Ultrastructure of the spermatozoa of five species of South African bivalves (Mollusca), and an examination of early spermatogenesis

Transmission electron microscopy of the spermatozoa of five species from three families of bivalves has shown that each species has a sperm with unique morphology. However, the morphology of the acrosomes of each species is typical of the subclass of bivalve to which they belong. An examination of spermatogenesis in the five species, along with a re-examination of material from six other species of bivalves, has revealed that pre-spermiogenic cells possess flagella. In addition, acrosome formation begins in the spermatocytes with the formation of proacrosomal vesicles in the Golgi body. During spermiogenesis the proacrosomal vesicles coalesce at the presumptive posterior of the spermatid, with a larger vesicle produced by the Golgi body. The single acrosomal vesicle eventually migrates to the anterior of the spermatid where it assumes its mature form. © 1994 Wiley-Liss, Inc.     

Deficiency in the omega-3 fatty acid pathway results in failure of acrosome biogenesis in mice

An omega-3 fatty acid, docosahexaenoic acid (DHA), is enriched in testicular membrane phospholipids, but its function is not well understood. The Fads2 gene encodes an enzyme required for the endogenous synthesis of DHA. Using Fads2-null mice (Fads2-/-), we found in our preceding studies that DHA deficiency caused the arrest of spermiogenesis and male infertility, both of which were reversed by dietary DHA. In this study, we investigated a cellular mechanism underlying the DHA essentiality in spermiogenesis. Periodic acid-Schiff staining and acrosin immunohistochemistry revealed the absence of acrosomes in Fads2-/- round spermatids. Acrosin, an acrosomal marker, was scattered throughout the cytoplasm of the Fads2-/- spermatids, and electron microscopy showed that proacrosomal granules were formed on the trans-face of the Golgi. However, excessive endoplasmic reticulum and vesicles were present on the cis-face of the Golgi in Fads2-/- spermatids. The presence of proacrosomal vesicles but lack of a developed acrosome in Fads2-/- spermatids suggested failed vesicle fusion. Syntaxin 2, a protein involved in vesicle fusion, colocalized with acrosin in the acrosome of wild-type mice. In contrast, syntaxin 2 remained scattered in reticular structures and showed no extensive colocalization with acrosin in the Fads2-/- spermatids, suggesting failed fusion with acrosin-containing vesicles or failed transport and release of syntaxin 2 vesicles from Golgi. Dietary supplementation of DHA in Fads2-/- mice restored an intact acrosome. In conclusion, acrosome biogenesis under DHA deficiency is halted after release of proacrosomal granules. Misplaced syntaxin 2 suggests an essential role of DHA in proper delivery of membrane proteins required for proacrosomal vesicle fusion.     

Sperm ultrastructure of the spider crab Maja brachydactyla (Decapoda: Brachyura)

This study describes the morphology of the sperm cell of Maja brachydactyla, with emphasis on localizing actin and tubulin. The spermatozoon of M. brachydactyla is similar in appearance and organization to other brachyuran spermatozoa. The spermatozoon is a globular cell composed of a central acrosome, which is surrounded by a thin layer of cytoplasm and a cup‐shaped nucleus with four radiating lateral arms. The acrosome is a subspheroidal vesicle composed of three concentric zones surrounded by a capsule. The acrosome is apically covered by an operculum. The perforatorium penetrates the center of the acrosome and has granular material partially composed of actin. The cytoplasm contains one centriole in the subacrosomal region. A cytoplasmic ring encircles the acrosome in the subapical region of the cell and contains the structures‐organelles complex (SO‐complex), which is composed of a membrane system, mitochondria with few cristae, and microtubules. In the nucleus, slightly condensed chromatin extends along the lateral arms, in which no microtubules have been observed. Chromatin fibers aggregate in certain areas and are often associated with the SO‐complex. During the acrosomal reaction, the acrosome could provide support for the penetration of the sperm nucleus, the SO‐complex could serve as an anchor point for chromatin, and the lateral arms could play an important role triggering the acrosomal reaction, while slightly decondensed chromatin may be necessary for the deformation of the nucleus. J. Morphol., 2010. © 2009 Wiley‐Liss, Inc.     

Ultrastructural aspects of spermatogenesis in Calyptogena pacifica Dall 1891 (Vesicomyidae; Bivalvia)

The process of spermatogenesis and spermatozoon morphology was characterized from a deep‐sea bivalve, Calyptogena pacifica (Vesicomyidae, Pliocardiinae), a member of the superfamily Glossoidea, using light and electron microscopy. Spermatogenesis in C. pacifica is generally similar to that in shallow‐water bivalves but, the development of spermatogenic cells in this species has also some distinguishing features. First proacrosomal vesicles are observed in early spermatocytes I. Although, early appearance of proacrosomal vesicles is well known for bivalves, in C. pacifica, these vesicles are associated with electron‐dense material, which is located outside the limiting membrane of the proacrosomal vesicles and disappears in late spermatids. Another feature of spermatogenesis in C. pacifica is the localization of the axoneme and flagellum development. Early spermatogenic cells lack typical flagellum, while in spermatogonia, spermatocytes, and early spermatids, the axoneme is observed in the cytoplasm. In late spermatids, the axoneme is located along the nucleus, and the flagellum is oriented anteriorly. During sperm maturation, the bent flagellum is transformed into the typical posteriorly oriented tail. Spermatozoa of C. pacifica are of ect‐aqua sperm type with a bullet‐like head of about 5.8 μm in length and 1.8 μm in width, consisting of a well‐developed dome‐shaped acrosomal complex, an elongated barrel‐shaped nucleus filled with granular chromatin, and a midpiece with mainly four rounded mitochondria. A comparative analysis has shown a number of common traits in C. pacifica and Neotrapezium sublaevigatum.     

Arylsulphatase and acid phosphatase activity associated with developing and ripe spermatozoa of the musselMytilus edulis

Summary Arylsulphatase and acid phosphatase activity were demonstrated cytochemically in spermatozoa of the marine musselMytilus edulis. Reaction product resulting from arylsulphatase activity was measured using an integrating microdensitometer and found to increase with incubation time and to be variable according to the pH of the incubation medium. Two peaks in activity, at pH 4.5 and 6.0 were evident for some experimental protocols suggesting the possibility of two isoenzymes; however, studies on the ultrastructural localization of the enzyme showed no difference between sites of activity for the two pH values. Ultrastructural localization of arylsulphatase showed activity associated with the Golgi body of developing spermatids and in particular within the proacrosomal vesicles but limited to the periphery of the acrosomal vesicle which is formed with the fusion of the proacrosomal vesicles. In spawned spermatozoa arylsulphatase activity was localized in association with the axial rod and subacrosomal material; activity also occurred along the outer acrosomal membrane and within the acrosomal vesicle and also associated with the acrosomal process following the acrosome reaction. Sulphate groups were demonstrated cytochemically within the vitelline coat of oocytes in the mantle tissue. These findings suggest that arylsulphatase could be one of the lysins previously demonstrated inM. edulis spermatozoa. Acid phosphatase activity was demonstrated in spawned spermatozoa around the nuclear envelope and along the outer acrosomal mambrane.     

Association of the developing acrosome with multiple small Golgi units, the Golgi satellites, in spermatids of the musk shrew, Suncus murinus

The spermatozoa of the musk shrew, Suncus murinus, have a fan-like giant acrosome with a diameter of approximately 20 mm. The aim of this study was to investigate how this giant acrosome is constructed in the musk shrew spermatid and, in particular, how the Golgi apparatus involved in acrosome formation behaves. The behaviour of the Golgi apparatus was monitored by confocal laser scanning microscopy with antibody against a Golgi-associated Rab6 small GTPase. In the early Golgi phase, small Golgi units, the Golgi satellites, localized as a large aggregate in the juxtanuclear cytoplasm. As acrosome formation progressed, the Golgi satellites gradually dispersed, associated with proacrosomal vesicles and an acrosomal vesicle, and finally became distributed as multiple small units over the whole surface of an acrosomal cap in the round spermatid. The mode of acrosome formation in musk shrews was distinctly different from that in rats and mice, in which the Golgi apparatus remains as a single unit throughout acrosome formation. In musk shrews, the proacrosomal vesicles formed successively by the Golgi satellites coalesced, one after another, into a potential acrosomal vesicle. This process may result in further enlargement of the acrosome. The results of the present study indicate that Golgi satellites are necessary for the biogenesis and development of the giant acrosome in musk shrew spermatozoa.     

Spermatozoa and spermatogenesis in the northern quahaug <Emphasis Type="Italic">Mercenaria mercenaria</Emphasis> (Mollusca,Bivalvia)

We studied the ultrastructure of spermatogenesis and spermatozoa in the northern quahaug, the clam Mercenaria mercenaria. Spermatogenetic cells gradually elongate. Mitochondria gradually fuse and increase in size and electron density. During spermatid differentiation, proacrosomal vesicles migrate towards the presumptive anterior pole of the nucleus and eventually form the acrosome. The spermatozoon of M. mercenaria is of a primitive type. It is composed of head, mid-piece, and tail. The acrosome shows a subacrosomal space with a short conical contour. The slightly curved nucleus of the spermatozoon contains fine-grained dense chromatin. The middle piece consists of a centriolar complex which is surrounded by four mitochondria. The flagellum has a standard “9 + 2” microtubular structure. The ultrastructure of spermatozoa and spermatogenesis of M. mercenaria shares a number of features with other species of the family Veneridae. M. mercenaria may be a suitable model species for further investigations into the mechanisms of spermatogenesis in the Bivalvia.     

Ultrastructure of the spermatogenesis of the cockle Anadara granosa L. (Bivalvia: Arcidae)

In this paper spermatogenesis and sperm ultrastructure of the cockle Anadara granosa are studied using transmission electron microscopy. The spermatocyte presents electron-dense vesicles and the arising axoneme that begins to form the flagellum. During spermatid differentiation, proacrosomal vesicles appear to migrate towards the presumptive anterior pole of the nucleus; eventually these vesicles become acrosome. The spermatozoon of Anadara granosa is of the primitive type. The acrosome, situated at the apex of the nucleus, is cap-shaped and deeply invaginated at the inner side. The spherical nucleus of the spermatozoon contains dense granular chromatin and shows invagination at the posterior poles. The centriole shows the classic nine triplets of microtubules. The middle piece consists of the centriolar complex surrounded by five giant mitochondria. It is shown that the ultrastructure of spermatozoa and spermiogenesis of Anadara granosa reveals a number of features that are common among bivalves. Received: 29 September 1998 / Received in revised form: 20 May 1999 / Accepted: 14 June 1999     

The acroplaxome is the docking site of Golgi-derived myosin Va/Rab27a/b- containing proacrosomal vesicles in wild-type and Hrb mutant mouse spermatids

Acrosome biogenesis involves the transport and fusion of Golgi-derived proacrosomal vesicles along the acroplaxome, an F-actin/keratin 5-containing cytoskeletal plate anchored to the spermatid nucleus. A significant issue is whether the acroplaxome develops in acrosomeless mutant mice. Male mice with a Hrb null mutation are infertile and both spermatids and sperm are round-headed and lack an acrosome. Hrb, a protein that contains several NPF motifs (Asn-Pro-Phe) and interacts with proteins with Eps15 homology domains, is regarded as critical for the docking and/or fusion of Golgi-derived proacrosomal vesicles. Here we report that the lack of an acrosome in Hrb mutant spermatids does not prevent the development of the acroplaxome. Yet the acroplaxome in the mutant contains F-actin but is deficient in keratin 5. We also show that the actin-based motor protein myosin Va and its receptor, Rab27a/b, known to be involved in vesicle transport, are present in the Golgi and Golgi-derived proacrosomal vesicles in wild-type and Hrb mutant mouse spermatids. In the Hrb mutant, myosin-Va-bound proacrosome vesicles tether to the acroplaxome, where they flatten and form a flat sac, designated pseudoacrosome. As spermiogenesis advances, round-shaped spermatid nuclei of the mutant display several nuclear protrusions, designated nucleopodes. Nucleopodes are consistently found at the acroplaxome- pseudoacrosome site. Our findings support the interpretation that the acroplaxome provides a focal point for myosin-Va/ Rab27a/b-driven proacrosomal vesicles to accumulate, coalesce, and form an acrosome in wild-type spermatids and a pseudoacrosome in Hrb mutant spermatids. We suggest that nucleopodes develop at a site where a keratin 5-deficient acroplaxome may not withstand tension forces operating during spermatid nuclear shaping.     

Ultrastructure of spermiogenesis in the American alligator,Alligator mississippiensis (Reptilia,Crocodylia, Alligatoridae)

Testicular samples were collected to describe the ultrastructure of spermiogenisis in Alligator mississipiensis (American Alligator). Spermiogenesis commences with an acrosome vesicle forming from Golgi transport vesicles. An acrosome granule forms during vesicle contact with the nucleus, and remains posterior until mid to late elongation when it diffuses uniformly throughout the acrosomal lumen. The nucleus has uniform diffuse chromatin with small indices of heterochromatin, and the condensation of DNA is granular. The subacrosome space develops early, enlarges during elongation, and accumulates a thick layer of dark staining granules. Once the acrosome has completed its development, the nucleus of the early elongating spermatid becomes associated with the cell membrane flattening the acrosome vesicle on the apical surface of the nucleus, which aids in the migration of the acrosomal shoulders laterally. One endonuclear canal is present where the perforatorium resides. A prominent longitudinal manchette is associated with the nuclei of late elongating spermatids, and less numerous circular microtubules are observed close to the acrosome complex. The microtubule doublets of the midpiece axoneme are surrounded by a layer of dense staining granular material. The mitochondria of the midpiece abut the proximal centriole resulting in a very short neck region, and possess tubular cristae internally and concentric layers of cristae superficially. A fibrous sheath surrounds only the axoneme of the principal piece. Characters not previously described during spermiogenesis in any other amniote are observed and include (1) an endoplasmic reticulum cap during early acrosome development, (2) a concentric ring of endoplasmic reticulum around the nucleus of early to middle elongating spermatids, (3) a band of endoplasmic reticulum around the acrosome complex of late developing elongate spermatids, and (4) midpiece mitochondria that have both tubular and concentric layers of cristae. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

Ultrastructure of spermatogenesis in Cerastoderma glaucum (Cardiacea) and Spisula subtruncata (Mactracea)

Summary

In Cerastoderma glaucum, Sertoli cells are rich in lipids, glycogen and lysosomes, and premeiotic cells exhibited nuage, a prominent Golgi complex and endoplasmic reticulum cisternae encircling the nucleus. The Golgi complex gives rise to proacrosomal vesicles during mid-spermiogenesis, and the round acrosomal vesicle, with a dense fibrillar core, migrates laterally while linked to the plasma membrane as it develops the subacrosomal material. In its final position, the vesicle becomes cap-shaped (0.6 μm) and differentiates into apical light and basal dense regions. The elongated and helicoidal nucleus (8–9.9 μm) has a thin tip (0.3 μm) that invades the subacrosomal space, and in the midpiece (0.8 μm) two of the four mitochondria extend laterally to the nucleus (1.5–2.1 μm). In Spisula subtruncata, Sertoli cells are rich in lipids, glycogen and phagocytosed sperm. Premeiotic cells exhibit nuage, a prominent Golgi complex that gives rise to proacrosomal vesicles from the leptotene stage and a flagellimi that is extruded at zygotene. The acrosomal vesicle forms during the round spermatid stage and differentiates into a large and dense basal region and an apical light region. It then migrates while linked to the plasma membrane by its apical pole. Development of the subacrosomal perforatorium is associated with nuage materials and endoplasmic reticulum vesicles. The mature cap-shaped (0.6 μm) acrosomal vesicle exhibits a large apical and irregular region with floccular contents and a basal dense region. The round nucleus becomes barrel-shaped (1.5 μm) and the midpiece (0.8 μm), with four mitochondria, contains a few glycogen particles.     

Differential expression of lysosomal associated membrane protein (LAMP-1) during mammalian spermiogenesis

The mammalian acrosome is a secretory vesicle of mature sperms that plays an important role in fertilization. Recent evidence had pointed out that some components found at endosomes in somatic cells are associated with the developing acrosome during the early steps of spermiogenesis. Moreover, the mammalian acrosome contains many enzymes found within lysosomes in somatic cells. In this work, we studied the dynamics of some components of the endosome/lysosome system, as a way to understand the complex membrane trafficking circuit established during spermatogenesis. We show that the cation independent-mannose-6-phosphate receptor (CI-MPR) is transiently expressed in the cytoplasm of mid-stage spermatids (steps 5-11). On the other hand, gamma-adaptin, an adaptor molecule of a complex involved in trafficking from the Golgi to lysosomes, was expressed in cytoplasmic vesicles only in pachytene and Cap-phase spermatids (steps 1-5). Our major finding is that the lysosomal protein LAMP-1 is differentially expressed during spermiogenesis. LAMP-1 appears late in spermatogenesis (Acrosome-phase) contrasting with LAMP-2, which is present throughout the complete process. Both proteins appear to be associated with cytoplasmic vesicles and not with the developing acrosome. None of the studied proteins is present in epididymal spermatozoa. Our results suggest that the CI-MPR could be involved in membrane trafficking and/or acrosomal shaping during spermiogenesis.     

Ultrastructure of spermiogenesis in the Cottonmouth,Agkistrodon piscivorus (Squamata: Viperidae: Crotalinae)

To date multiple studies exist that examine the morphology of spermatozoa. However, there are limited numbers of data detailing the ontogenic characters of spermiogenesis within squamates. Testicular tissues were collected from Cottonmouths (Agkistrodon piscivorus) and tissues from spermiogenically active months were analyzed ultrastructurally to detail the cellular changes that occur during spermiogenesis. The major events of spermiogenesis (acrosome formation, nuclear elongation/DNA condensation, and flagellar development) resemble that of other squamates; however, specific ultrastructural differences can be observed between Cottonmouths and other squamates studied to date. During acrosome formation vesicles from the Golgi apparatus fuse at the apical surface of the nuclear membrane prior to making nuclear contact. At this stage, the acrosome granule can be observed in a centralized location within the vesicle. As elongation commences the acrosome complex becomes highly compartmentalized and migrates laterally along the nucleus. Parallel and circum‐cylindrical microtubules (components of the manchette) are observed with parallel microtubules outnumbering the circum‐cylindrical microtubules. Flagella, displaying the conserved 9 + 2 microtubule arrangement, sit in nuclear fossae that have electron lucent shoulders juxtaposed on either side of the spermatids basal plates. This study aims to provide developmental characters for squamates in the subfamily Crotalinae, family Viperidae, which may be useful for histopathological studies on spermatogenesis in semi‐aquatic species exposed to pesticides. Furthermore, these data in the near future may provide morphological characters for spermiogenesis that can be added to morphological data matrices that may be used in phylogenetic analyses. J. Morphol. 2010. © 2009 Wiley‐Liss, Inc.     

Fine structural study of the spermatogenic cycle in Pitar rudis and Chamelea gallina (Mollusca,Bivalvia, Veneridae)

A comparative ultrastructural study of spermatogenesis was performed in the bivalve molluscs Pitar rudis and Chamelea gallina (Veneridae) from Turkey. Sertoli cells appeared to be rich in glycogen, lipid droplets and germ-cell phagolysosomes. Premeiotic cells exhibited nuage and a flagellum, with the Golgi complex and the rough endoplasmic reticulum originating proacrosomal vesicles during the pachytene stage. In round spermatids, the acrosomal vesicle migrated linked to the plasma membrane. In P. rudis, the acrosomal vesicle base formed a thin expansion that attached to the nuclear apex and was associated with development of the perforatorium. The cap-shaped acrosomal vesicle then differentiated into external and internal regions, and also into a small apical light region, although some cells exhibited an apical extension of the external component. On the contrary, two lateroapical light pouches developed in C. gallina. During spermiogenesis, chromatin became fibrillar and then condensed while the nucleus turned conical shaped in P. rudis or slightly curved in C. gallina. In P. rudis, the midpiece contained glycogen and four mitochondria, although five mitochondria were sometimes observed, whereas in C. gallina the midpiece contained four mitochondria. Comparison with other members of Veneroida shows a common ectaquasperm type, but novel findings in acrosome biogenesis.     

Differentiation of the acrosomal complex in ostrich (Struthio camelus) spermatids

The acrosomal complex of ostrich sperm consists of a small, cone-shaped acrosome and a slender, cylindrical perforatorium housed within a deep endonuclear canal. The perforatorium is almost exclusively endonuclear in location and is only covered by the acrosome at its point of origin in the apical subacrosomal space. The development of the acrosome is generally similar to that described in other non-passerine birds. Small proacrosomal granules (vesicles) emanating from the Golgi apparatus coalesce to form a large, membrane-bound acrosomal vesicle filled with homogeneous, electron-dense material. The acrosomal vesicle attaches to the nucleus via a shallow depression and subsequently collapses to form the typical cap-like acrosome of non-passerine birds. In ostrich spermatids the endonuclear canal becomes obvious when the collapsed acrosomal vesicle has assumed a dumbbell-shaped appearance. The perforatorium, which originates from moderately electron-dense material contained within the apical subacrosomal space, expands within the deepening endonuclear canal. The material of the perforatorium does not originate in the form of an obvious granule as in chicken and budgerigar spermatids. Indications are that in ostrich spermatids the developing acrosome plays a role in the shaping of the tip of the nucleus. The perforatorium, however, appears to represent a residual structure that has no specifically identified function. © 1996 Wiley-Liss, Inc.     

Targeting and fusion proteins during mammalian spermiogenesis.

Regulated exocytosis is controlled by internal and external signals. The molecular machinery controlling the sorting from the newly synthesized vesicles from the Golgi apparatus to the plasma membrane play a key role in the regulation of both the number and spatial location of the vesicles. In this context the mammalian acrosome is a unique vesicle since it is the only secretory vesicle attached to the nucleus. In this work we have studied the membrane trafficking between the Golgi apparatus and the acrosome during mammalian spermiogenesis. During bovine spermiogenesis, Golgi antigens (mannosidase II) were detected in the acrosome until the late cap-phase spermatids, but are not found in testicular spermatozoa (maturation-phase spermatids). This suggests that Golgiacrosome flow may be relatively unselective, with Golgi residents retrieved before spermination is complete. Surprisingly, rab7, a protein involved in lysosome/endosome trafficking was also found associated with the acrosomal vesicle during mouse spermiogenesis. Our results suggest that the acrosome biogenesis is associated with membrane flow from both the Golgi apparatus and the endosome/lysosome system in mammalian spermatids.     

The spermiogenesis and the early spermatozoa of <Emphasis Type="Italic">Armorloricus elegans</Emphasis> (Loricifera,Nanaloricidae)

The spermiogenesis consisting of five spermatid stages and the early spermatozoon has been investigated in Armorloricus elegans (Loricifera) with the use of transmission electron microscopy. The male reproductive system consists of three parts; testes, vasa deferentia and seminal vesicles. Caudally, the two seminal vesicles merge together in a ciliated duct and the excretory/gonadal—and digestive systems continue through the recto-urogenital canal, which opens via the lateral gonopores and the temporarily closed anal system. Spermiogenesis mainly occurs in the testes, whereas further maturation of the late spermatids and early spermatozoa occurs in the vasa deferentia and seminal vesicles. A maturation gradient (from spermatocytes to spermatozoa) is found from the posterior peripheral part of the testes to the anterior periphery and then centrally. During spermiogenesis the round nucleus becomes more osmiophilic and condensation of chromatin occurs. Later the nucleus elongates until it becomes rod-shaped in the early spermatozoa. In the second spermatid stage, a large vesicle is formed by saccules developed from the Golgi complex. This vesicle develops further and consists of three different osmiophilic parts with some crystal-like structures inside and is on the outside almost entirely surrounded by thick striated filaments. In the mid-piece the flagellum has a typical 9 × 2 + 2 axoneme and the two mitochondria are fused into a single sheet surrounding the flagellum. In the early spermatozoon stage an acrosomal-like cap structure with an acrosome filament appears proximal to the protruded rod-shaped nucleus. This cap is not formed by the Golgi complex and therefore might not be a true acrosome. Comparing the early spermatozoa of A. elegans with other cycloneuralians has shown some similarities with especially Kinorhyncha and Priapulida. These similarities are thought to be plesiomorphic.     

Spermatogenic ultrastructure in the anomalodesmatan bivalve Myochama anomioides (Mollusca: Myochamidae) – does the nucleus help position the ‘temporary’ acrosome?

Spermatogenic ultrastructure in the marine bivalve mollusc Myochama anomioides (Myochamidae) is described and contrasted with other bivalves, especially other euheterodonts. Small (0.1 μm diameter), primary proacrosomal vesicles produced in spermatocytes give rise to much larger (0.4 μm diameter) secondary proacrosomal vesicles in early spermatids, which in turn form the dished‐shaped, definitive acrosomal vesicle (diameter 1.0 μm) of later spermatids. The acrosomal vesicle acquires a deposit of subacrosomal material and comes to lie close to or in contact with the plasma membrane. The acrosomal complex (acrosomal vesicle + subacrosomal material) initially positions itself at the apex of the condensing, fibrous nucleus (the so‐called temporary acrosome position), but subsequently begins to move posteriorly. The condensing nucleus becomes markedly folded so that its apex is posteriorly orientated towards the migrating acrosomal complex and the midpiece (mitochondria and centrioles). The close spatial relationship of nuclear apex to acrosomal complex during this folding strongly suggests that acrosomal migration in M. anomioides is assisted, at least in part, by movement of the late spermatid nucleus. Similar nuclear folding has previously been demonstrated in an early stage of fertilization in another anomalodesmatan (Laternula limicola) raising the possibility that one event might be a reversal of the other.     

Acrosome differentiation in the ascidians Clavelina lepadiformis and Ciona intestinalis

The spermatozoa of both Clavelina lepadiformis and Ciona intestinalis have architectural features characteristic of ascidian spermatozoa that have been previously described. They have an elongated head (6 microm and 3 microm long, respectively) and a single mitochondrion that is closely applied laterally to the nucleus; they lack a midpiece. The acrosome of Clavelina lepadiformis spermatozoa is a moderately electron-dense, pear-shaped flattened vesicle, approx. 300 nm x 200 nm x 40 nm in length, width, and height, respectively. The acrosome of Ciona intestinalis spermatozoa is a moderately electron-dense, round flattened vesicle with an electron-dense plate in its central region. It is approx. 200 nm x 200 nm x 50 nm in length, width, and height, respectively. During spermiogenesis in both ascidians, several proacrosomal vesicles (50-70 nm in diameter) appear in a blister at the future apex of the spermatids. These vesicles appear to be associated with the inner surface of the plasma membrane enclosing the blister. They come into contact with each other along the inner surface of the plasma membrane and fuse to form a horseshoe-shaped acrosomal vesicle, which becomes a round, flattened vesicle during further differentiation. Some speculations about the mechanism of acrosome differentiation, the possible role of the acrosome during fertilization, and in the speciation of ascidians are presented.     

Assembly of spermatid acrosome depends on microtubule organization during mammalian spermiogenesis

The acrosome is a secretory vesicle attached to the nucleus of the sperm. Our hypothesis is that microtubules participate in the membrane traffic between the Golgi apparatus and acrosome during the first steps of spermatid differentiation. In this work, we show that nocodazole-induced microtubule depolarization triggers the formation of vesicles of the acrosomal membrane, without detaching the acrosome from the nuclear envelope. Nocodazole also induced fragmentation of the Golgi apparatus as determined by antibodies against giantin, golgin-97 and GM130, and electron microscopy. Conversely, neither the acrosome nor the Golgi apparatus underwent fragmentation in elongating spermatids (acrosome- and maturation-phase). The microtubule network of round spermatids of azh/azh mice also became disorganized. Disorganization correlated with fragmentation of the acrosome and the Golgi apparatus, as evaluated by domain-specific markers. Elongating spermatids (acrosome and maturation-phase) of azh/azh mice also had alterations in microtubule organization, acrosome, and Golgi apparatus. Finally, the spermatozoa of azh/azh mice displayed aberrant localization of the acrosomal protein sp56 in both the post-acrosomal and flagellum domains. Our results suggest that microtubules participate in the formation and/or maintenance of the structure of the acrosome and the Golgi apparatus and that the organization of the microtubules in round spermatids is key to sorting acrosomal proteins to the proper organelle.