Paper - On the idiosome, golgi apparatus, and acrosome in the male germ cells
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Bowen RH. On the idiosome, golgi apparatus, and acrosome in the male germ cells. (1922) Anat. Rec. 14(1): 159-180.
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On the Idiosome, Golgi Apparatus, and Acrosome in the Male Germ Cells
Robert H. Bowen Department of Zoology, Columbia University
The terms idiosome, Golgi apparatus, and acrosome have been rather generally applied in recent years to three characteristic structures of the male germ cells of animals, where they have proved a source of error and confusion for practically all students of spermiogenesis. Indeed, our understanding of these structures is now so beclouded that, even in recent text-books of cytology, they have usually been dismissed with a few casual comments, for the most part incorrrect. The one point upon Which there seems now to be fairly general agreement is that these three cytoplasmic components are definite and distinct parts of the cell, unrelated, in any immediate Way at least, to mitochondria, chromidia, or other formed elements of the cytoplasm. In other Words, the structures considered in this paper are to be thought of as parts of the cell comparable from the standpoint of morphological identity with the mitochondria, the centrioles, and perhaps even the nucleus. Recent studies have made it increasingly clear that the chief source of past errors is to be found in the technical methods which Were. formerly employed almost exclusively for the study of spermatogenesis. But since the problems of the cytoplasm have been attacked with a better appreciation of the fundamental diﬂiculties involved, there has been an immediate clarification of many disputed points. My own interests happen to have centered in this field, and as a result of studies on the formation of the sperm in a variety of animals I have arrived at a fairly clear idea of What seem to me the fundamental features in the morphology of the idiosome, Golgi apparatus, and acrosome in the male germ cells. I propose, therefore, in this paper to attempt a synthesis of my own and others’ results, with a View to putting the available facts into a connected story which may serve as a supplementary chapter to the current text-book descriptions of spermiogenesis. That this account will prove to be correct in all respects is rather improbable, for our knowledge of the facts in different animal groups is still fragmentary and in many cases quite unsatisfactory; but I do hope to arrive at some scheme, satisfactory at least in a descriptive way, which may serve as a point of departure for further researches. No attempt will be made to treat the subject from an historical standpoint except in the interests of clearness, and illustrations will be drawn wherever possible from my own material, with which I am most familiar. The account will be limited to the conditions found in typical, ﬂagellate sperms, since these are the only ones so far adequately investigated. I shall endeavor, also, to bring out particularly those features which still require further research for their elucidation.
When the testis of almost any animal (with the exception of many insects)——-a mollusc or salamander, for example-~is fixed in one of the orthodox cytological ﬂuids, such as strong Flemming, and stained with Fe-hematoxylin and a counterstain in accordance with the usual procedure, there will be found in the cytosome of each spermatocytez a roughly spherical mass which takes the stain somewhat more darkly than the surrounding cytoplasm. Under good conditions, it is possible to demonstrate the centrioles Within this mass, located centrally and usually in the form of paired granules (fig. 1A). To this spherical body Meves, in 1896, gave the name of idiozom, a term which was to stand “ fuer die spezifisch beschaffene Huelle, welche die Centralkoerper in den maennlichen Samenzellen umgib‘:..” Subsequently Meves (’03) suggested the substitution of ce*ntr0theca;, a term which never succeeded in displacing its predecessor in general usage. More recently, Regaud (’10) has suggested the spelling idiosome, a change which has much to recommend it, and will be adopted in this paper. It is clear from Meves’ remarks in adopting the term centrotheca, that he regarded the association of the idiosomic substance with the centrioles as the fact of primary importancema view with which I cannot agree for reasons that will presently appear.
- 1 The acrosome, being characteristic of the developing sperm, is not of course present as such in the spermatocyte stages. This section deals, therefore, only with the idiosome and the Golgi apparatus.
- 2 Primary spermatocytes are referred to unless specifically stated to the contrary.
- 3 The usual English spelling is idiozome.
Fig. 1 Diagram to illustratcd by various to and the centrioles and D, frequent app aran larly after chromeosm c and Golgi apparatus in In
illustrate the structure of the idiosome and Golgi apparatus in the primary spermatocyte as demon l methods. Only the outlines of the nucleus and cytosome are indicat-c(l. The idiosome is s'tippled, presented as simple gramiles. A, after the oust-omary fixation with ﬂuids c-0nt.ainin.g acetic acid; B cc after treatment. by the i1nprcgnat.io11 methods for demonstrating the Golgi apparatus; C, particuiixatives with acetic acid absent or present in Very small amount; E, scattered condition of the idiosome y insects as demonstrated by many of the special technical methods for the Golgi apparatus.
The real nature of the idiosome was from the very beginning obscure, and the fact that it was spherical and contained the centrioles immediately led to a most unfortunate confusion with the so—called attraction sphere of Van Beneden (’83) and the archoplasm of Boveri (’88). These latter terms Were applied by their sponsors to the unusual protoplasmic diff erentiations which arise in connection with the centrioles during the division of certain animal eggs, but Which, as has been repeatedly urged by various workers (for example, Duesberg, ’20), have nothing whatever to do with the idiosome. The grounds for this distinction will appear more clearly in subsequent paragraphs. The point to be noted is that the terms archoplasm and attraction sphere have a very special and limited application, if indeed they have any very good basis in fact at all, and in any event are not to be used as indicative of any direct relationship with the idiosome.
This very simple condition of the idiosome‘! and its surroundings is characteristic of material which has been fixed in fluids containing a considerable amount of acetic acid. If the same kind of material (mollusc or salamander) be fixed in ﬂuids in which acetic acid is absent or present only in small amount, such as Benda’s modified Flemming or Flemming without acetic, the results are rather strikingly different. In a spermatocyte (stained Wiilh Fe-hematoxylin) from such a preparation (fig. 1C), the idiosome itself appears much as before, but closely applied to its periphery appear a number of small, crescentic rods which stain very sharply. These rods have of course long been known under a great variety of names (Archoplasmaschleifen, Centralkapsel, Pseudochromosomen, formazioni periidiozomiche, Nebenkern, etc.), since in certain animals—the molluscs particularly~—they are often preserved in more or less perfect condition even after fixation in strong Flemming. They are, as Gatenby and others have recently shown, the representatives of the Golgi apparatus in the spermatocyte. The term apparatus is a very unhappy one, for in this case, as in so many others, the Golgi material apparently forms no connected apparatus at all. It would be in the interests of clarity to refer to this substance simply as the Golgi rods or some equivalent descriptive expression. It is important to note that these rods are actually separate units, and one does not get the impression that they are interconnected in any Way by secondary fusions.
- 4 I have not thought it Worth while to include a discussion of the internal structural features which have been occasionally described in the idiosome. The meaning of these, and particularly their homologies, is as yet by no means clear. The reader who is interested in these features is referred to the papers of Papanicolaou and Stockard (’19) and Gatenby and Wbodger (’21)on the idiosome of the guinea-pig.
Again, in material prepared by one of the impregnation methods, for example, the silver reduction techniques of Golgi or Cajal, the idiosome and its surroundings may assume a somewhat different aspect. The idiosome substance itself may be more or less invisible, but its position is clearly marked by the Golgi apparatus which encloses it. The material of the Golgi apparatus is blackened intensely in successful preparations and has sometimes been described as forming a true network or reticulum, comparable to the classical figures of the ‘apparato reticolare interno’ (fig. 1B). This appearance of the Golgi apparatus has been described by numerous authors as characteristic of many somatic cells after impregnation by silver nitrate or osmic acid, but the occurrence of the Golgi material in reticular form has rarely been described in spermatocytes. Perroncito (’10) thus describes and figures the Golgi apparatus in Paludina (a mollusc) ; but it should be noted that Gatenby ("18) has figured the Golgi apparatus in the same animal as made up of separate rodlets as in figure 1C.
Finally, after the same impregnation methods referred to in the preceding case, the Golgi apparatus may appear as a shell, closely applied to the idiosome and often (always?) incomplete on the side adjacent to the nucleus (fig. 1D). The shell itself may be more or less solid, or may appear as though made up “of a number of filaments of varying thickness.” Duesberg (’20) has described such a structure for the Golgi apparatus in the spermatocytes of the opossum, and I have found more or less similar conditions in the salamander after silver nitrate (Cajal) impregnation. Furthermore, following osmic acid impregnation, I have found (salamander) that the ‘shell’ is sometimes entirely disrupted and the fragments scattered about in the cytoplasm in the vicinity of the idiosome.
How are these conflicting appearances of the Golgi apparatus to be harmonized; or, indeed, is there any reason to believe that the Golgi apparatus is always developed in the spermatocytes on the same structural plan? Unfortunately, we are not in position to give a categorical answer to these questions, chieﬂy because the majority of workers have so far been content to study the Golgi apparatus by very limited technical means. It is accordingly difficult to say in any particular case whether the results are descriptively correct or mere technical distortions of the truth. However, my own experience leads me to believe that the discrepancies in the published accounts of the Golgi apparatus, in the spermatocytes at least, are largely traceable to the untrustworthy results of the admittedly ca~ pricious impregnation methods (silver and osmic acid). Gatenby’s work on Paludina and my own results on Plethodon (a salamander) seem to me to indicate unmistakably that the probable source of our difficulties is to be accounted for along the following lines. When the material is preserved in one of the chrome—osmic mixtures, which are generally agreed to be among the best-known cytoplasmic fixatives, the condition of the Golgi apparatus is almost invariably that of a cluster of rodlets applied to the surface of the idiosome (fig. 1C). After a silver—nitrate impregnation the Golgi rods may undergo a slight disintegration resulting in their running together to form an apparent network or reticulum (fig. 1B), while still further disintegration or ‘smearing’ of the impregnation deposits may transform the Golgi apparatus into a more or less complete ‘shell’ (fig. 1D), or even disrupt the rods completely and scatter the fragments in the neighboring cytoplasm. finally, after fixation in acetic—acid mixtures, the entire Golgi apparatus may be completely disintegrated and dissolved (fig. 1A). That the actual condition of the Golgi apparatus in the living spermatocyte is most nearly comparable to a group of separate rodlcts is strongly supported not only by the general appearances which accompany what cytologists call ‘good fixation,’ but by the additional fact that fragmentary observations on living material seem to point in the same direction.5
- 5 I wish to state specifically that this at-tempted explanation of the various appearances of the Golgi apparatus in the spermatocytes after different types of technique is not to be extended inconsiderately to conditions in somatic cells as well, in which the Golgi apparatus is most commonly described as having the form of a reticulum or network. The facts now known about the possible effects of technical treatment ought, however, to lead to a more critical scrutiny of the actual conditions in cells of all kinds. The facts suggest that in tissue cells, whose tenure of life is presumably a long one, the Golgi rods normally undergo more or less fusion resulting in the reticular condition so frequently described.
It is clear from what has now been stated that the idiosome and the Golgi apparatus form in many spermatocytes a topographical unit, the center of which is occupied by the centrioles. Two questions at once suggest themselves: 1) is the close relation of the idiosomic substance and Golgi material merely a chance coincidence of location and, 2) what relation, if any, exists between these two materials and the centrioles, which would justify the retention of Meves’ original definition of an idiosome and the consequent confusion with the attraction sphere and archoplasm?
It is possible to give at least a partial answer to these questions by reason of the unique conditions which obtain in many (perhaps all) insects. In the insects the idiosome seems sometimes to occur as a compact body in the very early spermatocytes, but eventually (in all known cases) and frequently (the Hemiptera furnish particularly good examples) from the very earliest spermatocyte stages onwards, the idiosome is represented only by many separate masses of idiosomic material each accompanied by a Golgi rodlet, the whole constituting a so—called Golgi body (Bowen, ’20). These are at first definitely concentrated toward one pole of the cell and are thus directly comparable to the compact idiosome already considered. We are obviously dealing with an idiosome the constitutent parts of which have become separated from one another. In the growth period the Golgi bodies begin to migrate away from their definitely polar position (fig. 1E), and eventually become scattered throughout the cytoplasm as I have described fully in the Pentatomidae (Bowen, ’20). However, at no time is there any indication of a separation of the Golgi rodlets from the masses of idiosomic substance.
The behavior of the centrioles during these same stages offers a number of points of interest. In the very early primary spermatocytes the behavior of the centriole is fully known in comparatively few forms, but presumably the centriole from each spindle pole of the last spermatogonial division is always carried over directly into the succeeding primary spermatocyte. In the mollusc and salamander this centriole soon divides, but the parts remain close together until the maturation prophases are well advanced, and are located, as noted above, in the idiosome. In the Hemiptera, however, the centrioles behave very differently, for after dividing, the daughter centrioles begin at once to migrate around the nucleus (fig. 1E) until eventually they occupy diametrically opposite positions on the nuclear membrane. Meanwhile they each divide precociously in anticipation of the two rapidly succeeding maturation divisions. Throughout the migration of the centrioles and until the divisions of maturation begin, the Golgi bodies maintain no striking topographical relations to the centrioles.
The conditions in the insects furnish, it seems to me, an excellent basis for the interpretation of the idiosome structures as a whole. We are to think of the Golgi bodies (Golgi rodlet plus idiosomic fragment) as the units of idiosome construction. These bodies tend, for some unknown reason, to collect in the vicinity of the centrioles, just as other parts of the cell (mitochondria, for example) may be polarized toward the centrioles when these bodies are together. If the centrioles remain thus together, the Golgi bodies likewise remain massed around them, giving rise to the familiar idiosome and Golgi apparatus complex of many authors. If, however, the centrioles separate, the polarization of the cell is apparently destroyed, and the idiosome fragments migrate out into the cytoplasm, together with the mitochondria, and become scattered throughout the cell in haphazard fashion. It becomes, therefore, perfectly clear that the important and primary relation is that which exists between the Golgi material and the idiosomic substance, while their joint relation to the centrioles is merely a topographic one. The basis for Meves’ definition of the idiosome thus breaks down entirely and it is evident that we must look for a rigorous definition of the idiosome in terms of its interrelation to the Golgi apparatus rather than to the centrioles. I have suggested elsewhere (Bowen, ’20) that this relation may possibly be of a permanent nature, the two components forming essentially a unit cell structure. finally, this interpretation clears up much of the current misconception which obtains concerning the protoplasmic differentiations that sometimes surround the centrioles, such as the archoplasm, attraction sphere, centrosome, etc. These differentiations (so far as they have any existence in fact) have presumably a true organic relationship to the cellular centers, being connected with the processes of spindle formation, while the relation of the idiosomic substance to the centrioles is purely topographical and frequently does not obtain at all. Certain aspects of this interpretation will be further clarified by comparison with the following sections.
The Maturation Divisions
It is not my intention in this paper to go extensively into the phenomena exhibited by the idiosome and Golgi apparatus during the maturation divisions, in part because the details are as yet insufficiently known, but chieﬂy because the earlier workers almost invariably failed to observe any of the details by reason of the technique employed, and there is, accordingly, little to confuse the reader in the literature so far published. I wish, however, .to indicate certain broad features of the division phenomena as an introduction to the spermatid stages, which have long been a source of inextricable confusion, due largely to ignorance of the fate of the idiosome and Golgi apparatus during the division phases.
From the facts now available it appears that there are two (possibly three) methods of distributing the Golgi apparatus in the spermatocyte divisions, the method being perhaps correlated with the previous condition of the idiosome. In the case of the idiosome which early loses its compact form (fig. IE), the separate Golgi bodies undergo more or less fragmentation, prior to mitosis, which involves both the Golgi rods and the idiosomic substance. The resulting fragments (so—called dicty— osomes) gradually collect around the poles of the spindle during the metaphase, each pole receiving an approximately equal quantity (Bowen, ’20). During the telophase, when the centrioles are separating in preparation for the second maturation division, the dictyosomes are dispersed throughout the cell and are again accumulated around the spindle poles at the subsequent metaphase. During the telophase they may again become more or less scattered in the cytoplasm.
In the case of the idiosome which retains its compact form until immediately before the maturation divisions (figs. 1A to D), the end result is similar, but the intermediate steps are somewhat different. In this type, immediately preceding division, the idiosome and Golgi rodlets are divided into two groups which migrate with the centrioles to the spindle poles, where they are found at the metaphase just as in the preceding case (see, for example, Ludford and Gatenby, ’21). Subsequently they become scattered throughout the cytoplasm and in the secondary spermatocytes are again assembled around the centrioles in compact form. The process is repeated in the second maturation division.
It is possible that a third type of division occurs in which the collection of the Golgi fragments around the poles is omitted, as Duesberg (’20) has described in the opossum. But the evidence for this is very scanty and further research may prove it quite erroneous.
In this brief outline of the division phenomena, I have purposely omitted mention of several important points which are still unsettled. ‘The first of these involves the possible fragmentation of the Golgi rodlets prior to their accumulation around the centrioles at metaphase. A breaking-up or division of the Golgi rodlets certainly occurs in the Hemiptera which I have studied, and I have thought that there was good evidence of the same thing, in the salamander. Gatenby, on the other hand, insists that the Golgi rods are merely sorted out intact. The question is an exceedingly difficult one to settle on account of the inadequate technical methods now available, but it is one Well Worth further study. The second point in dispute has to do with the fate of the idiosomic substance during division. In the Hemiptera the dictyosomes are too small to analyze satisfactorily, and it is therefore impossible to say whether or not they retain the essential structure of the Golgi bodies through the division stages. From the fact that the idiosomic material can be found associated with the Golgi substance as soon as the dictyosomes have again fused to form aggregates sufficiently large to study accurately, I have concluded that in all probability each dictyosome is essentially a miniature Golgi body. Each Golgi fragment would thus be accompanied by a small mass of idiosomic material, and the distribution of both substances would accordingly be identical. No specific stain for the idiosomic material is at present known, and since it cannot be satisfactorily made out in small amounts, the question cannot be immediately solved. In the idiosome of the compact type, it seems certain that the substance accompanies the Golgi material to the spindle poles, but its subsequent fate is again obscured by the scattering of the Golgi pieces, and though it reappears with the latter around the centrioles in the late telophase, its exact intermediate behavior is uncertain. Ludford and Gatenby (’21) believe that it becomes separated from the Golgi material in the interval—a view to which their figures do not lend particular support, and which to my mind runs counter to all the other available evidence.
It will be clear from the above that the exact behavior of the Golgi apparatus and the idiosome in the maturation divisions is very inadequately understood, but nevertheless that the Golgi material certainly, and the idiosomic material probably, is distributed to the resulting spermatids in a form identical, at least topographically, with the same materials of the original idiosome-Golgi complex of the spermatocytes. Furthermore, this opportunity may be taken to point out again that, as can be inferred from the above account, the idiosome has not been observed to take any part in the formation of spindle and asters during mitosis. This, I believe, is practically conclusive evidence that the idiosome is a cellular differentiation absolutely unrelated to attraction spheres or archoplasm, whose presumed function it is to provide material for the construction of the achromatic figure.
At the close of the second maturation division the Golgi apparatus is present in each spermatid in the form of numerous, scattered pieces or dictyosomes. These may begin at once a process of fusion, so that the individual pieces become fewer in number, but larger in size. Presently they assume proportions not unlike the Golgi bodies of the growth period, and it is clear that each one is composed of the characteristic Golgi and idiosomic materials, arranged exactly as in the spermatocytes. According to the account of Ludford and Gatenby, the dictyosomes would perhaps undergo no fusion whatever, since they are, by description, complete Golgi bodies throughout the division stages. In any event, it is agreed that the Golgi apparatus and accompanying idiosome are usually present in the earliest spermatids in the form of more or less scattered Golgi bodies.
From this point on, however, the behavior of the Golgi bodies is subject to certain differences which may be conveniently classified under two characteristic types—types which recall the similar arrangements of the Golgi apparatus and the idiosomic material in the primary spermatocytes. These two types I propose to designate as follows: I, simple or fused type;
II, compound or multiple type. These two types will be considered separately.
Type I, Simple 07" fused
In the great majority of animals thus far examined, the behavior of the Golgi bodies in the spermatid follows type I. In this type the Golgi bodies, formed as noted in the preceding paragraphs, gradually draw together, their idiosomic portions apparently fusing until eventually a compact mass is formed comparable to, and indeed directly homologous with, the characteristic idiosome of the primary spermatocytes as developed in the salamander and mollusc.
The mass thus formed may assume a variety of appearances exactly similar to those of figures 1A to D, and probably, in part at least, for the same reasons. Thus, in material fixed in ace-tic—acid mixtures the Golgi apparatus is completely dissolved, leaving only a spherical mass, the idiosome, or ‘sphere’ of many authors (fig. 2A). In material fixed especially in the chrome—osmium ﬂuids with little or no acetic acid, the Golgi rodlets are sometimes clearly demonstrated, applied to the surface of the idiosome just as in the spermatocytes (fig. 2B). Very rarely a net-like Golgi apparatus comparable to that of figure 1B has been described (Perroncito, ’10), but this is decidedly unusual. Very frequently, after impregnation methods, the Golgi material forms a solid shell enclosing the idiosomic substance, though here again the shell is incomplete on the side toward the nuclear membrane (fig. 20). As a matter of fact, I have preparations of hemipteran testes in which, after the appropriate technique, all these various appearances (except the reticular condition of the Golgi apparatus) are clearly visible. The diagrams in figures 2A, B, and C are taken from these preparations. I had originally thought that the ‘shell’ condition shown in figure 2C represented the final step in the fusion of the Golgi elements begun immediately after the second maturation division was completed. Recently, however, I have succeeded in demonstrating the presence (in Euschistus) of separate Golgi rods, as shown in figure 2B, so that the condition of figure 2C is perhaps to be explained as in part an artifact.
The Golgi apparatus-idiosorne complex may be located in almost any part of the spermatid; but in insects, in which the mitochondria are aggregated into a solid, spherical mass or nebenkern, the idiosome is characteristically found near the angle between nucleus and nebenkern, and sometimes in a definite relation to the centrioles (figs. 2A to C). But the centrioles, it is almost universally agreed, are never located within the idiosome as is so characteristic of arrangements in the spermatocytes. Here, again, is a striking piece of evidence which tends to show that Meves’ conception of the relation between idiosome and centrioles is erroneous. and that the relation between Golgi apparatus and idiosome is the essential thing.
The facts thus far elucidated show clearly the basis of many of the older descriptions of the spermatid idiosome. The acetic acid in the fixative, or other technical failings, caused the Golgi apparatus to be overlooked entirely during the spermatocyte divisions. The idiosome, once reconstituted in the spermatid, became visible again with the older methods, and was called an idiosome because it looked like its predecessor in the spermatocytes of the same or some other animal. What was once a mere guess is now kndwn to have the best of basis in fact. Furthermore, the appearance of this body near the nebenkern in insects was the obvious source of the long confusion which resulted from tracing the idiosome to spindle remnants, mitochondrial derivatives, and every other possible origin. The acrosome of the mature sperm is to be derived from the complex formed thus by the idiosome and Golgi apparatus, which I shall accordingly refer to henceforth as the acroblast. The acroblast is the exact equivalent of the idiosome plus Golgi apparatus of the spermatocytes.
fig. 2 Diagram to illustrate the history of the idiosome and Golgi apparatus in the differentiation of the sperm. The idiosome and the mitochondria (nebenkern) are stippled; the centiroles and t-ail fila.ments are indicated, together with the outlines of nucleus and cytosome. A, acrosorne; 0;, acroblast; N, nebenkern. A, B, C, typical early spermatid (Hemiptera; Bowen, ’20), showing idiosomic conditions directly comparable to those of figures 1A, C’, D; D, differentiation of the vesicular type of acrosome (Hemiptera; Bowen, ’ 20); E, differentiation of the granular type of acrosome (Mollusca; Seliitz, ’1G); F, deposition of the migratory type of vesicular acrosome (Hemiptera; Bowen, ’20). The arrows indicate the direction of subsequent movement of the aerosomc and acroblast (Golgi remnant); G, deposition of the stationary type of vesicular acrosome (Ceuthophilus; Bowen, ’22); I, deposition of the stationary type of granular acrosome (Mollusca; Sehitz, ’16). In figures G, and I the arrow indicates the direction of subsequent movement of the acroblast.
The origin of the acrosome from the acroblast is essentially similar in all animals belonging to Type I, but the nature of the acrosome itself makes possible a subdivision on morphological grounds, as follows: Type I, vesicular; type II, granular.
The vesicular type of acrosome is of very common occurrence, appearing in typical form in many Mammalia, Hemiptera, and Amphibia. In this case the acrosome makes its first appearance as a small, clear, bubble—like vesicles either Within the idiosomic substance of the acroblast or on that part of its periphery from which the Golgi material is lacking. This vesicle gradually enlarges and soon projects conspicuously on one side of the acroblast (fig. 2D). The relation of the vesicle to the nucleus varies greatly in different forms, but the acrosome—acroblast complex seems in general to occupy some position near the spermatid nucleus. In the Hemiptera (fig. 2D) the vesicle is always in contact with the nuclear membrane; in Locusta (Otte, ’07) the acroblast may be in contact with the nuclear membrane, While in Ceuthophilus (Bowen, ’22) no particular relation seems regularly to occur. The ultimate size of the vesicle in proportion to the acroblast varies greatly in different animals, but the meaning of these differences is not known. Within the vesicle a darkly staining granule is characteristically developed, the granule being located typically on the inner wall of the vesicle at the point where it is in contact with the nucleus (fig. 2D). For convenience in description, I propose to call the vesicle, the acrosomal vesicle, and the granule, the acrosomal granule; together they form the acrosome.
- In some animals several such vesicles may be produced simultaneously, the separate rudiments thus developed being subsequently merged to form a single Vesicle.
The granular type of acrosome is of much less frequent occurrence than the vesicular type, having been worked out with any completeness only in Mollusca (see, for example, Schitz, ’16, on Columbella and Gatenby, ’19, on Paludina). In this type there is developed on the periphery of the acroblast a small, darkly staining granule, instead of the clear vesicle characteristic of the preceding case (fig. 2E). No definite relation between the nuclear wall and the acrosome seems to exist.
The homologies which presumably exist between the vesicular and granular types of acrosome are not at present evident, for we do not know whether the granular acrosome is to be compared with the vesicular acrosome as a whole, or only with the acrosomal granule which is developed within it. It is of course possible that the granular type may be produced by faulty differentiation, and that its fundamental similarity to the vesicular type is obscured by the stain. Furthermore, we do not understand in either case the exact source of the acrosome.
Apparently it is produced in both cases by a differentiation of the acroblast material, but whether this involves both idiosome and Golgi apparatus, or is confined to the idiosome alone is not known. The important point to note is that the acrosome is a secondary product of the acroblast and that neither idiosomic nor Golgi material goes directly into its formation. After the acrosome is completely formed, the acroblast is separated from it (figs. 2F to I), gradually moves back through the cytoplasm of the sperm tail, and, after undergoing degenerative changes, is probably eventually cast out of the sperm in all cases together with other débris of spermiogenesis. After the separation of the acroblast from the acrosome, I have suggested (Bowen, ’ 20) for it the name Golgi remnant, as a term of convenient description. The Golgi apparatus and idiosome are thus lost from the completed sperm, in which they are represented only by their differentiation product, the acrosome. Weigl (’12), and Gatenby and Woodger (’21), however, claim that in some mammals at least, a part of the Golgi material is left behind as a permanent contribution to the middle-piece of the sperm; but the exact status of this material is, I think, at present a matter of reasonable doubt.
The ultimate role of the acrosome is, in any case, to act as an apical piece for the sperm head. It must, in other words, be applied eventually to the anterior surface of the nucleus, this final location being attained, as a rule, not later than the earliest stages in the elongation of the nucleus (if any occurs) to form the sperm head. The method of deposition varies according to the location of the acrosome at the time when the acroblast is separated from it. On this basis, two types of acrosomal deposition can be recognized, as follows: Type I, migratory; type II, stationary.
In the migratory type the acrosome is developed at some point removed by a greater or less distance from the apical pole of the nucleus, and the acroblast is cast off with the aerosome still at some distance from its final place of fixation (fig. 2F). In this case the completed acrosome is left resting at some point on the nuclear wall, whence it migrates (as indicated by the arrow in fig. 2F) around the nucleus to its definitive position. This migratory type is especially characteristic of the Hemiptera (Bowen, ’20), in which the vesicular type of acrosome occurs. I do not know of any examples of this migratory type in the granular type of acrosome.
In the stationary type, the acroblast and acrosome move together to the anterior surface of the nucleus. Here the acrosome is deposited in situ (figs. 2G and I), and the acroblast (Golgi remnant) is then gradually separated from it, behaving subsequently exactly as in the migratory type. Examples of the stationary type among vesicular acrosomes (fig. 2G) are quite common, those of the mammal (see, for example, Gatenby and Woodger, ’21), Ceuthophilus (Bowen, ’22), and probably the salamander (Bowen, ’22) being representative examples. The molluscs present many examples of the stationary, granular type (fig. 21) of acrosome (cf. Schitz, ’16, and Gatenby, ’19).
The exact fate of the acrosomal granule in the vesicular type is in most cases unknown, though it seems, in Hemiptera for example, to be related to a darkly staining part which forms the tip of the acrosome. Acrosomes, particularly of the vesicular type, may undergo a great variety of differentiations, the case of Locusta (Otte, ’07) being an especially good example; but the various shapes and structural features are not known to possess any points of general interest.
In figure 2 I have endeavored to represent in partially diagrammatic form the various types of behavior which occur in the simple or fused method of acrosomal formation. Beginning witha common type of acroblast (figs. 2A to C’), either of two lines of development may be followed, leading to the production of an acrosome of either the vesicular or granular type (figs. 2D and E). The acrosome completed, it may either be deposited in situ or migrate to its definitive location subsequent to its deposition (figs. 2F, G and I).
Type II, Compound or multiple
The compound or multiple method of forming the acrosome is very incompletely known and seems, furthermore, to be of rather infrequent occurrence. At any rate, it has been observed very rarely, though this fact may be due to the much greater difficulties involved in its proper demonstration. In this type the Golgi bodies in the spermatid undergo little or no fusion (fig. 3), so that a single acroblast such as is figured in figures 2A to O is never produced. Instead, each of the Golgi bodies is apparently a small acroblast in itself»——a probable interpretation which is borne out by the fact that in the fused type the acroblast is occasionally bipartite, each partial acroblast forming an acrosome proportional in size to the available material. The deposition of the partial acrosomes from these multiple acroblasts results presumably in the gradual building up of the complete acrosome. This process could undoubtedly be carefully analyzed in an animal which produced a vesicular type of acrosome from multiple acroblasts of sufficient size, and my observations on the Lepidoptera suggest that some of the moths may furnish material suitable for the purpose.
Thus far only two cases of multiple acroblasts are known, viz., the grasshopper (Bowen, ’22) and the moths and butterfiiesfi In the grasshopper the acrosome is apparently of the granular type (fig. 3), but all the parts concerned are so minute I that thus far the exact details of the formation of the acrosome have not been made out. The granular acrosome is very small and is formed near the base of the head, moving thence around the nucleus to its apical side just prior to the drawing out of fig. 3. Diagram to illustrate the formation of the acrosome from the multiple type of acroblast (acrididae; Bowen, ’22). The arrows indicate the direction of the later movements of acrosome and acroblast. A, acrosome; N, nebenkern (mitochondria). the sperm head. It thus belongs Without question to the migratory type. In the Lepidoptera the acrosome is of the vesicular type, and is sometimes very large. In Pygaera the acrosomal granule itself is of extraordinary size and prominence. Here again the acrosome seems to be of the migratory type, but again the exact relation of the Golgi bodies to the parts of the acrosome has not been conclusively demonstrated. However, the multiple origin of the acrosome is clear, and the connection of the Golgi bodies with its formation is likewise beyond doubt. Thus far all my observations tend to bear out the accuracy of the interpretation of the multiple type of acroblast which has been outlined above.
- The notes here given on the Lepidoptera are largely based on a study of Pygaera and Callosarnia, the results of which I have not yet published.
Aside from the single and multiple types of acroblast, an intermediate condition is an obvious possibility, but I am not acquainted with any form in which the facts are actually indicative of such a condition.
In the following table I have classified some of the betterknown examples of acrosome formation according to the types which have been outlined in this paper.
TYPE OF ACROBLAST
KIND OF ACROSOME
PLACE OF DEPOSITION
Insecta Hemiptera Coleoptera Orthoptera
A mphibia Uro delia
M am mal in
l 3 I
M ultiple Single Multiple Single
Granular Vesicular Vesicular
Migratory Stationary Migratory
Stationary (‘E’) Stationary
The nomenclature employed in this paper with reference to the spermatid structures may be outlined as follows:
lacrosomal granule 180 ROBERT H. BOWEN
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