Formation of the acrosome and basal body during spermiogenesis in a marine snail,Nerita picea (Mollusca: Archaeogastropoda)

In Nerita picea the proacrosomal granule is formed basally in the early spermatid from one large cisterna of the Golgi body, with which the other Golgi-derived vesicles fuse. After the proacrosomal granule has attached to the plasma membrane and invaginated to form a cup shape, one cisterna of endoplasmic reticulum inserts into the open end and deposits a granular secretion on the inner surface. Subsequently, the proacrosome migrates along the plasma membrane to the apex of the nucleus, but the Golgi body remains basal, as occurs in other archaeogastropods and also many polychaete annelids. However, the final shape and structure of the acrosome is similar to that of mesogastropods. The annulus attaches the distal centriole to the plasma membrane early in spermiogenesis. The production of the flagellum by the distal centriole not only expands the plasma membrane posteriorly but moves the centriolar complex to the nucleus, causing an invagination of the plasma membrane where it is bound by the annulus. During proacrosome migration, the Golgi body secretes a dense tube around the flagellum, and the mitochondria fuse into two spheres at the base of the nucleus. The nuclear plug that closes off the intranuclear canal until this stage rapidly reorganizes itself into two tubes of material inside the canal. The centrioles continue flagellar production, break away from the annulus, and move deep into the intranuclear canal where they fuse together to form the basal body of the sperm. In the maturing spermatid, the two mitochondria fuse into a single sheath that spirals around the flagellum. The annulus does not migrate posteriorly but remains anterior to the midpiece, which is unusual for a filiform sperm. Spermiogenesis in Nerita picea has features in common with both archaeogastropods and mesogastropods but also has some unique features. These observations lend credence to the idea that the Neritidae are a transitional group between Archaeogastropoda and Mesogastropoda.     

The ultrastructure of amberjack (Seriola dumerilii) sperm

Scanning and transmission electron microscopy were used to investigate the fine structure of sperm of the Mediterranean amberjack Seriola dumerilii. Each spermatozoon has an ovoid head which lacks an acrosome, a short, irregularly-shaped midpiece and a long flagellar tail. The midpiece houses eight spherical mitochondria, which are separated from the axoneme by the cytoplasmic canal. The centrioles are arranged approximately at right angles to each other. The proximal centriole lies inside, and the distal centriole outside, the nuclear fossa. The flagellum is inserted eccentrically into the head and is tangential to the nucleus, so that the spermatozoon is asymmetrical. It contains the conventional 9 + 2 axoneme, shows intratubular differentiations in the A microtubules of doublets 1, 2, 5 and 6, and possesses one pair of lateral fins. On the basis of its ultrastructural organization, the amberjack sperm resembles type II sperm as defined previously, except for the presence of the proximal centriole inside the nuclear fossa. This could result from a partial rotation of the nucleus during spermiogenesis.     

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     

Fine structure of the spermatozoa of Crassostrea gigas (Mollusca,Bivalvia)

We describe sperm ultrastructure and acrosome differentiation during spermiogenesis in Crassostrea gigas (Mollusca Bivalvia). The sperm cell is a uniflagellated cell of the primitive type. The head region contains a rounded or conical nucleus surmounted by small acrosome. This organelle consists of a membrane-bound acrosomal granule, the contents of which have a homogeneous density, except in the anterior region, which is positive for PTA. The acrosome also surrounds the perforatorium, which includes oriented fibrillar elements: this is the axial body. The middle piece contains four mitochondria encircling two perpendicular centrioles. The distal centriole is provided with a system of mechanical fixation to the plasma membrane, consisting of nine fibers in radial arrangement. The tail flagellum, about 50 m?m long, contains the usual microtubular axoneme. © 1993 Wiley-Liss, Inc.     

Sperm ultrastructure and spermiogenesis in two Exogone species (Polychaeta, Syllidae, Exogoninae)

Abstract. The spermatozoa of Exogone naidina and E. dispar are characterized by a prominent bell-shaped acrosome, a spheroidal nucleus, and a conventional flagellum. During spermiogenesis, the acrosomal vesicle undergoes conspicuous modifications leading to its final bell shape with a posterior opening. The subacrosomal material initially shows radiating filaments but in mature sperms it appears as a meshwork of electron-opaque material. The acrosomal axis is oblique with respect to the main longitudinal sperm axis. The chromatin is arranged in electron-opaque strands in the early spermatids, then becomes amorphous, and is finally organized in filaments in mature sperms. Centrioles are orthogonally arranged beneath the nucleus and fibers radiate from the distal centriole to contact the plasma membrane and the single mitochondrion. The latter is located eccentrically on the side of the nucleus opposite the acrosome. A disk-shaped structure is evident beneath the distal centriole. The flagellar axoneme has a 9+2 microtubule pattern. A conspicuous glycocalyx surrounds the flagellar plasma membrane, and an electron-lucent space is present between these two structures at the distal tip of the flagellum. We compare the sperm morphology of these two species of Exogone with that described in other members of the subfamily Exogoninae. The fine structure of these two species supports the occurrence of an ent-aquasperm type within Exogoninae, in accordance with the brood strategy present within this subfamily. The mode of reproduction is of taxonomic importance for defining subfamilies within Syllidae, and is likely also of phylogenetic significance. Because epitoky is probably plesiomorphic, the ent-aquasperm type found in Exogoninae can be considered a derived feature within Syllidae.     

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.     

Comparative Ultrastructure of Cnidarian Sperm

Marine Cnidarian sperm consist of a head containing the nucleusand several electron dense vesicles. These vesicles aie anteriorto the nucleus in the Hydrozoan and Scyphozoan sperm and adjacentto the posterior legion of the nucleus in the Anthozoan sperm.They are Golgi in origin and do not coalesce. They may be aprimitive acrosome. Mitochondria may number four to five inthe Hydrozoan and Scyphozoan sperm while they form an aggregatein the Anthozoans. Both centrioles persist in the mature spermatozoa.The distal centriole gives rise to the long flagellum and hasassociated with it nine pericentriolar processes.     

The behaviour of centrioles and the formation of the flagellum in rooster and drake spermatids

Summary Developmental changes in the formation of the centrioles and flagellum during spermiogenesis in the rooster and drake were studied.Changes in the length and thickness of the wall of the centrioles were observed from an early stage of spermatid development. Before the proximal centriole is attached to the nucleus microtubules were observed near the centrioles joined to them. At this stage of spermatid development changes on the nuclear membrane were observed at a place where the proximal centriole is attached to the nucleus. At the later stage of spermatid differentiation three to five dense extensions in the space of the nuclear invagination and dense bodies or granules near the distal centriole were present. The anterior part of the newly formed flagellum is covered by a cytoplasmic membrane displaying extension which is approximately 1.3 m long. Slight differences between the two species were observed.     

Ultrastructural observations on spermiogenesis in the peanut worm,Themiste pyroides (Chamberlin, 1920) (Sipuncula; Sipunculidea)

Summary

The process of spermiogenesis and the ultrastructure of the spermatozoa in the peanut worm, Themiste pyroides, from the Sea of Japan were observed with electron microscopy (SEM and TEM). The testes are composed of groups of spermatogonia and are covered by peritoneal cells. Clusters of spermatocytes are released from the testes into the coelomic fluid. Connected by intercellular bridges, the spermatocytes within a given cluster develop asynchronously. Proacrosomal vesicles and a flagellum appear in spermatocytes. Spermatids in the clusters retain the intercellular connections. During spermiogenesis, the acrosomal vesicle, formed by coalescence of small proacrosomal vesicles in the basal part of the spermatid, migrates to the apical part of the cell to form a conical-shaped acrosome. The basal concavity lying above the nucleus is filled with subacrosomal substance. The midpiece contains four mitochondria, two centrioles, and some residual cytoplasm with dark glycogen-like granules. A peculiar annulus structure develops around the base of the flagellum. The distal centriole has a pericentriolar complex consisting of radially oriented elements. Before the spawning process, the spermatozoa are filtered throughout the ciliary nephrostomal funnel into the excretory sac of paired nephridia where they are stored for a short time. The sperm are released into the sea water via nephridiopores. Spermatozoa remaining in the coelomic fluid after spawning are resorbed by amoebocytes. This species from Vostok Bay is characterized by a prolonged spawning period from June to early October. The reproductive strategy of T. pyroides is discussed in comparison with that of Thysanocardia nigra, the latter having a unique pattern of packaging of the spermatozoa, resulting in the formation of spermatozeugmata, as a reproductive adaptation to the very short spawning period.     

Spermiogenesis in caecilians Ichthyophis tricolor and Uraeotyphlus cf. narayani (Amphibia: Gymnophiona): analysis by light and transmission electron microscopy

Spermiogenesis, known as spermateleosis in lower vertebrates, is the transformation of the round spermatid into a highly specialized spermatozoon with a species-specific structure. Spermateleosis and sperm morphology of two species of caecilians, Ichthyophis tricolor and Uraeotyphlus cf. narayani, from the Western Ghats of Kerala, India, were studied using light and transmission electron microscopy. Spermateleosis is described in early, mid-, and late phases. During the early phase, the spermatid nucleus does not elongate, but the acrosome vesicle is Golgi-derived and its material is produced as a homogeneous substance rather than as discrete granules. In development of the acrosome, the centrioles shift in position to the lower half of the cell. The acrosomal vesicles take the full shape of the acrosome with the establishment of the perforatorium in midphase. An endonuclear canal develops and accommodates the perforatorium. The incipient flagellum is laid down when the proximal centriole attaches to the posterior side of the nucleus and the distal centriole connects to the proximal centriole, which forms the basal granule of the acrosome. The axial fiber also appears during midphase. The mitochondria shift in position to the posterior pole of the cell to commence establishment of the midphase. Late phase is characterized by nuclear condensation and elongation. Consequently, the final organization of the sperm is established with the head containing the nucleus and the acrosome. The undulating membrane separates the axoneme and axial fiber. Most of the cytoplasm is lost as residual bodies.     

玫瑰无须鲃精子的超微结构

透射和扫描电镜研究显示玫瑰无须售巴（Puntius conchonius）的精子由头、中片和尾三部分组成。头部无顶体,呈球形或卵圆形,主要由细胞核组成,核内染色质致密。核前端几乎无细胞质存在,核膜紧密靠近细胞质膜,而在核的后端有少量细胞质存在。在核后端偏于一侧处有一个浅的核后凹,中心粒复合体部分地镶嵌于其中,中心粒复合体由近端中心粒和远端中心粒组成,二者呈钝角形排列,鞭毛从远端中心粒的末端发出。中片由前边的主要部分——领和后边细薄的袖套构成。领内含有数个不规则分布的线粒体包埋于细胞质中,袖套的长短、粗细差别较大,有的精子没有袖套。由于与鞭毛的不对称连接,使得头部及中片均呈不对称型。尾是一根细长的鞭毛,尾丝具有典型的“9＋2”微管结构,尾部两侧均无侧鳍。与鲤科其它鱼精子相比,该鱼精子的主要特征是具有长短不一的袖套,领内有不同数量的液泡,且有些空泡向外界开口呈孔状。袖套的长短与领内液泡化水平似有某种相互联系,这也许与精子的老化程度有关[动物学报51（5）：892—897,2005]。     

尼罗罗非鱼精子形成中核内囊泡的释放

通过透射电镜观察了尼罗罗非鱼的精子形成过程。尼罗罗非鱼精子细胞在成熟过程中,细胞核中出现由双层生物膜构成的囊泡。囊泡中均匀分布着电子密度低的物质。该囊泡逐渐从细胞核内排到细胞核外。在此过程中细胞核不但排出不参与染色质浓缩的物质,还将多余的核膜排出。进入袖套的囊泡可以留在精子的袖套中,而排到核前方和核侧面的囊泡继续以出芽的方式排出精子细胞。尼罗罗非鱼成熟精子的头部仅有染色质高度浓缩的细胞核。细胞核前     

Spermatozoa ultrastructure in Sciaenidae and Polynemidae (Teleostei:Perciformes) with some consideration on Percoidei spermatozoa ultrastructure

Spermatozoa ultrastructure was studied in five marines (Paralonchurus brasiliensis, Larimus breviceps, Cynoscion striatus, Micropogonias furnieri, Menticirrhus americanus, Umbrina coroides, Stellifer rastrifer), and one freshwater (Plagioscion squamosissimus) species of Sciaenidae and one species of Polynemidae (Polydactylus virginicus). The investigation revealed that, in all species, spermatozoa display a round head, a nucleus containing highly condensed, filamentous chromatin clusters, no acrosome, a short midpiece with a short cytoplasmic channel, and a flagellum showing the classic axoneme structure (9+2) and short irregular lateral fins. In Sciaenidae, the spermatozoa are type II, the flagellar axis is parallel to the nucleus, the lateral nuclear fossa is double arched, the centriolar complex is outside the nuclear fossa, the proximal centriole is anterior and perpendicular to the distal centriole, and no more than ten spherical (marine species) or elongate (freshwater species) mitochondria are observed. Polynemidae spermatozoa are of the intermediate type with the flagellar axis eccentric to the hemi-arc-shaped nucleus, and exhibit no nuclear fossa, the centriolar complex close to the upper nuclear end, the proximal centriole lateral and oblique to the distal centriole, and one large ring-shaped mitocondrion. The data available show that no characteristic is exclusively found in the spermatozoa of members of the Sciaenidae family when compared to other Percoidei with type II spermatozoa. However, three characteristics were exclusively found in Polynemidae: (1) the hemi-arched nucleus; the positioning of the centrioles; and (2) the ring-shaped mitocondrion. The interrelationships between Sciaenidae and Polynemidae as well as between these two families and other Percoidei are herein discussed.     

Ultrastructure of spermiogenesis in the gastropod Calliotropis glyptus Watson (Prosobranchia: Trochidae)with special reference to the embedded acrosome

Testicular spermatozoa and sperm development in the archaeogastropod Calliotropis glyptus Watson (Trochoidae: Trochidae) are examined using transmission electron microscopy and formalin-fixed tissues. During spermiogenesis, the acrosome, formed evidently through fusion of Golgi-derived proacrosomal vesicles, becomes deeply embedded in the condensing spermatid nucleus. Two centrioles (proximal and distal), both showing triplet microtubular substructure, are present in spermatids—the distal centriole giving rise to the sperm tail and its associated rootlet. During formation of the basal invagination in the spermatid nucleus, centrioles, and rootlet move towards the nucleus and come to lie totally within the basal invagination. Mitochondria are initially positioned near the base of the nucleus but subsequently become laterally displaced. Morphology of the mature spermatozoon is modified from that of the classic primitive or ect-aquasperm type by having 1) the acrosome embedded in the nucleus (the only known example within the Mollusca), 2) a deep basai invagination in the nucleus containing proximal and distal centrioles and an enveloping matrix (derived from the rootlet), 3) laterally displaced periaxonemal mitochondria, and 4) a tail extending from the basal invagination of the nucleus. Implantation of the acrosomal complex and centrioles within imaginations of the nucleus and lateral displacement of mitochondria effectively minimize the length of the sperm head and midpiece. Such modifications may be associated with motility demands, but this remains to be established. The unusual features of C. glyptus spermatozoa, though easily derivable from ‘typical’ trochoid sperm architecture, may prove useful in delineating the genus Calliotropis or tracing its relationship to other genera within the trochid subfamily Margaritinae.     

Spermatozoal Ultrastructure in Branchiostoma moretonensis Kelly, a Comparison with B. lanceolatum (Cephalochordata) and with other Deuterostomes

The spermatozoon of Branchiostoma moretonensis closely resembles that of B. lanceolatum and, though near the primitive sperm, allows recognition of a cephalochordate sperm type. This has: a bell-shaped acrosome; diffuse subacrosomal material not structured as an acrosome rod; sub-ovoidal nucleus with shallow anterior concavity, deep tubular posterior fossa (endonuclear canal) and condensed but lacunate chromatin; single asymmetrical, postnuclear mitochondrion almost completely or completely encircling the centrioles; mutually perpendicular proximal and distal centrioles of the triplet type, with the distal forming the basal body of the flagellum and the proximal (always?), as in B. moretonensis, with a spur-like extension (striated rootlet) into the nuclear fossa; flagellum tilted relative to the longitudinal axis (and endonuclear canal) of the nucleus; a 9 + 2 axoneme with hollow tubules and both dynein arms present on the doublets; and scattered glycogen granules, numerous around the distal centriole. The mitochondrion of B. moretonensis is C-shaped in transverse section, as in urochordates, but cephalochordate sperm resemble those of echinoderms, and specifically holothuroids, more closely. The occurrence of flagellar rootlets and composite mitochondria in various animal groups is discussed. The term paramorphy is proposed for parallel and convergent acquisition of an identical character: symparamorphy where acquisition is by parallelism and alloparamorphy where it is by convergence; the two terms represent, however, extremes of a continuum. Superficially similar but structurally different characters acquired by convergence are termed analogomorphis.     

A light and electron microscopical study of spermatogenesis in Hydra cauliculata

Morphological changes in the interstitial cells were studied during their differentiation into spermatozoa. Development of the spermatogonium involves an increase in nuclear and nucleolar size, and the formation of a dense mass of cytoplasmic ribosomes. The mature spermatozoon has a relatively simple structure. The head consists of a bullet shaped, homogeneous nucleus, which lacks an acrosome but bears distal membrane specializations. The middle piece is composed of four large spherical mitochondria at the base of nucleus. A single flagellum projects from one of the two centrioles lodged between the mitochondria. The flagellum appears early during development in the primary spermatocyte. During spermiogenesis microtubules associated with the basal body flagellum complex appear to define the axis of chromatin condensation.     

The ultrastructural organization of the mature spermatozoon of Luidia clathrata (Say) (Echinodermata: Asteroidea)

The sperm of Luidia clathrata are morphologically typical of asteroid sperm. The head is spherical and contains the nucleus and acrosomal complex. The nucleus has an anterior indentation in which rests the acrosomal complex. There is no evidence of a centriolar fossa along the posterior border of the nucleus. The acrosome is a cup-shaped structure containing a less electron dense central region. The periacrosomal material is homogeneous in nature, and the subacrosomal specialization of the periacrosomal materials appear as bands of varying electron density. The middle piece is an annular band of mitochondria which surrounds the proximal and distal centrioles. The centrioles exhibit the typical nine triplet arrangement. Both the centrioles and the axoneme project to one side of the middle piece region. Associated with the distal centriole is an elaborate pericentriolar process.     

Fine structure of spermatozoa in the gilthead sea bream (Sparus aurata Linnaeus, 1758) (Perciformes, Sparidae)

Scanning and transmission electron microscopy were used to investigate the fine structure of the sperm of the sparid fish Sparus aurata L. The mature spermatozoon of gilthead sea bream belongs, like that of the other sparid fish, to a "type I" as defined by Mattei (1970). It has a spherical head which lacks an acrosome, a short, irregularly-shaped midpiece and a long cylindrical tail. The nucleus reveals a deep invagination (nuclear fossa) in which the centriolar complex is located. The two centrioles are approximately perpendicular to each other and show a conventional "9+0" pattern. The proximal centriole is associated with a cross-striated cylindrical body lying inside a peculiar satellite nuclear notch which appears as a narrow invagination of the nuclear fossa. The distal centriole is attached to the nuclear envelope by means of a lateral plate and radial fibres made of an electron-dense material. The short midpiece houses one mitochondrion. The flagellum is inserted perpendicularly into the base of the nucleus and contains the conventional 9+2 axoneme.     

Sperm ultrastructure in the inseminating Macropsobrycon uruguayanae (Teleostei: Characidae: Cheirodontinae)

Macropsobrycon uruguayanae is a small, inseminating characid (tetra) of the tribe Compsurini. Although spermatozoa can be found within the ovarian cavity close to oocytes, the exact moment of fertilization has not yet been determined. Spermatozoa have moderately elongate nuclei with electron-dense chromatin. During spermiogenesis, nuclear rotation takes place. Elongate mitochondria with lamellar cristae are found posterior to the nucleus. Centrioles are parallel to one another with the proximal centriole slightly anterior to the longer distal one. The anterior tip of the proximal centriole is located within a shallow nuclear fossa. Electron-dense spurs are associated within the anterior and posterior ends of the distal centriole. Striated centriolar rootlets radiate both anteriorly and posteriorly from the distal centriole. Nine longitudinal accessory microtubules surround the axoneme in the proximal flagellum. The flagellum has a typical 9 + 2 axoneme with no intratubular differentiation. Atypical spermatozoa are also found in the testicular lumen. These cells resemble spermatozoa in most aspects, except that their nuclei are variable in shape, with the granular chromatin less electron-dense than that seen in spermatozoa. The origin and function of these cells could not be determined. The specializations seen in the spermatozoa are discussed as possible adaptations related to the habit of insemination.     

Ultrastructural studies of spermatozoa in three bivalve species with notes on evolution of elongated sperm nucleus in primitive spermatozoa

Sperm ultrastructure and spermiogenesis of the three bivalve species Musculus discors, Nucula sulcata, and Dreissena polymorpha have been studied. During spermatid differentiation in Musculus discors and Nucula sulcata the nucleus attains an elongated rod-like shape. The spermatozoon from Nucula sulcata was found to have a cup-shaped acrosome and five mitochondria surrounding two centrioles in the middle piece. The spermatozoa from Musculus discors has a long complex acrosome. From the distal centriole striated processes extend and attach to the plasma membrane. The spermatozoon of the fresh water species Dreissena polymorpha agrees in all main features with those of other invertebrate groups with external fertilization. It is thus of the primitive type with barrel-shaped nucleus and four to five mitochondria1 spheres in the middle piece. The acrosome is a prominant, complex structure at the apex of the mature spermatozoon. A comparison of sperm ultrastructure among bivalves indicates that there is a certain correlation between the evolution of the elongated sperm nucleus and large, yolk-rich eggs. In species with an elongated sperm nucleus the increased egg size has often led to a lecithotrophic or direct development. The elongated nucleus is a slight modification of the primitive type. There is a great variation in acrosome structure among bivalve spermatozoa, reflecting diverging functional demands at fertilization of the eggs.