 and 55 Transitional forms between the islet and acinus cell, the reappearing of mitochondrial filaments around the nucleus. X2400. 56 to 58 Various phases of mitosis. X2400. 59 Amitotic cell-division. X2400.


PLATE 6

EXPLANATION OF FIGURES



60 and 61 Sections of the pancreas, showing the distribution of internal secreting cells which either are scattered solitarily or form smaller or larger groups. The smallest black spot shows a solitary islet cell; cell groups or islets are delineated in various sizes according to the number of cells constituting them. Figure 60, X180; figure 61, XIOO.


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[[Category:Historic Embryology]][[Category:1920's]][[Category:Pancreas]][[Category:Endocrine]]
[[Category:Draft]]