Paper - Some features of the histogenesis of the thyreoid gland in the pig (1910)
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Moody RO. Some features of the histogenesis of the thyreoid gland in the pig. (1910) Anat. Rec. 4: 429-.
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Some Features of the Histogenesis of the Thyreoid Gland in the Pig
Robert Orton Moody
From The Hearst Anatomical Laboratory Of The University Of California
That the thyreoid gland of pig has its origin in a median and two lateral elements which unite early in embryonic life to form a one lobed gland, lying ventrad of the trachea, was definitely determined by Born and confirmed by other investigators. But concerning certain features of its histogenesis, different views have been expressed. The development of the connective tissue framework, the processes and relation of follicle and colloid formation and some other disputed points are the subjects of this investigation.
Wolfler, one of the earlier investigators, is quoted by Lustig as follows:
The epithelial vesicles are formed from masses of round or elongated cells having large, round nuclei surrounded by very little protoplasm. Towards the end of the foetal period and after birth the peripheral elements of these groups of cells dispose themselves in a circle and assume a cubical form, while the central elements become at first granular, then degenerate and disappear in the pale granular mass that fills the lumen of the vesicle thus formed, which is lined with epithelium. Lustig then adds "concerning the form, size and general characteristics of the epithelial masses and their transformation, my observations agree entirely with those of Wolfler."
Hertwig describes the formation of the vesicles as follows:
"The cords acquire a narrow lumen around which the cylindrical cells axe regularly arranged. Then there are formed on the cords at short intervals enlargements, which are separated by slight constrictions. By the deepening of these constrictions the whole network is finally subdixdded into numerous, small, hollow, epithelial vesicles or follicles, which are separated from one another by highly vesicular embryonic tissue. Subsequently the follicles increase in size, especially in the case of man. This is due to the secretion by the epithelial cells of a considerable quantity of colloid, which is poured into the cavity of the follicles."
Souli6 and Verdun in their study of the development of the thyreoid in rabbits and moles, referring to a rabbit embryo of 15 nun. say: "The cords which constitute the median thyreoid no longer present a uniform caliber throughout their entire length; at intervals they show swellings which are hollow ampullae lined with cubical epithelium. This is the first appearance of the follicles of the gland."
Tournaux and Verdun, describing the thyreoid in a human embryo of 32.4 mm. say: "The cell cords have not a regularly cylindrical form but carry throughout their length spherical or ovoid enlargements, in which there are central cavities. The cords average 30-40 microns in diameter, increasing to 80 at the level of the dilatations, which are formed of small polyhedral cells heaped on each other around the central excavation. Ill many places the wall of the vesicle appears thickened in the form of a bud, which gives the external surface a varicose appearance."
Thus it is seen that Wolfler and Lustig found in the pig and some other animals that the formation of the follicle and the colloid are synchronous, late in foetal life, both are formed by the degeneration of the central portion of masses of cells. Souli6, Toilrnaux and Verdun, however, find that in man, rabbit and mole, follicles appear early in foetial life, formed from swellings on, or enlargements of the primitive cell coluilins, and that the formation of collpid takes place at a later period. Hertwijg offers another slightly different view: thai a lumen firist appears ill the 6ords, upon which alternate enlargements and constrictions occur later to form the follicles. * '
Embryo pigs in the earlier stages, 5 to 35 millimeters in length, were fixed in Zenker's fluid, cut in serial sections 5-10 microns thick and stained with Mallory's connective tissue stain, as modified by Sabin or with haematoxylin and congo red.
From older embryos, 40-280 millimeters in length, the glands were removed, fixed in Zenker's, or in van Gehuchten's fluid, and prepared as above for general study. For the further study of the connective tissue framework, two methods of digestion were used: Flint's method of piece digestion for the demonstration of the framework of organs and Hoehl's method of the digestion of thin sections on slides, every alternate section being kept without digestion for control, as suggested by Clark.
In Flint's method, which the author characterizes as "tedious at the best," the time element is most variable and uncontrollable. Of two sections equally thick, cut from the same gland, carried through all stages of digestion in the same containers, one digests in three or four weeks, while the other takes as many months. The one that digests more slowly usually appears brown after a few days, while the other retains its normal color and becomes more transparent. Both eventually yield satisfactory results. The process of fat extraction may be omitted with embryonic tissues, thereby shortening the time required for digestion by ten or twelve days. It is especially desirable to omit a second extraction in older tissues, not only on account of saving time, but also to avoid injury to the sections, which adhere closely to the walls of the paper box container, so that it is almost impossible to remove them without more or less destruction of the delicate tissues. The most satisfactory method of removing the pieces of gland from the paper box is to open the latter and immerse it in a dish containing digesting fluid, after which gentle shaking may free the tissue. The use of any other mechanical force usually results in some distortion or tearing.
To ensure success with this method certain precautions must be observed. All glassware, corks, etc., must be chemically clean as the presence of even a minute quantity of certain reagents interferes with or entirely inhibits the digestive process. All fluids should be carefidly filtered, for any small particle of foreign matter may become entangled in meshes of the digesting tissue and greatly interfere with the study of the framework. When changing the fluid it is not necessary nor advisable to remove all of it from the vessel containing the sections, but enough should be left in the dish to float them in order to avoid distortion and tearing of the tissues. If for staining or any other process the specimens are to be transferred from one dish to another, a spoon with a small bowl placed at right angles to the handle is desirable.
An excellent picture of the coarser framework of organs can be seen by the use of the stereoscopic microscope, long before digestion is complete. It is advantageous to study and draw the sections at this stage, because as digestion proceeds, in spite of every precaution, delicate tissues may become twisted or torn and the complete picture ruined. The specimen may be removed from the digestive fluid, washed in water, put in glycerine, studied, rewashed in water and replaced in the fluid to complete digestion. The transfer from water to glycerine and back to water should be made through several dilutions of increasing strength. After digestion is completed the structure of the framework may be more strongly brought out by staining the tissue with aniline blue. It is possible to use the oil immersion to advantage in studjdng the finer details of thick sections.
While using Hoehl's method of digesting sections on slides, it was found that with a slight addition to the technique sections 200 microns thick may be prepared. These sections are fastened to the slide in the following manner: after removing the paraffine in the usual way sections are placed in absolute alcohol for a few minutes and then put on the slide. A fine camel's hair brush dipped in thin celloidin is put at four equidistant points of the periphery of the section and from each point is drawn quickly toward the edge of the slide. The four celloidin bands thus made hold the section to the slide, not only during digestion but also through the subsequent processes of staining and mounting.
By this method it is possible to study the framework of embryonic organs in three dimensions with the various powers of the monocular microscope, whereas in young embryos even the gross structure is so small that piece digestion and the stereoscopic microscope fail to reveal it.
After digestion, to avoid injury to the tissues, all fluids used in washing, staining and dehydrating must be put on the slide at the edge of the section drop by drop and allowed to spread slowly. Pieces of blotting paper used to absorb the fluids should never be placed on the tissues.
Pig embryo 6 mm in length
In 5 embryos of this length, the median element of the thyreoid gland is a compact sjmcytium forming a bi-lobed elongated mass of irregular outline, Ijdng in the mesodermal sjmcytium on the ventral and lateral walls of the aorta, at about the level of the second gill-arch. It is still attached to the ventral wall of the pharynx by a cord of cells forming a pedicle that varies from 30 to 75 microns in length. The entire length of the gland, including the pedicle, varies from 75 to 155 microns. The two lobes may lie in close contact, with only a thin layer of mesodermal sjmcytium between them or they may be separated throughout their whole length by a blood vessel as well as the sjmcytium.
The line of division between the two lobes corresponds with the median line of the body, so that the lobes lie one each side of this plane. This line commonly terminates at the caudal end of the pedicle, but may extend throughout its entire length to the ventral wall of the pharjmx (fig. 1). This condition together with the fact that the lateral elements of the gland are paired, suggests that at this stage the thjrreoid of pig is a paired organ.
The median element as a whole, following closely the contour of the aorta, has the shape of a piece of gutter, concave dorsad, convex ventrad. The surface in contact with the wall of the aorta is smooth, but the convex surface is studded with cell masses, varying greatly in size and shape.
The parenchjona of the gland is a sjmcytium with large, round or oval nuclei, which in two embryos are evenly distributed in an abundant cytoplasm (fig. 2).
In the other three embryos, a differentiation has taken place into an outer layer of closely crowded, elongated, oval nuclei^ radially arranged in a scanty protoplasm, and an inner area of smaller, rounder nuclei with abundant protoplasm (fig. 1).
Fig. 1 Frontal section of thyreoid of pig embryo 6 mm. in length. Magnified 175 diameters. An, median thyreoid element. N, nucleated red blood corpuscles. P£, epithelium of pharnyx. S, mesodermal syncytium.
This change when it has taken place, remains a characteristic feature until the median element is invaded by blood vessels in embryos 13-15 nmi. in length. Neither size, shape nor staining properties distinguish the nuclei of the parenchyma from those of the surrounding mesoderm.
The mesodermal syncytium consists chiefly of round or oval nuclei and endoplasm. With Mallory's stain blue exoplasmic fibrils may be seen forming from the endoplasm, which has a pinkish tinge. Fibrils of exoplasm follow closely the contour of the gland forming a delicate investment, from which fibrils may be seen passing into the parenchyma, not penetrating deeply, but surrounding one or two nuclei or passing between them. In addition to these delicate fibrils larger strands of exoplasm enter with blood vessels that pass through the gland. From the walls of these vessels or from these strands and occasionally from the wall of the aorta fibrils of exoplasm extend into the parenchyma (fig. 3). These vessels arise from the aorta and pass directly through the median element without giving any branches to the gland.
FiQ. 2 Transection of thyreoid of pig embryo 5 mm. in length. Magnified 555 diameters. An, median thyreoid element. Ao, aorta. N, nucleated red blood corpuscles. S, mesodermal syncytium.
Pig embryo 6.5 mm in length
The median thyreoid elements of two embryos are still connected by a pedicle to the wall of the pharynx, but only in one of them is it definitely bi-lobed. In the other it is extremely irregular in shape, being much cut up by the blood vessel winding through it. A branch from the aorta passes through the bi-lobed element, but this is the last stage prior to the general vascularization of this element in which blood vessels are found within the gland. Increase in size is the only noticeable difference between the gland in these and in earlier embryos.
Fig. 3 Transection of thyreod of pig embryo 5 mm. in length. Magnified 435 diameters. An, median thyreoid element. Ao, aorta. B, bloodvessel. N, nucleated blood carpusele. S, mesodermal syncytium. EF, exoplasmic fibrils.
Pig embryo 7 mm in length
Bom describes the median element of the thyreoid of embryo pigs at this age as follows: Aus einer kleinen Vertiefung zieht ein Epethelialstrang ventralwarts in der Lange von 0.1 mm. der sich zu einer von hinten her loffelartig ausgeholten Epithelmasse verbreitert. Die ausgeholte Mitte derselben ist sehr diinn so dass es oft den Anschein hat, als theile sich der Epithelstrang in zwei bogenformig divergirende Aeste. Im Innern der seitlichen Enden waren Lumen erkennbar. This description indicates that the median element is bi-lobed in appearance only, but this investigation shows that the division into two lobes is real and definite, in this as in younger and older embryos. It also shows that no lumen such as Born describes is present in the median element at this or any other stage. It is true, however, that the pedicle has at this time separated from the wall of the pharynx.
Pig embryo 10 mm in length
The changes that take place in the median element and the siUTOunding mesodermal syncytium during the development of the embryo from 7 to 10 mm. in length are chiefly those of rapid growth. At 10 nun. the cytoplasm is relatively less abundant and the nuclei more so than in earlier stages and many of the nuclei in both syncytia are in some phase of karyokinesis. There are around the periphery of the median element blood vessels that do not penetrate the parenchyma.
Pig embryo 12-16 mm in length
At 12 mm. begins the invasion of the median element by blood vessels. Sometimes the direct connection is seen between blood vessels without and within the gland, but frequently none was found between these extra-parenchymal vessels and spaces within, which contain nuclei of mesodermal origin and fibrils of exoplasm and appear to be blood vessels (fig. 4).
This invasion proceeds rapidly until embryos are 15 mm. in length, when the bi-lobed condition and differentiation of the parenchymal nuclei into a distinct central and peripheral area no longer exists, but the nuclei are similar in shape and uniformly distributed throughout the parenchymal syncytium. The parenchyma is cut into many islands of various shapes and sizes by the blood vessels as is pictured by Bom.
Fig. 4 Transection of thyreoid of pig embryo 13 mm. in length. Magnified 555 diameters. An, median thyreoid element. B, blood vessel. CN, mesodermal nuclei. EF, exoplasmic fibrils. N, nucleated blood corpuscles. S, mesodermal syncytium.
The lateral elements of the thyreoid, which arise from the ventral ends of the fourth gill arch, are now flask-shaped and still attached to the arch by a constricted neck, which as it has no lumen may be called a pedicle. These elements are formed of one or more layers of nuclei in a syncytial protoplasm lying in the mesodermal syncytium and surrounding a central cavity. Arising from this syncytium and continuous with it, fibrils of exoplasm pass centrad, forming an intra-parenchymal exoplasmic framework.
Fig. 5 Transection of the lateral element of the thyreoid of pig embryo 15 mm. in length. Magnified 555 diameters. B, blood vessel. EF, exoplasmic fibrils. PS, parenchymal syncytimn.
Holmgren has described an intercellular connective tissue framework supporting the epithelial cells of the mucous membrane of the oesophagus in Hirudo medicinalis and Proteus anguineus.
The study of these early embryos shows that the median element of the thyreoid begins as a syncytial outgrowth from the wall of the pharynx, having no intra-parenchymal framework of exoplasm and no lumen, while the lateral elements arising later in the development of the embryo have both an intra-parenchymal framework and a central lumen.
Pig embryo 15-20 mm in length
The changes in the median element during this period are an increase in the parenchymal and exoplasmic syncytia and a relatively greater increase in the number of blood vessels. In the lateral elements the rapid increase of nuclei has almost destroyed the intra-parenchymal framework, so that fibrils, cut ends of fibrils and nuclei of mesodermal origin, scattered here and there, are all that remain. The lumen has also been obliterated and these elements have gradually moved towards and finally united with the median element, so that in embryos 20 nrni. in length the thyreoid gland is a single mass. But on account of the latter origin of the lateral elements they have not yet been invaded by blood vessels and can therefore be readily distinguished from the median element.
FiG. 6 Transection of thyreoid of pig embryo 35 mm. long. Magnified 750 diameters. BC, blood corpuscles. EF, exoplasmic fibrils. PS, parenchymal eyncytiiim.
Pig embryo 20-34 mm in length
Rapid growth accompanied by comparatively gradual changes mark this period of development. The restoration of the intraparenchjonal framework of exoplasm in the lateral parts and the completion of the framework in the median part take place. The increase of the vascular system in the latter is so rapid that in most embryos blood vessels appear to form the greater part of this portion of the gland. The invasion of the lateral elements by the vascular system begins in embryos 26 mm. long and proceeds slowly, so that in pigs 34 mm. long the greater vascularity of the median part still sharply differentiates it from the others.
Pig embryo 36 mm in length
In sections stained by Mallory's method or with hematoxylin and Congo red, the peri-glandular connective tissue has all the forms of nuclei usually found during the transformation of endoplasm into exoplasm and of exoplasm into fibrillae. The large vesicular variety of nuclei predominates but the small darker staining form is abundant. There is a definite capsule varying in density. ^ Laterally, where it is crowded between the parenchyma of the gland and large blood vessels and dorsally, where it lies between the parenchyma and the trachea it is more dense than ventrally where the pressure is less.
Within the capsule the connective tissue syncytium permeates that of the parenchyma, forming an intra-parenchymal framework of exoplasmic fibrils and nuclei of the small dark-staining variety. Probably the large vesicular nuclei are also present but are not differentiated from the nuclei of the parenchyma. The interlacing fibrils of exoplasm that form the intra-parenchymal framework are continuous with those of the capsule and with those of the walls of the blood vessels within the gland (fig. 6).
These vessels are still much more nmnerous in the median than in the lateral elements. This is, however, the last stage of the series in which this differentiation is found.
Beginning with embryos of this size, the method of pancreatic digestion already described may be used with advantage in studying the development of the connective tissue framework. This method verifies the facts already established by the study of undigested, stained material.
The digestion of sections for a few hours removes all nuclei, both of the parenchymal and of the connective tissue syncytia, leaving undigested the stroma of the red blood corpuscles and the fibrillated exoplasm. The extra-parenchymal exoplasm shows a fine reticular structure which by condensation fonrs the capsule of the gland (fig. 7). The further development of this capsule is similar to the process described by Flint for that of the submaxillary gland.
Within the gland the fibrillated exoplasm forms a network with round or oval meshes approximating in size one or more of the parenchymal nuclei in imdigested specimens of the same age.
Pig embryo 4S mm in length
So far serial sections of the embryo have been used, but beginning with this stage the gland is removed before fixation. It is small, approximately spherical mass, about .5 mm. in diameter. The development of the vascular system has been more rapid in the peripheral than in the central portion of the gland, obliterating the distinction that has hitherto existed between the parts formed from the lateral and median elements.
In the periglandular connective tissue many of the nuclei are of the large vesicular type, strongly resembling those of the parenchymal syncytium, within the gland the connective tissue nuclei are smaller and stain more deeply. The uniformity and continuity of the intra-parenchymal framework is beginning to disappear, while definite thickenings of this framework, here and there, foreshadow the formation of the follicular walls. There is no other indication of follicles; the cords of cells have no constrictions nor any lumen. However, there are in the parenchymal syncytium occasional droplets of colloid between the nuclei. This colloid is not formed by the degeneration of nuclei, as described by W6lfler, for the parenchymal nuclei have a perfectly normal appearance. In pigs then the appearance of colloid precedes the formation of the follicle, and is produced by the activity of the parenchyma (fig. 8).
FiQ. 7 Transection of thyreoid of pig embryo 35 mm. long. Digested on the slide, stained with methylene blue. Magnified 187 diameters. B, undigested red blood corpuscles. EF, intra-parenchymal framework.
Pig embryo 60 mm in length
Sections stained with hematoxylin and congo red show that the parenchyma still exists as a syncytium, but occasional nuclei show more or less isolated masses of protoplasm about them. There are, however, as yet no cell membranes. Mallory's stain emphasizes the connective tissue and shows clusters of parenchymal nuclei surrounded by stronger strands of fibrillated exoplasm. The rapid increase of parenchymal nuclei has still further broken down the mesodermal network, but strands of exoplasm may still be seen scattered here and there among the nuclei. Drops of colloid have increased in number and size, but there are still many masses of cells in which there is no appearance of colloid (fig. 9).
There is no evident determining factor as to where these drops of colloid appear. They may be separated by one or by many nuclei, or they may be close together with only a bit of protoplasm intervening; they may occur close to blood vessels or more remote from them.
Digested specimens confirm the story already told. Isolated areas with stronger strands of connective tissue fibrils around them contain a reticulum of finer fibrils. In some of these areas where the continuity has been broken, the finer fibrils have been washed away during preparation.
Pig embryo 70 mm in length
At this stage are found the first follicles with completed walls (fig. 10). These are few in nuuxber and only seen in sections stained by Mallory's method. Digested specimens show a framework enclosing irregular spaces of varying sizes and shapes, none of which are as small as the follicles. Delicate strands of fibrillated exoplasm extend from this framework into the spaces forming incomplete partitions, which ultimately become follicular walls. These first formed follicles differ from those in the adult in the syncytial character of the epithelial lining, which is a single layer of nuclei surrounded by protoplasm. Between some of these nuclei fibrillated exoplasm may still be seen (fig. 10).
The colloid drops are increasing in mmiber and size throughout the gland and the rapid increase in nuclei is completing the breaking down of the intra-parenchymal network.
Fig. 8 Section of thyreoid of pig embryo 45 mm. in length. Magnified 500 diameters. B, blood capillary. BC, blood corpuscles. C, drops of colloid. PS, parenchymal syncytium. EF, exoplasmic fibrils.
Fig. 9 Section of thyreoid of pig embryo 60 mm. in length. Magnified 500 diameters. B, blood capillary. BC, blood corpuscle. C, drops of colloid. EF, exoplasmic fibrils. PS, parenchymal syncytium.
Fig. 10 Section of thyreoid of pig embryo 70 mm. in length. Magnified 500 diameters. B, blood vessel. BC, blood corpuscle. F, follicle.
Pig embryo 100 mmin length
At this age the secretion of colloid is abundant throughout the syncytium. The growth of connective tissue has been rapid, resulting in the formation of many complete and incomplete follicles . In some follicles the nuclei are not arranged in a definite outer layer so that they do not encircle the colloid, which is separated in these places from the wall of the follicle by protoplasm alone.
Fig. 11 Section of thyreoid of pig embryo 100 mm. in length. Magnified 372 diameters. B, blood vessel. BC, blood corpuscle. M, follicle wall. C, colloid.
The size of the colloid drops seems to bear no definite relation to the development of the connective tissue wall of the follicle, many of the larger drops lie in masses of nuclei without follicular walls, while some of the smaller drops are enclosed in a complete follicle.
Fibrillated exoplasm is now rarely seen between the nuclei assembled around a drop of colloid. It is more common among the masses and columns of cells not differentiated into follicles, but even here it is disappearing.
Some blood vessels have developed walls of considerable thickness from which large strands of connective tissue pass into the parenchjrma in such a way as to suggest future lobulation.
No differentiation is now to be seen between the central and the lateral parts of the gland in vascularity, colloid formation or connective tissue development (fig. 11).
Pig embryo IOO-I4O mm in length
During the period in which the embryo is increasing in length from 100 to 140 mm. the rapid formation of follicles by the growth of septa, and the increase of colloid continue, accompanied by a corresponding increase in the syncytium of the gland. In embryos about 140 mm. in length distinct cell outlines are first found in the parenchyma. These appear in the older follicles and are not seen in the undifferentiated cell-masses which are, however, not nimierous. Hence it is clear that colloid is formed for a considerable time while the gland is a syncytium.
Pig embryo 170 mm in length
The division of the syncytium into follicles is essentially complete. Branching follicles, such as Streiff as described in man, now begin to appear and are found in all later stages. The transformation of the parenchymal syncytium into cells has proceeded rapidly. Digested sections show the follicle walls to be formed of reticulated connective tissue, the fibrils of which may readily be seen with higher powers.
In pieces of the thyreoid prepared according to Flint's method, stained with aniline blue, mounted in glycerine, the framework of the gland may be seen to a considerable depth. These preparations show septa of connective tissue passing from the walls of some blood vessels to become continuous with the walls of other vessels or with the capsule of the gland.
The transfonnation of the syncytium into cells is completed, and in section the gland is seen to be made of follicles, the definite inter-foUicular framework carrying a rich supply of blood vessels, and masses of cells that have been called resting cells lying here and there between the follicles. The parenchymal epithelium is of the low cuboid variety with no differentiation into chief and colloid cells as is described by Langendorflf.
Fig. 12 Transection, 1 mm. thick, of thyreoid of pig 2 days old. Magnified 31 diameters. Mounted in glycerine, and drawn with the aid of the stereoscopic microscope. B, blood vessel. C, capsule. S, septa.
The follicles are losing their earlier globular shape and are becoming more polyhedral in form. Digested sections show a marked increase in the number of connective tissue fibrils in the follicle wall, which results in amuch finer meshedreticulum. Block digestion of transections of the entire gland shows an almost kidney shaped outline, the connective tissue entering at the hilum with blood vessels, and apparently dividing the gland into irregular lobules. These septa, however, are not continuous throughout the gland so that the lobulation is incomplete. The size and shape of the follicles is well shown (fig, 12).
Fig. 13 A digested free hand section about 1 mm. thick of thyreoid of adult pig. Drawn with the stereoscopic microscope and reflected light. Magnified 25 diameters. B, blood vessel. F, follicle. M, follicle wall. S. connective tissue septum.
The follicles have increased in size and number and the consequent crowding has further developed their polygonal form. Their walls have increased in thickness and their component fibers are larger and stronger. These changes are readily seen with the stereoscopic microscope in sections 1 mm. thick (fig. 13).
With greater magnification may be seen the connective tissue fibrils and the reticular structure of the walls as well as the coarser network of the septa and of the capsule. There are also in some preparations small round and oval openings in the follicular walls distinctly unlike the openings between the .meshes of the connective tissue (fig. 14).
Fig. 14 Part of 13 highly magnified. M, follicle wall. S, septum. C, capsule, cf, cut follicle. F, follicle.
The median element of the thyreoid of the embryo pig in the earlier stages, is a distinctly bi-lobed syncytium with neither an inter-nuclear .mesodermal framework nor a lumen. The mesodermal sjmcytium enters the parenchymal syncytium in two ways : it is carried in by blood vessels, and passes directly in from the surrounding mesoderm. The vascularization of this element takes place in eml^ryos about 14 mm. in length.
The lateral elements are also syncytial in character, but have an intra-syncytial framework of exoplasm and a central lumen. This framework disappears after the union of the lateral and median elements, which takes place in embryos about 20 mm. in length.
The lateral and median elements can be distinguished by the diiference in vascularization until the embryos are about 35 mm. in length.
The intra-parenchymal framework of exoplasm is present throughout the gland in embryos about 35 mm. long, but as such soon disappears.
Colloid is first formed early in embryonic life, before the formation of follicles and while the parenchyma is still a syncytium. In pig embryos colloid is not formed by cell degeneration.
The follicles, first found in embryos 70 mm. in length, are formed from the parenchyma by the ingrowth of connective tissue from the walls of blood vessels and from the capsule and by the strengthening of portions of the intra-parenchymal exoplasmic framework.
Epithelial cells formed from the parenchymal syncytium are seen first in embryos about 140 mm. in length. The transformation of this syncytium into epithelium is completed before birth.
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