Acrosomal Granule Research Articles

The subacrosomal granule and its evolution during spermiogenesis in a lizard

Some aspects of spermiogenesis have been studied in the testis of the teiid lizard Cnemidophorus lemniscatus lemniscatus by electron microscopy. Shortly after the acrosomal vesicle is lodged in a nuclear concavity of the spermatid, a dense granule differentiates in the center of the subacrosomal space. It is cone-shaped and shows a longitudinal striation. Its base applies to the acrosomal membrane and, through this, to the acrosomal granule. Its rounded vertex causes a depression of the nuclear membranes which, initially juxtaposed, separates at this point to form a vesicle. The granule develops and becomes a rod when spermiogenesis is advanced and the subacrosomal space has taken the form of a secondary cap. The rod is cylindrical, retains its original striation and has a convex acrosomal end. It encloses the vesicle formed by the nuclear envelope in its base and follows the apex of the nucleus. Meanwhile, the acrosomal granule loses its identity and the acrosomal cap is filled with a dense substance, in which a fringe of translucent material differentiates. This fringe lies in the dorsal and apical margins of the acrosome and is incompletely divided by longitudinal crests of the dense acrosomal substance. A projection of the Sertoli cell forms an accessory cap which envelops the acrosome and is in turn covered by the cytoplasm of the spermatid, constituting an intricate association. Two reflex membranes underlie the plasmalemma in the outer surface of the projection of the Sertoli cell. They are continuous with one another at their ends and with the cell membrane in the edge of pores. In the peripheral cytoplasm of the spermatid facing the accessory cap, numerous microtubules run longitudinally. By means of thin membranes some are interconnected or connected with the plasmalemma, from which they seem to originate.

Electron microscope studies of spermatogenesis in Branchiobdella pentodonta whitman (Annelida, oligochaeta).

In Brallchiobdella pentodonta Whitman meiosis begins in follicles containing 16 spermatogonia. In each follicle the spermatogonia are connected by cytoplasmic bridges to a central anuclear cytoplasmic mass or cytophorus. They develop synchronously. Synaptonemal complexes are present in the primary spermatocytes. Spermatids contain a large globoid paranuclear body consisting of an acrosomal granule and coiled tubules which evidently receive the contents of the acrosomal granule and are considered the acrosome carrier. The spermatids separate from the cytophorus only when differentiation is completed. The ripe spermatozoon is relatively long. It has anteriorly the coiled tubules, followed by the nucleus, the mitochondrial sphere and the distal centriole from which the flagellum originates, A coiled ribbon-like structure encloses the flagellum along its entire length while a manchette of microtubules surrounds all the other structures of the sperm.

Ontogenesis of Spermatozoa Autoantigens in Guinea Pigs

The ontogenetic appearance of three independant spermatozoa autoantigens (S, P and T) has been studied in guinea pig germinal cells by immunofluorescence and comparison with cytology and histological structures during early maturation of seminiferous tubule cells. The maturation of 120 testes from 60 guinea pigs studied from day 1 to day 50 after birth has shown an evolution in 3 periods. During the first, or negative, period (day 1 to day 25), only spermatogonia (from day 1) and spermatocytes I (from day 16) are present. No significant PAS-positive formations are seen and no autoantigen is detected. During the second, or transitional, period (day 26 to 29), spermatocytes II and spermatids appear as well as paranuclear PAS-positive golgian proacrosomal and acrosomal granules. At the same time, the three autoantigens S, P and T are detected on the same PAS-positive formations with a frequency that increases from day to day. During the third, or positive, period (from day 30) all testes present cells with PAS-positive formation, progressive maturation of acrosomes in spermatids and appearance of spermatozoa (present on day 39) leading to the adult structure of seminiferous tubules. The three autoantigens are constantly present during that period. The simultaneous appearance of the 3 antigens in haploid germinal cells (spermatids and possibly spermatocytes II) as an early expression of cytodifferentiation and their total absence from diploid germinal cells (spermatogonia and spermatocytes I) seem to be of biological significance.

Fine structure of an elongated dorso-ventrally compressed echinoderm (holothuroidea) spermatozoon.

The spermatozoon of Cucumaria pseudocurata is unique among those of the echinoderms in that it is tabloid in shape, i.e., elongated and dorsoventrally compressed. The sperm consists of a dorsal surface which contains an extensive striated rootlet-like structure located within a dorsal groove and a ventral surface which contains a medially situated acrosome. A single mitochondrion lies at the base of the nucleus. The flagellum is unusual in that a 9 + 3 tubular arrangement is observed in the mid-tail region. The acrosome consists of an acrosomal granule bounded by a limiting membrane and a surrounding periacrosomal layer. The granule is irregular in shape with the anterior-posterior surfaces flaring out, forming pockets in the periacrosomal material. The ventral granule surface bulges forming a close association with the plasma membrane. The dorsal surface is indented. Ventral to the depression (within the granule) is a small area containing a particulate-fibrous material. To the inside of the granule limiting membrane there is a second membrane-like structure (incomplete) which extends from the anterior-posterior surfaces around the dorsal face of the granule. Dorso-medial to the granule the periacrosomal layer contains a particulate-fibrous region lodged within the granule depression. This material is presumably the precursor of the acrosomal filament. Prominent cytoplasmic folds extend off from the basal flagellar region. The proximal and distal centrioles are situated perpendicular to one another within the mitochondrion. Centriolar satellite materials are associated with both centrioles. Toward the base of the tail the satellite of the distal centriole consists of nine radiating arms extending at an angle of 45° to the axis of the centriole. Each arm terminates in a dense thickening. The striated rootlet extends anteriorly from the distal centriole to just below the level of the acrosome.

Electron microscopic cytochemical study of phosphatases during spermiogenesis in Chinese hamster.

Ultrastructural localization of several nonspecific and nucleoside phosphatase activities during spermiogenesis was studied in Chinese hamster testes. Acid phosphatase activity was found in dense bodies, multivesicular bodies, and Golgi apparatuses of both spermatogenic cells and Sertoli cells, while no alkaline phosphatase activity was observed in the seminiferous tubule, although peritubular tissue was positive. During early stages of acrosome formation, acid phosphatase activity was demonstrated in the Golgi apparatus and in acrosomal granules. This enzyme could not be detected in the granule when the acrosomal cap was formed. Thiamine pyrophosphatase, inosine diphosphatase, and acid phosphatase were detected in Golgi complex of the early spermatid, but none of these three enzymes were detected following caudal migration of the apparatus. This phenomenon indicates a change in the physiologic status of the Golgi apparatus during spermatid maturation. Thiamine pyrophosphatase, inosine diphosphatase, and adenosine triphosphatase activities were localized on the cell surface and between intercellular spaces of spermatogenic cells and Sertoli cells throughout the seminiferous tubular cycle. In the spermatid, however, the three enzymes were not observed on the plasma membrane in exonemal canal. Furthermore, along the opposing plasma membrane of adjacent spermatogenic cells mere traces or no enzyme activities were present. The phenomenon indicates a membrane specialization and suggests a metabolic relationship between Sertoli cells and spermatogenic cells.

Fine structure of an unusual spermatozoon of a brooding sea cucumber, Cucumaria lubrica.

The sperm of Cucumaria lubrica consists of an elongated head, 1.5 microns (n) in diameter and 5.2 μ in length; a mitochondrial midpiece of 1.9 μ in length; and a tail of 65 μ. The acrosome contains an irregularly circular granule which is located in an anterior nuclear depression. The granule is surrounded by a complete limiting membrane in most of the sperm examined. Occasional sperm contain granules surrounded by incomplete membranes. The acrosomal granule consists of a homogeneous material except for an electron-dense sphere displaced anteriorly. A subacrosomal layer consisting of a homogeneous reticular material encircles the granule. This layer contains several vesicular structures and a fibrous component immediately posterior to the granule.The nucleus measures 6.8 μ in length and 1.4 μ at its greatest diameter, but tapers to a diameter of 0.5 μ at the posterior end. A single mitochondrial mass surrounds the posterior 1.6 μ of the nucleus. The proximal and distal centrioles, lying perpendicular to each other, are found posterior to the nucleus. Microtubules are observed in close association with the satellite of the distal centriole.The adaptive significance of this unusual sperm is discussed.

Spermiogenesis in the Nine-Banded Armadillo

The testes of mature armadillos were fixed by either perfusion or immersion. The morphology of the seminiferous tubules and the process of spermiogenesis were studied.The developing spermatids are generally oval in shape and contain a centrally placed nucleus. A well-developed Golgi apparatus, scattered mitochondria, centrioles, smooth endoplasmic reticulum, and a chromatoid body are observed within the cytoplasm. Granules formed within the Golgi appear to coalesce to form the acrosomal granule, which is enclosed within a vesicle (Fi,g. 1). The acrosome adheres to the nucleus at the anterior pole of the developing spermatid. The acrosome vesicle collapses and extends over the anterior two-thirds of the nucleus (Fig. 2). As this vesicle expands, the Golgi continues to release granules into the vesicle. Concomitant with acrosome formation, the centrioles and chromatoid body migrate to the posterior pole of the developing cell.

The ultrastructure of testicular spermatozoa in two species of Rana

The morphology of testicular spermatozoa of two anuran amphibians, Rana pipiens and Rana clamitans , has been investigated with the electron microscope. The nuclei, middle pieces, and tails of both species are similar. Both species have elongated, cylindrical nuclei. The middle pieces contain individual mitochondria and two centrioles, the distal one being continuous with the axial filament. The tails consist of the 9 + 2 axial filament pattern. The acrosomes of R. clamitans spermatozoa consist of homogeneous saclike structures, overlapping the anterior portion of the nuclei. No definite structure can be assigned to a mature acrosome of R. pipiens spermatozoa. Acrosomal granules, stacks of membranes and membrane-bound vesicles have all been observed on fully elongated, condensed nuclei of what appear to be mature spermatozoa. These structures are confined to the anterior tip of the nucleus with no overlap.

Changes in the golgi apparatus during spermiogenesis in the rat.

AbstractThe fine structure of the Golgi apparatus in rat spermatids at various steps of spermiogenesis was investigated in routinely stained preparations as well as in sections stained with the periodic acid‐silver technique for the demonstration of glycoprotein.In steps 1–7 spermatids, a layer of cisternae of endoplasmic reticulum envelops the Golgi apparatus. Between this layer and the stack of Golgi saccules, small coated vesicles (500–600 Å) are associated with smooth‐surfaced tubular profiles. These are irregular expansions of the first or outermost Golgi saccule. The next few saccules of the stack are regularly arranged and similar to one another. The last or innermost saccule is thick and produces coated buds, similar to the large coated vesicles (900–1200 Å) which are found nearby. Large smooth vesicles with a dense core, known as proacrosomic vesicles, are also found in this location at steps 1 and 2. These at step 3 fuse into the single acrosomic vesicle, which in turn at step 5 transforms into the head cap (while the dense core of the vesicles becomes the acrosomic granule).Using the PA‐silver technique on steps 1–7 spermatids, a gradation of staining is observed in Golgi saccules, with the most intense reaction in the innermost saccule, its coated buds and the large coated vesicles nearby. Intense staining is also noted in the head cap, while staining of lesser intensity is observed in the acrosomic granule.In older spermatids (steps 8–18) the Golgi apparatus gradually loses most of the features just described and appears to consist primarily of stacks of saccules with dilated edges. These are only weakly stained by the PA‐silver technique.These observations are interpreted as follows. Material produced in the endo‐plasmic reticulum would be transferred by small coated vesicles to the outermost saccule of the Golgi stacks. During saccule migration toward the inner aspect of the stacks, the synthesis of glycoprotein would be completed. The transfer of this glycoprotein via the large coated vesicles would cause the proacrosomic vesicles to grow into the acrosomic vesicle which later would become the head cap.

Comparative histochemical observations on the contributions of the acrosomal vesicle and granule to the acrosomal cap in the spermatozoa of mammals.

By applying the periodic acid-Schiff technique to frozen sections of material fixed in weak Bouin's solution and extracted with hot pyridine, the contributions of the acrosomal vesicle and granule to the acrosomal cap have been observed in the maturing spermatozoa of the American opossum, guinea pig, rabbit, marmoset and chimpanzee. In the spermatids of the opossum, the acrosomal granule is not developed and the thin, homogeneous carbohydrate layer of the acrosomal cap is derived from the concentrated material of the acrosomal vacuole. In the guinea pig, both the acrosomal vesicle and granule make the greatest contributions to the carbohydrates of the acrosomal cap; these continue to maintain their identity in the head of mature sperm. In the rabbit, marmoset and chimpanzee, the carbohydrate material of acrosomal vesicle origin is less developed and constitutes a thin layer in the lateral portions of the acrosomal cap; the carbohydrate material in its anterior portions is mainly derived from the acrosomal granule which becomes flattened; there is also a slight thickening of the intermediate layer of the acrosomal cap in this region. The distinction between the carbohydrate contributions from the acrosomal vesicle and granule disappears in the fully formed acrosomal cap which shows a homogeneously PAS-positive intermediate layer. In the chimpanzee, the acrosomal vesicle makes a relatively very small contribution to the carbohydrate layer of the acrosomal cap which is mainly formed by spreading of material from the acrosomal granule.

Spermatogenesis and the structure of the mature sperm in Nucella Lapillus (L)

ABSTRACT The testis of Nucella consists of numerous tubules, all directed inwards and joining to form a common testicular duct. In a single tubule the spermatogonia lie round the periphery. Mature sperm line the lumen of the tubule. Cells in the same stage of spermatogenesis are grouped together and all members of a group pass through spermatogenesis in phase. Staining with fast green before and after treatment with Van Slyke reagent indicates a change from lysine-rich to arginine-rich histone in the maturing spermatid. Sperm of Nucella are motile throughout their length. The sperm are thread-like and about 80 μ long. The head is Feulgen-positive and about 40 μ long. The mid-piece lies behind the head and is about 8 μ long. The flagellum runs from the front end of the head to the tip of the tail; in the head it is completely surrounded by the nucleus. The spermatogonia contain two centrioles situated near the nucleus and a conspicuous Golgi complex. There are synaptinemal complexes in spermatocyte nuclei in the synapsis stage. In the early spermatid the centriole pushes a tube through the nucleus. This tube is lined by nuclear membrane and is occupied by the anterior portion of the flagellar shaft. The nucleus elongates and the nucleoprotein condenses into strands arranged helically along the long axis of the nucleus. These strands fuse to form lamellae, which disappear in the mature sperm. Mitochondria aggregate at the base of the early spermatid nucleus and form a loose spiral around the flagellar shaft. The outer mitochondrial membranes fuse. The mid-piece of the mature sperm consists of a large tubular mitochondrion enclosing a portion of the flagellar shaft. At the early spermatid stage a pro-acrosomal granule is formed from a large Golgi complex. From this the acrosome develops; it consists of a cone and an acrosome granule. There are two sets of microtubules associated with the acrosome, one lying within the cone, the other outside the cone and separated from it by a ‘ragged membrane’. The microtubules of the outer set extend backwards along the head for two-thirds of its length. The centriole which gives rise to the flagellar shaft lies at the anterior end of the head and is separated from the acrosome by a thin layer of nucleoprotein and a double layer of nuclear envelope. There is no second centriole or derivative thereof in the mature sperm. In the tail groups of coiled fibres are associated with each pair of the peripheral flagellar fibrils.

Acrosome morphogenesis in Lumbricus terrestris.

Acrosome morphogenesis commences in the juxtanuclear cytoplasm at the posterior end of spermatids of Lumbricus terrestris. A dense rod-shaped structure and the Golgi apparatus together participate first in forming an acrosome vesicle that contains the acrosome granule, and somewhat later shape the conical base of the acrosome in the cytoplasm beneath the vesicle. Cytoplasmic flow may account for the migration of the immature acrosome to the apical surface of the nucleus of the spermatid. Manchette microtubules play a key role in the final modelling of the acrosome. Sheathed by the manchette the acrosome elongates to 3–4 times its pre-attachment length. The conical base of the acrosome then extends anteriorly to enclose the acrosome vesicle. A dense rod emerging from the rod-shaped granule occupies an indentation of the base of the acrosome vesicle. The mature acrosome of Lumbricus is an extremely complex structure about 5–7 microns long and is bounded by the plasmalemma of the spermatozoon.

MORPHOLOGY OF GAMETE MEMBRANE FUSION AND OF SPERM ENTRY INTO OOCYTES OF THE SEA URCHIN

Sea urchin gametes predominate in molecular studies of fertilization, yet relatively little is known of the subcellular aspects of sperm entry in this group. Accordingly, it seemed desirable to make a detailed examination of sperm entry phenomena in sea urchins with the electron microscope. Gametes of the sea urchins Arbacia punctulata and Lytechinus variegatus were used in this study. Samples of eggs containing 2 to 8 per cent oocytes were selected and fixed with osmium tetroxide in sea water at various intervals after insemination. Fixed specimens were embedded in Epon 812, sectioned, and examined with an electron microscope. An apical vesicle was observed at the anterior end of the acrosome. The presence of this structure, together with other observations, suggested that initiation of the acrosome reaction in sea urchin sperm involves dehiscence of the acrosomal region with the subsequent release of the acrosomal granule. Contact and initial fusion of gamete membranes was observed in mature eggs and oocytes and invariably involved the extended acrosomal tubule of the spermatozoon. Only one spermatozoon normally enters the mature egg. The probability of locating such a sperm in ultrathin sections is exceedingly low. Several sperm do normally enter oocytes. Consequently, observations of sperm entry were primarily restricted to the latter. The manner of sperm entry into oocytes did not resemble phagocytosis. Organelles of the spermatozoon were progressively divested of their plasma membrane as they entered the ground cytoplasm of the oocyte fertilization cone. Initiation of the acrosome reaction, contact and initial fusion of gamete membranes, and sperm entry into oocytes of sea urchins conform to the Hydroides-Saccoglossus pattern of early fertilization events as described by Colwin and Colwin (13).

DEVELOPMENT OF THE ACROSOMIC SYSTEM OF THE SPERMATOZOON IN THE NORWEGIAN LEMMING (LEMMUS LEMMUS).

An electronmicroscopic study on the development of the acrosome system of the spermatozoon in the Norwegian lemming (Lemmus lemmus) has been performed. The proacrosomal granules were found to be few and to consist of a dense center and a less dense periphery, and the acrosomal vesicle to contain stainable material of the same structure but of lower density than that of the acrosomal granule. Like in the guinea pig, two zones of different density exist throughout acrosome development and are visible also in mature spermatozoa. A large osmiophilic formation consisting of saccular, tubular, and lamellar structures, was found between the apex of the condensed nucleus and the acrosome and was identified as perforatorium.

Studies on the acrosome: VII. Formation of the acrosomal process in sea urchin spermatozoa

The acrosomal granule of the sea urchin spermatozoon is capped by a thin trigger layer; around its inner side are several layers of membrane precursor substances. On exposure to dissolved egg jelly, the trigger layer expands and disintegrates. Within the membrane produced by the precursor layers, the acrosomal lumen undergoes a volume increase which pushes outward the central part of the membrane to form the acrosomal process. Fibrous precursor is organized into an axial core within the process, and the granule substance spreads over its outer surface. These changes occur within one second. It is suggested that an increase in the permeability of the sperm plasma membrane initiates these changes.

ROLE OF THE GAMETE MEMBRANES IN FERTILIZATION IN SACCOGLOSSUS KOWALEVSKII (ENTEROPNEUSTA). I. THE ACROSOMAL REGION AND ITS CHANGES IN EARLY STAGES OF FERTILIZATION.

Previous electron microscope studies of sperm-egg association in the annelid Hydroides revealed novel aspects with respect to the acrosomal region. To determine whether these aspects were unique, a comparable study was made of a species belonging to a widely separated phylum, Hemichordata. Osmium tetroxide-fixed polyspermic material of the enteropneust, Saccoglossus, was used. The acrosomal region includes the membrane-bounded acrosome, with its large acrosomal granule and shallow adnuclear invagination, and the periacrosomal material which surrounds the acrosome except at the apex; here, the acrosomal membrane lies very close to the enclosing sperm plasma membrane. After reaching the egg envelope, the spermatozoon is activated and undergoes a series of changes: the apex dehisces and around the resulting orifice the acrosomal and sperm plasma membranes form a continuous mosaic membrane. The acrosomal granule disappears. Within 7 seconds the invagination becomes the acrosomal tubule, spans the egg envelopes, and meets the egg plasma membrane. The rest of the acrosomal vesicle everts. The periacrosomal mass changes profoundly: part becomes a fibrous core (possibly equivalent to a perforatorium); part remains as a peripheral ring. The basic pattern of structure and sperm-egg association in Saccoglossus is the same as in Hydroides. Previous evidence from four other phyla as interpreted here also indicates conformity to this pattern. The major role of the acrosome is apparently to deliver the sperm plasma membrane to the egg plasma membrane.

Localization of thiamine pyrophosphatase activity in the golgi apparatus of a mollusc, Helix aspersa.

Spermatids of the snail Helix aspersa were studied after fixation in buffered osmium tetroxide and after applying Novikoff and Goldfischer's method (15) for demonstrating thiamine pyrophosphatase (TPPase) activity both with the light and the electron microscope. The appearance of cells in the light microscope after localizing the enzyme is very similar to the appearance after the application of classical Golgi techniques. The electron microscope shows the "dictyosomes" to consist of non-granular membranes, vesicles, and vacuoles typical of the ultrastructure of the Golgi apparatus. Sites of TPPase activity are localized by deposits of lead phosphate, and are found between the membranes of the Golgi apparatus, in the small vesicles, in multivesicular bodies often found associated with it, but not within the large Golgi vacuoles. Heavy deposits are found on the caudal part of the nuclear envelope, but not in the acrosomal granule. It is suggested that TPPase may act as an intermediary in acrosome formation by the Golgi apparatus or "acroblast" of this cell. The finding of diphosphatase activity in the Golgi apparatus of an invertebrate is suggested as additional evidence for the existence of a homology between the Golgi apparatus of all animal cells.

Fine structure of the spermatozoon of Hydroides hexagonus (Annelida), with special reference to the acrosomal region.

This paper describes in some detail the structure of the acrosomal region of the spermatozoon of Hydroides as a basis for subsequent papers which will deal with the structural changes which this region undergoes during fertilization. The material was osmium-fixed and mild centrifugation was used to aggregate the spermatozoa from collection to final embedding. The studies concern also the acrosomal regions of frozen-thawed sperm prepared by a method which previously had yielded extracts with egg membrane lytic activity. The plasma membrane closely envelops four readily recognizable regions of the spermatozoon: acrosomal, nuclear, mitochondrial, and flagellar. The acrosome consists of an acrosomal vesicle which is bounded by a single continuous membrane, and its periphery is distinguishable into inner, intermediate, and outer zones. The inner and intermediate zones form a pocket into which the narrowed apex of the nucleus intrudes. Granular material adjoins the inner surface of the acrosomal membrane, and this material is characteristically different for each zone. Centrally, the acrosomal vesicle is spanned by an acrosomal granule: its base is at the inner zone and its apex at the outer zone. The apex of the acrosomal granule flares out and touches the acrosomal membrane over a limited area. In this limited area the adjoining granular material of the outer zone is lacking. The acrosomal membrane of the inner zone is invaginated into about fifteen short tubules. The acrosomal membrane of the outer zone is closely surrounded by the plasma membrane. At the apex of the acrosomal region a small apical vesicle is sandwiched between the plasma membrane and the acrosomal membrane. Numerous frozen-thawed specimens and occasional specimens not so treated show acrosomal regions at the apex of which there is a well defined opening or orifice. Around the rim or lip of this orifice plasma and acrosomal membranes may even be fused into a continuum. The evidence indicates that the apical vesicle and the parts of the plasma and acrosomal membranes which surround it constitute a lid, and the rim of this lid constitutes a natural "fracture line" or rim of dehiscence. Should fracture occur, the lid would be removed and the acrosomal vesicle would be open to the exterior.

Changes in the spermatozoon during fertilization in Hydroides hexagonus (Annelida). I. Passage of the acrosomal region through the vitelline membrane.

In the previous paper the structure of the acrosomal region of the spermatozoon was described. The present paper describes the changes which this region undergoes during passage through the vitelline membrane. The material used consisted of moderately polyspermic eggs of Hydroides hexagonus, osmium-fixed usually 9 seconds after insemination. There are essentially four major changes in the acrosome during passage of the sperm head through the vitelline membrane. First, the acrosome breaks open apically by a kind of dehiscence which results in the formation of a well defined orifice. Around the lips of the orifice the edges of the plasma and acrosomal membranes are then found to be fused to form a continuous membranous sheet. Second, the walls of the acrosomal vesicle are completely everted, and this appears to be the means by which the apex of the sperm head is moved through the vitelline membrane. The lip of the orifice comes to lie deeper and deeper within the vitelline membrane. At the same time the lip itself is made up of constantly changing material as first the material of the outer zone and then that of the intermediate zone everts. One is reminded of the lip of an amphibian blastopore, which during gastrulation maintains its morphological identity as a lip but is nevertheless made up of constantly changing cells, with constantly changing outline and even constantly changing position. Third, the large acrosomal granule rapidly disappears. This disappearance is closely correlated with a corresponding disappearance of a part of the principal material of the vitelline membrane from before it, and the suggestion is made that the acrosomal granule is the source of the lysin which dissolves this part of the vitelline membrane. Fourth, in the inner zone the fifteen or so short tubular invaginations of the acrosomal membrane, present in the normal unreacted spermatozoon, lengthen considerably to become a tuft of acrosomal tubules. These tubules are the first structures of the advancing sperm head to touch the plasma membrane of the egg. It is notable that the surface of the acrosomal tubules which once faced into the closed acrosomal cavity becomes the first part of the sperm plasma membrane to meet the plasma membrane of the egg. The acrosomal tubules of Hydroides, which arise simply by lengthening of already existing shorter tubules, are considered to represent the acrosome filaments of other species.

Electron Microscopic Study of Mature Bovine Spermatozoa

Summary Characteristics of the normal bovine spermatozoa as revealed by electron micrographs have been reported. Detail of the head area reveals that the acrosomic granule is an integral structure within the acrosomic cap of the mature spermatozoa. The acrosomic cap overlaps the porous-appearing posterior nuclear cap. The midpiece containing outer axial fibrils of at least two different dimensions appears to have a diameter at least as great as the thickness of the head. Possible contractile elements of the fibrils are closely associated with the basal granules in the neck. Elements of the tail area also have been described.