Spermiogenesis in Polyplacophora,with special reference to acrosome formation (Mollusca)

Summary The present study examines spermiogenesis, and in particular the formation of the acrosome, in ten species of chitons belonging to four families. This study emphasizes the formation of the acrosome but brings to light several other structures that have received little or no mention in previous studies. The process of spermiogenesis is essentially similar in each species, although Chaetopleura exhibits some significant differences. In early spermiogenesis the Golgi body secretes numerous small pro-acrosomal vesicles that gradually migrate into the apical cytoplasm. The chromatin condenses from granules into fibres which become twisted within the nucleus. A small bundle of chromatin fibres projects from the main nuclear mass into the anterior filament; this coincides with the appearance of a developing manchette of microtubules around the nucleus that originates from the two centrioles. Radiating from the distal centriole is the centriolar satellite complex, which is attached to the plasma membrane by the annulus. The distal centriole produces the flagellum posteriorly and it exits eccentrically through a ring of folded membrane that houses the annulus. Extending from the annulus on one side of the flagellum, in all but one species, is a dense fibrous body that has not been previously reported. The proximal centriole lies perpendicular to the end of the distal centriole and is attached to it by fibro-granular material. Pro-acrosomal vesicles migrate anteriorly through the cytoplasm and move into the anterior filament to one side of the expanding nucleus. Eventually these vesicles migrate all the way to the tip of the sperm, where they fuse to form one of two granules in the acrosome. In mature sperm the nucleus is bullet-shaped with a long anterior filament and contains dense chromatin with occasional lacunae. The mitochondria vary in both number and position in the mature sperm of different species. Both centrioles are housed eccentrically in a posterior indentation of the nucleus, where the membranes are modified. The elongate flagellum tapers to a long filamentous end-piece that roughly corresponds to the anterior filament and may be important in sperm locomotion for hydrodynamic reasons. An acrosome is present in all ten species and stained positively for acid phosphatase in three species that were tested.     

STUDIES OF SPERMIOGENESIS IN A NEMATODE, NIPPOSTRONGYLUS BRASILIENSIS

The fine structure of the developing spermatids and the mature sperm of Nippostrongylus brasiliensis was investigated. Immature spermatids are found at one end of the tubelike testis, and the mature sperm at the other. The spermatid has a prominent nucleus, with the chromatin clumped at the margin. It also contains a pair of centrioles, located near the nucleus. The cytoplasm is filled with ribosomal clusters, but it lacks an organized Golgi area or endoplasmic reticulum. Besides the normal mitochondria, the spermatid has specialized mitochondrionlike inclusions with dense matrix, few broad cristae, and a crystalloid structure always facing the nucleus. As spermiogenesis proceeds, the nucleus elongates, comes to lie at one end, and later evaginates to form a separate head structure, leaving the mitochondria and other cytoplasmic organelles in a broad cytoplasmic region. The nuclear material becomes filamentous and spiral, and the centrioles come to lie at one end near the junction of the head and the cytoplasmic portion of the sperm. Microtubules are found in the cytoplasmic region extending from the tubelike nucleus. The specialized mitochondria are about eighteen in number, and are arranged in rows in staggered groups of three around the microtubules in the cytoplasmic region. The mature sperm is aflagellate and lacks an acrosome. No movement of the sperm was ever observed.     

东方扁虾精子发生的超微结构

应用电镜技术研究了东方扁虾（Thenus orientalis)精子发生的全过程,精原细胞呈椭圆形,其染色质分布较均匀,线粒体集中于细胞一端形成“线粒体区”。初级精母细胞较大,染色质凝聚成块,次级精母细胞核质间常出现大的囊泡,胞质内囊泡丰富而线粒体数量却明显减少,早期精细胞核发生极化、解聚,部分胞质被抛弃。中期精细胞外观呈金字塔形,分为三区;正在形成的顶体位于塔顶,核位于塔基部,居间的细胞质基质内富含膜复合物,后期精细胞顶体进一步分化。形成顶体帽和内、外顶体物质等三个结构组份。成熟精子核呈盘状或碗状,具有5-6条内部充满微管的辐射臂。     

Fine structure of spermiogenesis with special reference to the spermatid morulae of the freshwater mussel Prisodon alatus (Bivalvia,Unionoidea)

Within the testicular cysts of the mussel Prisodon alatus are numerous somatic host cells described as Sertoli cells (SC), each containing a variable number of young spermatid morulae. Among them, several free spermatid morulae, spermatids, and spermatozoa were observed. Each free spermatid morula is surrounded by an external membrane. The early spermatids enclosed within the morulae have dense and homogeneous chromatin, and the cytoplasm occupies little space around the nucleus. Later, during spermiogenesis, the SC show lysis and disrupt to liberate the spermatid morulae. The membrane of the free morula is then disrupted, releasing the young spermatids. The SC disappear just after the appearance in the testis of a large number of free young spermatids. The nucleus of each free spermatid becomes gradually smaller and denser by the appearance of a granular pattern of condensed chromatin. During the maturation phase of the spermatids, the cytoplasm becomes more voluminous, and mitochondria and centrioles are more evident. Then, flagellogenesis occurs, and the nucleus gradually condenses into thicker strands. In the mature sperm, the apical zone has a disc-shaped acrosomal vesicle and the midpiece contains five mitochondria and two centrioles located at the same level. The flagellum has the common 9+2 microtubular pattern. The results are discussed with particular reference to Sertoli cells and clusters of spermatid morulae with those of species of closely related taxa in the bivalves. J. Morphol. 238:63–70, 1998. © 1998 Wiley-Liss, Inc.     

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 ultrastructure of the spermatozoon and spermiogenesis in Cryptocotyle lingua (digenea: Heterophyidae)

During spermiogenesis two lateral flagellar processes and a median process arising from the apex of the zone of differentiation, fuse to form the elongated unipartite spermatozoon. Two axial units, therefore, with the ‘9+1’ pattern of microtubules are incorporated into the spermatozoon. The nucleus, in the head region, contains dense lamellar subunits arranged in a spiral in the long axis. These are formed by condensation of the chromatin during spermiogenesis. The single elongated mitochondrion, resulting from early fusion of small mitochondria, extends through the head and middle regions of the spermatozoon. Peripheral microtubules, present originally in the zone of differentiation, are arranged in straight dorsal and ventral rows, along the length. β glycogen particles accumulate in the spermatozoa after they have separated from the residual cytoplasm. Spermatozoa are present in the testes on the second day after infection of the bird host and accumulate in the vesicula seminalis from the third day onwards.     

Ultrastructure of spermatogenesis and mature spermatozoa in the flatworm Prosthiostomum siphunculus (Polycladida,Cotylea)

This is the first study investigating spermatogenesis and spermatozoan ultrastructure in the polyclad flatworm Prosthiostomum siphunculus. The testes are numerous and scattered as follicles ventrally between the digestive ramifications. Each follicle contains the different stages of sperm differentiation. Spermatocytes and spermatids derive from a spermatogonium and the spermatids remain connected by intercellular bridges. Chromatoid bodies are present in the cytoplasm of spermatogonia up to spermatids. During early spermiogenesis, a differentiation zone appears in the distal part of spermatids. A ring of microtubules extends along the entire sperm shaft just beneath the cell membrane. An intercentriolar body is present and gives rise to two axonemes, each with a 9 + “1” micro‐tubular pattern. Development of the spermatid leads to cell elongation and formation of a filiform, mature spermatozoon with two free flagella and with cortical microtubules along the sperm shaft. The flagella exit the sperm shaft at different levels, a finding common for acotyleans, but so far unique for cotylean polyclads. The Golgi complex produces numerous electron‐dense bodies of two types and of different sizes. These bodies are located around a perinuclear row of mitochondria. The elongated nucleus extends almost along the entire sperm body. The nucleus is wide in the proximal part and becomes narrow going towards the distal end. Thread‐like chromatin mixed with electron‐dense intranuclear spindle‐shaped bodies are present throughout nucleus. The general sperm ultrastructure, the presence of intranuclear bodies and a second type of cytoplasmic electron‐dense bodies may provide characters useful for phylogenetic analysis.     

Sperm nucleomorphogenesis in the cephalopod Sepia officinalis

Sperm nucleomorphogenesis in the cephalopod Sepia officinalis is the product of the interaction between perinuclear microtubules and condensing chromatin. This interaction occurs during spermiogenesis and is established through the nuclear membrane. As in other cephalopod species, the perinuclear microtubules are transient structures. In the case of S. officinalis, they begin to appear in the basal area of the early spermatid and progress from there, establishing contact with the external nuclear membrane and follow a defined, but not symmetric, geometry. Thus, the microtubules accumulate preferentially in one area of the nuclear membrane which we refer to here as the "dorsal zone". Later, the microtubules will be eliminated before the mature spermatid migrates to the epidydimis. The chromatin is condensed within the nucleus following a complex pattern, beginning as fibro-granular structures until forming fibres of approximately 45 nm diameter (patterning phases). From this stage on, an increase in the chemical basicity of DNA-interacting proteins is produced, and chromatin fibres coalesce together, being recruited to the dorsal zone of the membrane, where there is a higher density of microtubules. This last step (condensation phases) allows the chromatin fibres to be arranged parallel to the axis of the elongating nucleus, and more importantly, is deduced to cause a lateral compression of the nucleus. This lateral compression is in fact a recruitment of the ventral zone toward the dorsal zone, which brings about an important reduction in nuclear volume. The detailed observations which comprise this work complement previous studies of spermiogenesis of Sepia and other cephalopods, and will help to better understand the process of cellular morphology implicated in the evolution of sperm nuclear shape in this taxonomic group.     

Ultrastructure of sperm and spermatogenesis of Lobatostoma manteri (Trematoda, Aspidogastrea)

Mature sperm has two axonemes of the 9 + '1' pattern incorporated in the sperm body, a row of peripheral microtubules interrupted along part of the sperm by the axonemes, some microtubules in the interior of the sperm and a long lateral extension (lobe) of the sperm body, an elongate nucleus and mitochondrion, and many dense rod-like structures. A supporting rod extends underneath a specialized region consisting of alternating thin and thick transverse rows of irregular dense patches, and with surface ridges around (all or) most of the surface of the sperm. Primary spermatocytes in the prophase of the first meiotic division have synaptonemal complex(es), and are rich in mitochondria. In early spermiogenesis, mitochondria are arranged around the surface of the nucleus, a dense layer appears at one pole of the nucleus, close to an apposed dense layer at the cell membrane in which a row of microtubules develops. The intercentriolar (= central) body develops close to the nucleus. The fully developed intercentriolar body has a regular striation and is located perpendicular and close to the surface of the nucleus. Two flagella extend into the space surrounding the outgoing median process, their basal bodies are located perpendicular to the intercentriolar body and their cross-striated rootlets extend along the surface of the rounded nucleus. At a later stage, rootlets and flagella become more parallel with the intercentriolar body, the nucleus and the fused mitochondria migrate into the median process, and the flagella become incorporated into the median process (= sperm body). The outgrowing spermatozoa are connected to the cytoplasm of the cytophore by dense arching membranes. Finally, rootlets of flagella are resorbed and the spermatozoa are pinched off close to the basal bodies. Two species (Lobatostoma and Multicotyle) of the same family differ strongly in the type of spermiogenesis, although their mature sperm is of the same basic type, i.e. spermiogenesis is not necessarily more useful for phylogenetic considerations than sperm structure.     

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.     

Development of the sperm head in the caddis fly Potamophylax rotundipennis (Insecta,Trichoptera)

The restructuring of the sperm head has been examined in a caddis fly, Potamophylax rotundipennis (Limnephilidae), using light and electron microscopy. The roughly spherical nuclei of young spermatids are transformed into needle-shaped elements in advanced spermatids. During this process, the nuclei transiently become sickle-shaped. Prominent structural changes occur within the nucleus during spermiogenesis. The chromatin of spherical and slightly elongated nuclei has an amorphous appearance, then coarse granules become apparent, chromatin threads are visible in fully elongated nuclei and finally lamellar elements appear. During the changes in chromatin texture, a dense layer, the chromatin rim, develops transiently. This feature of the chromatin surface is interpreted as the structural expression of exchanges between nucleus and cytoplasm. A microtubular manchette is formed at the cytoplasmic face of the nuclear envelope. Whereas the manchette covers the full perimeter of the nucleus in early stages of elongation, gaps in the palisade of microtubules appear before the nuclear diameter decreases and needle-shaped nuclei develop. It is possible that the intermittent deployment of manchette microtubules is involved in reducing the nuclear diameter towards the end of nuclear elongation. The delayed detachment of the chromatin from the posterior pole of the nucleus, observed at the onset of nuclear clongation, points to local modifications of the nuclear envelope responsible for the connection of the centriole adjunct and the flagellum with the posterior pole of the nucleus.     

Fine structure of the spermatozoa of Chiton marginatus (mollusca: Amphineura), with special reference to nucleus maturation

The spermatozoon of Chiton marginatus is a long uniflagellate cell displaying structural features of “modified sperm.” The nucleus presents a conical shape with a long apical cylindrical extension. The chromatin is homogeneously dense. Scattered inside the condensed nucleus, a few nuclear lacunae are visible. The acrosomal complex is lacking. Some mitochondria are located in a laterofrontal structure side by side with the nucleus. The typical midpiece is absent. The cytoplasm forms a thin layer around the nucleus and the mitochondria. The proximal centriole is in a basal nuclear indent. The distal centriole serves to form the axoneme tail with the usual microtubular pattern. During nuclear maturation, the early spermatid nucleus is spherical and contains fine granular chromatin patches. The nuclear envelope shows a deposit of dense material at the base of the nucleus, forming a semicircular invagination occupied by a flocculent mass. In middle spermatid stage, the chromatin gets organized in filaments, coiled as a hank, attached over the inner surface of the basal thickening of the nuclear envelope. The nucleus starts to elongate anteroposteriorly. At the pointed apical portion of the spermatid, a group of microtubules is observed seeming to impose external pressure to the nucleus giving rise to the long apical nuclear point. The mitochondria have a basal position. Late spermatids have an elongated conical nucleus. The chromatin filaments are further condensed, and lacunae appear inside the nucleus. Some mitochondria migrate to a lateral position.     

The spermatogenesis and the sperm structure of Terebrantia (Thysanoptera, Insecta)

Spermatogenesis and the sperm structure of the terebrantian Aeolothrips intermedius Bagnall are described. Spermatogenesis consists of two mitotic divisions; the second is characterized by the loss of half of the spermatids, which have pyknotic nuclei. Early spermatids have two centrioles, but when spermiogenesis starts, a third centriole is produced. The three basal bodies give rise to three flagella; later these fuse into a single flagellum which contains three 9 + 0 axonemes. The basal bodies are surrounded by a large amount of centriole adjunct material. During spermiogenesis this material contributes to the shifting of the three axonemes towards the anterior sperm region parallel to the elongating nucleus, and it is transformed into a dense cylinder. In the mature spermatids the three axonemes amalgamate to create a bundle of 27 doublet microtubules. Near the end of spermiogenesis the dense cylinder of the centriole adjunct lies parallel to the nucleus and the axonemes. It ends where the mitochondrion appears at half-sperm length. We confirm that Terebrantia testes have a single sperm cyst; their sperm are characterized by a cylindrical nucleus, three axonemes fused into one, a small mitochondrion and a short cylindrical centriole adjunct which corresponds to the dense body described in a previous work. The acrosome is lacking. At the midpoint of the anterior half of the sperm the outline of the cross-section is bilobed, with the nucleus contained in a pocket evagination of the plasma membrane. These characters are discussed in light of a comparison between Tubulifera and Terebrantia.     

Ultrastructure of spermiogenesis and synthesis of enigmatic cytoplasmic inclusions in the spirally shaped spermatozoon of the polychaete Spio setosa (Annelida: Spionidae)

The sperm of Spio setosa (Polychaeta, Spionidae) are known to be very unusual in form; here, spermiogenesis and the structure of the spermatozoon in this species are described by transmission electron microscopy. While spermiogenesis is similar to that described for many other polychaetes, two notable exceptions to this process include the synthesis of abundant ring‐shaped and tubular, membrane‐bounded cytoplasmic inclusions in the midpiece, and the differentiation of a spirally shaped sperm head. Spermatids develop as free‐floating tetrads in the male's coelom. A microtubular manchette does not develop during chromatin condensation and nuclear elongation, and the spiral acrosome forms as a single Golgi‐derived vesicle that migrates anteriorly to become housed in a deep anterior nuclear fossa. Early in spermiogenesis, numerous Golgi‐derived, membrane‐bounded cytoplasmic inclusions appear in the cytoplasm; these ultimately occupy the sperm midpiece only. The mature spermatozoon in the male has a 15‐μm‐long head consisting of a nucleus coiled like a spring and a spiral acrosome with differentiated substructure, the posterior two thirds of which sits in an anterior nuclear fossa. The midpiece is wider than the rest of the spermatozoon and contains 9–10 spherical mitochondria surrounding the two centrioles, as well as numerous membrane‐bounded conoid and tubular cytoplasmic inclusions. The axoneme has a 9 + 2 arrangement of microtubules. By contrast, stored sperm in the female's seminal receptacles have lost the midpiece inclusions but contain an abundance of glycogen. The function of the midpiece inclusions remains unresolved, and the significance of their absence in stored sperm within the seminal receptacle and the appearance of midpiece glycogen stores remains unclear and requires additional investigation.     

Studies on the polymorphic spermatozoa of a marine snail. 2. Genesis of the eupyrene sperm

Spermiogenesis of the eupyrene sperm in the snail, Fusitriton oregonensis, was studied with light and electron microscopes. Endoplasmic reticulum, which encircles the nucleus in each spermatid, appears to connect with the Golgi body and to interconnect between adjacent spermatids via cytoplasmic bridges. It is suggested that as the Golgi body migrates around the nucleus the endoplasmic reticulum may circulate with it. The alignment of the proacrosome with the nucleus is effected by a 180° rotation of the Golgi body, after which it separates and migrates posteriorly with the residual cytoplasm. Each sperm possesses a well-developed intracellular digestive system as indicated by multivesicular bodies, residual bodies, and myeloid figures. Autophagy begins in the residual cytoplasm before it is released from the middle piece. Microtubules are found outside the nucleus and mitochondria during the final stages of spermiogenesis, when elongation is almost complete. These microtubules appear to be involved in the final shaping and twisting process, in which torsion is locked in the nucleus and the mitochondria spiral around the axoneme. The annulus attaches the distal centriole to the plasma membrane in the early spermatid and as flagellar production begins they move towards the implantation fossa at the base of the nucleus. There are two centrioles in the early spermatid, the distal centriole and procentriole. The small procentriole fuses with the distal centriole in the intranuclear canal to form the centriolar cap of the basal body. This cap is pushed through the end of the nuclear tube and is separated from the subacrosomal space by only the nuclear membranes.     

Fine structural observations on nuclear maturation during spermiogenesis in Hydra littoralis

Cytodifferentiation during spermiogenesis in Hydra littoralis was studied at the fine structural level. Concentration of nuclear material as well as specific orientation of granular and filamentous nuclear elements are apparent in two regions of the early spermatid: where the nuclear envelope is in contact with mitochondrial membranes at one pole of the cell and at an opposite region where the nucleus is closely apposed to the plasma membrane. Ultimately the mass of condensed nuclear material becomes concentrated at the mitochondrial pole of the cell. Additional electron-dense material is extruded from the nucleus into a large vacuole which is in continuity with the nuclear membrane as well as associated with Golgi lamellae and vesicles. Eventually all residual cytoplasm is sloughed, leaving the nucleus, mitochondria, and flagellum. These observations are suggestive of nucleocytoplasmic interactions during development, especially influences of mitochondria and plasma membranes on chromatin condensation.     

中国雨蛙精子形成的研究

中国雨蛙的精子形成过程中,细胞核的浓缩经历了５个时期。从第１期进入第２期,染色质纤维增粗并聚集成卷曲的柱状结构。从第２期进入第３期,染色质纤维进一步增粗,细胞核逐渐伸直成柱状。进入第４期,染色质紧密聚集,纤维之间间隙很小。进入第５期,染色质纤维聚集成均匀的致密结构。伴随着染色质的浓缩,核膜数次更新,核内不参与浓缩的物质渐次从核中排出,核中出现一串核泡。顶体在染色质未浓缩之前（第１期）开始分化,由一     

Fibrogranular Material,Annulate Lamellae and Microtubules During Spermiogenesis in Drosophila melanogaster

During spermiogenesis in Drosophila melanogaster, a “perinuclear plasm’ accumulates between the fenestrated portion of the nuclear envelope and an adjacent lamella of ER in the young spermatid. Microtubules appear within the perinuclear plasm and become especially concentrated in a nuclear concavity. Cytoplasmic pores are present locally within the lamella of ER. In addition, localized or discrete bodies composed of fibrogranular material become closely associated with single pore complexes in the lamella of ER. A close association exists between pore complexes (annulate lamellae), the small granular and fibrillar subunits of the fibrogranular bodies, polyribosomes and the nuclear-associated microtubules during much of spermiogenesis. While the fibrogranular material becomes less concentrated during spermiogenesis, the number of pore complexes in a single section increases such that two, three or even four short annulate lamellae are intercalated within many longitudinally oriented microtubules which are present in the furrow of the spermatid nucleus. Structural relationships observed between cytoplasmic pores (annulate lamellae), fibrogranular bodies, polyribosomes and microtubules are discussed in relation to information about the timing of RNA and protein synthesis. This study extends previous observations about the distribution and structural variations of annulate lamellae elsewhere in the spermatid cytoplasm.     

Fine structure of spermatozoa and spermatogenesis ofSchizomus palaciosi,Reddell and Cokendolpher, 1986 (Arachnida: Uropygi,Schizomida)

Summary Spermatogenesis ofSchizomus palaciosi occurs in cysts in paired tubular testes located ventrally in the opisthosoma. Only few germ cells comprise one cyst. In early spermiogenesis an acrosomal complex composed of a spherical vacuole and a short acrosomal filament is established opposite of which a 9×2+3 flagellum emerges from a flagellar tunnel. The latter, however, is only a short-lasting structure. A manchette of microtubules surrounds nucleus and part of the acrosomal vacuole. The alterations in the arrangement of the microtubules during spermiogenesis are described. The spermatid finally is an elongate cell with a slender acrosomal vacuole on top of the helical nucleus. A deep implantation fossa filled with dense material is encountered. The acrosomal vacuole is accompanied by an intricate paracrosomal lattice structure not known at present of otherArachnida. This structure disappears during final spermiogenesis. The acrosomal filament (perforatorium) reveals filamentous subunits arranged in a regular pattern. Large ovoid mitochondria do not establish a distinct middle piece. Finally the elongate spermatid is coiled to form the mature spherical spermatozoon.The results are discussed under functional and taxonomical aspects.     

Cytodifferentiation during spermiogenesis in Lumbricus terrestris

The structural changes during spermiogenesis were studied on developing spermatids in seminal vesicles and receptacles of Lumbricus terrestris fixed in glutaraldehyde-osmium tetroxide and embedded in Epon-Araldite. The centriole plays a prominent role in the morphogenesis and organization of the microtubules of the manchette and flagellum. Microtubules arising from the centriole extend anteriorly to encase the developing middle piece, the nucleus, and the acrosome. The manchette not only provides a supporting framework for the cell during elongation, but also may provide the motive force for the elimination of both nucleoplasm and cytoplasm. The manchette participates in segregation and elimination of the nuclear vesicle that contains the nonchromatin nucleoplasm. Compartmentalization and conservation may also be a function of the manchette since those elements which remain within the framework of microtubules are retained, while all the cytoplasm outside the manchette is discarded. At maturation, the endoplasmic reticulum plays a key role in dismantling the manchette and reducing the cytoplasm external to it. During the early stages of middle-piece formation, six ovoid mitochondria aggregate at the posterior pole of the spermatid nucleus. Concurrent with manchette formation, the mitochondria are compressed laterally into elongate wedge-shaped components, and their outer limiting membranes fuse to form an hexagonal framework that surrounds the dense intramitochondrial matrices. Dense glycogen granules are arranged linearly between the peripheral flagellar tubules and the outer membrane of the mature sperm tail.