Spermiogenesis and sperm structure in the crabUca tangeri (Crustacea,Brachyura), with special reference to the acrosome differentiation

Summary Early spermatids of the crabUca tangeri consists of the nucleus of granular chromatin and the cytoplasm, which contains a proacrosomal vesicle in close association with membrane lamellae. In the mid spermatids an invagination of the acrosomal vesicle membrane gives rise to the formation of the perforatorium, a spindle-shaped tubule which encloses tubular membranous structures. The pair of centrioles located at the base of the acrosome is not directly involved in perforatorial differentiation. The acrosomal vesicle shows a heterogeneous content composed of the operculum, the thickened ring, and three layers of different materials concentrically arranged around the perforatorium. During the late spermatid stage the nuclear profile differentiates numerous slender arms and the chromatin arranges into fibers. Membranous tubules from the cytoplasm become incorporated into the tubular structures of the perforatorium. The mature spermatozoon has the typical structure of the branchyuran sperm, with a complex acrosome, cupped by the nucleus, and a thin cytoplasmic band intervening between the former main elements. The centrioles are degenerate. The nuclear arms are unusually numerous (more than 20) and lack microtubules or microtubular derivatives.     

The Spermatozoon of Siboglinum (Pogonophora)

The spermatozoon and some spermatid stages of Siboglinum (Pogonophora) have been examined by light and electron microscopy. In the spermatozoon a helical acrosome, a helical nucleus and a “body” with axonema follow each other in normal sequence. Head and tail are joined by a very short neck region containing two modified centrioles. The posterior portion of the nucleus is surrounded by a mitochondrial sheath consisting of three tightly wound mitochondrial helices. In the main portion of the tail the 9+2 unit is sorrounded by a granular sheath of dense material. In the neck region a centriole adjunct develops into a dense substance containing about nine rods. At an early stage, when the centriolar apparatus and flagellum become associated with the nucleus, three large mitochondria with fairly regular cristae are seen at the base of the nucleus. A well developed Golgi apparatus is present in early stages. Rows of microtubules are observed encircling the spermatid nucleus. Compared with the primitive type of spermatozoon the pogonophore sperm shows elongated and specialized nucleus, acrosome and mitochondria. It is concluded that the ancestral form must have had a fairly primitive spermatozoon and that evolution has proceeded towards a modified sperm with complicated spiral structure in connection with the evolution of a modified biology of fertilization, viz. specialized spermatophores. It is not known how the spermatophore discharges the spermatozoa nor how the spermatozoa find their way to the eggs. Two kinds of sperms are produced in the gonads of Siboglinum. The atypical sperm is smaller than the typical one.     

Organization of the Acrosome and Helical Structures in Sperm of the Aplysiid, Aplysia kurodai (Gastropoda, Opisthobranchia)

Spermiogenesis in the aplysiid, Aplysia kurodai (Gastropoda, Opisthobranchia) was studied by transmission electron microscopy, with special attention to acrosome formation and the helical organization of the nucleus and the other sperm components. In the early spermatid, the periphery of the nucleus differentiates into three characteristics parts. The first part is that electron-dense deposits accumulate on the outer nuclear envelope. This part is destined to be the anterior side of the sperm because a tiny acrosome is organized on its mid-region at the succeeding stage of spermiogenesis. The second part, in which electron-dense material attaches closely to the inner side of the nuclear envelope, is the presumptive posterior side. A centriolar fossa is formed in this part and the axoneme of the flagellum extends from the fossa. A number of lamellar vesicles derived from mitochondria assemble around the axoneme and form the flagellum complex. The third part is recognized by the chromatin which condenses locally along the inner nuclear envelope. During development of the spermatid, this part extends to form a spiral nucleus accompanied by chromatin condensation and formation of microtubular lamellae outside the extending nucleus.
Finally, in the mature sperm, a tiny, spherical acrosomal vesicle is detected at the apex. The slender nucleus, overlapping both the primary and secondary helices which are composed of different structural elements, winds around the flagellum axoneme.     

斑节对虾精子发生的超微结构

斑节对虾精子发生划分为精原细胞、初级精母细胞、次级精母细胞、精子细胞和精子五个阶段。精子发生中，从精原细胞到精子，染色质经历了从以异染色质为主变为高度凝聚态，再经解聚为弥散絮状的变化过程。同时，核从具有完整核膜变为核膜不完整。成熟的的精子含有核仁。顶体由高尔基囊泡逐渐演化而成，并向外伸长成为棘突。这是斑节对虾精子发生的主要特征。     

Ultrastructural analysis of spermiogenesis in Admetus pomilio (Arachnida,amblypygi)

The mature spermatozoon of Admetus pomilio is a spherical cell containing nucleus and tightly coiled flagellum. In early spermatids the Golgi apparatus forms the acrosomal vesicle and at the opposite side the distal centriole gives rise to the axonemal complex of the sperm tail. As the nucleus elongates, chromatin forms twisted filaments and the spermatid nucleus takes on a helical form. Microtubules are juxtaposed with the nucleus envelope, which is separated from a central chromatin mass by an electron lucid region. A long perforatorium, located on the border of the chromatin mass, runs helically in the nucleus from the centriolar region to subacrosomal space. During tail elongation, the anterior part of the axoneme is surrounded by a long, spiral mitochondrial sheath. In the late spermatid, chromatin filaments appear twisted and become aggregated. The nucleus and flagellum undergo further contortions in which the nucleus coils and the flagellum winds up into the body of the cell and coils in a regular fashion. The mitochondrial sheath surrounds about 2/3 of the 9 + 3 axoneme. These features of spermatid ultrastructure resemble those in the primitive Liphistiomorpha.     

Spermatogenesis in a Free-Living Marine Nematode Neochromadora poecilosoma (Chromadorida,Chromadoridae) Studied Using TEM

Spermatogenesis and the structure of mature spermatozoa were studied using TEM in a free-living marine chromadorid nematode Neochromadora poecilosoma from the Sea of Japan. In spermatocytes, fibrous bodies (FB) develop; in spermatids, the synthetic apparatus lies in the residual body, while the nucleus, mitochondria, and FB are located in the main cell body (MCB). The nucleus consists of a diffuse chromatin of fibrous structure, which is not enclosed in a nuclear envelope. In the spermatid stage, the development of FB is completed, and immature spermatozoa from the proximal region of the testis do not show any structural differences from the MCB of spermatids. The mature spermatozoa are polarized cells. They attach to the uterus wall by a pseudopod filled with filaments of the cytoskeleton; in the MCB of spermatozoon, there is a nucleus surrounded by mitochondria and osmiophilic bodies. The spermatozoa of N. poecilosoma show typical ultrastructure features of sperm cells found in most studied nematodes (amoeboid nature and the absence of axoneme, acrosome, and nuclear envelope). However, no aberrant organelles characteristic of nematode spermatozoa were found throughout sperm development in N. poecilosoma and other chromadorids.     

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     

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     

白肛海地瓜精子发生及形态的超微结构

通过透射和扫描电镜观察了白肛海地瓜(Acaudina leucoprocta)的精子发生过程及其形态结构,揭示了白肛海地瓜精子发生时期一系列变化,其精子发生分为精原细胞、初级精母细胞、次级精母细胞、精细胞、成熟精子5个时期。精原细胞体积最大。精母细胞染色质开始凝集。精细胞前顶体颗粒形成。白肛海地瓜成熟精子的超微结构为原生型,由头部、中部、尾部组成,头部圆形,最前端为顶体,核染色质凝集成团块状,中部是线粒体和中心粒复合体融合成1个超大结构,尾部长约60μm,尾部鞭毛横切面为典型的"9+2"型结构。     

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.     

中国绿螂精子的超微结构

应用透射电镜技术观察了中国绿螂(Glaucomya chinensis)精子的超微结构。精子为典型的原生型,包括头部、中段和尾部三部分。头部由顶体和细胞核组成。顶体呈倒"V"字型。细胞核呈长圆柱形,没有核前窝,具有核后窝。中段由4个线粒体环绕中心粒而成。尾部细长,为典型的"9+2"结构。文中还讨论了双壳类精子形态结构的种属间差异。     

Is the sperm type of the Nemertodermatida close to that of the ancestral Platyhelminthes?

Results from a transmission electron microscope study of the spermiogenesis and spermatozoon of Meara stichopi (Nemertodermatida, Platyhelminthes) indicate that the sperm type of the Nemertodermatida has evolved from the primitive metazoan sperm type rather than from an aberrant biflagellar sperm type as found in many other flatworms. The spirally coiled mitochondrial derivative in the mature spermatozoon develops from two large oval mitochondria in the early spermatid stages. A single flagellum grows out from a peripheral basal body adjacent to a perpendicularly placed accessory centriole. The basal body moves to a distal depression of the nucleus, and becomes equipped with an anchoring fibre apparatus. Most of the flagellum becomes axially incorporated into the developing spermatid. No trace of a second flagellum was found in any stage of the spermiogenesis. Rounded vesicles appear around the proximal, tapering end of the elongating nucleus. Most probably these vesicles form a thin acrosomal structure in the mature spermatozoon. No dense bodies, characteristic of many other ‘turbellarian’ flatworm sperm types, were found. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

平疣桑椹石磺精子结构观察

通过电光学显微镜和透射电子显微镜观察了平疣桑椹石磺精子的形态及其超微结构。平疣桑椹石磺成熟精子属于进化型,由头部、中段和末段组成。头部由顶体和精核构成,顶体长约0.7μm,呈细奶嘴状,内含物分布均匀,电子密度稍低于细胞核。顶体基部与精核前端紧密相连,无间隙。精核长约3.8μm,宽约1.0μm,核质高度浓缩,电子密度高,无核泡,纵切似辣椒状,核后端内凹形成核后窝。中段加长,结构复杂,线粒体演化成线粒体鞘,螺旋状包绕轴丝。精子末段由轴丝及包绕轴丝的质膜组成,轴丝为典型的“9＋2”结构。比较了平疣桑椹石磺精子与相关腹足类精子结构的异同,进一步证实了腹足纲贝类精子结构之间的区别主要在于顶体有无及形态,精核的长短与外形、中段线粒体的数目及其排列方式等。     

Ultrastructural Study of Spermatogenesis and Mature Spermatozoa of Paravortex cardii (Platyhelminthes, Dalyellioida)

The spermatogenesis and mature spermatozoon of Paravortex cardii were studied by transmission electron microscopy. Meiotic divisions occur without cytokinesis and the spermatid nuclei appear embedded in a common cytoplasmic mass. The mature spermatozoon is filiform, very regular in contour and circular in cross-section. A tubular lining of microtubules lying close to the plasma membrane is found along the spermatozoon. Rows of spherical glycogen particles with helical arrangement lay internal to the cortical microtubules. The spermatozoon of P. cardii may be divided into two regions, nuclear and cytoplasmic regions. The nuclear region contains an elongated nucleus with a densely packed nuclear material. The mitochondria are distributed throughout the cytoplasmic region; they pack tightly together and often fuse to form one large one. This spermatozoon lacks both acrosome and the so-called dense bodies. A ciliary or centriolar apparatus was not observed. Accordingly, the spermatozoon of P. cardii is considered to be aflagellate in type. Spermatozoa are compared among flatworms, and some considerations on the significance of their ultrastructure for phylogeny in the Platyhelminthes are tentatively given.     

SPERMIOGENESIS IN THE PULMONATE SNAIL, EUHADRA HICKONIS II. STRUCTURAL CHANGES OF THE NUCLEUS

In accordance with the characteristic shape of the nucleus and degree of condensation of the nuclear substance, spermiogenesis in Euhadra hickonis can be roughly divided into four stages. The chromatin in the highly polymorphic nucleus of the first stage, early spermatid, forms relatively thick (ca. 50 nm) fibrils which associate here and there into irregular clumps. In the next stage, the spermatid nucleus becomes conspicuously spherical, its contents appear more finely homogeneous and the irregular clumps of chromatin are few. In the third stage, the nucleus gradually takes on an ellipsoidal shape as the antero-posterior axis shortens. The anterior part of its envelope becomes structurally modified in preparation for the adherence to it of the developing acrosome, and an implantation fossa forms posteriorly at the center of a second area where the nuclear envelope has been modified. The diameter of the chromatin fibrils again increases and those near the implantation fossa become oriented perpendicular to the nuclear envelope.
As the nucleus elongates in the fourth stage, a concentric sheath of microtubules closely surrounds it. These appear to depolymerize as the nuclear elongation proceeds, so that they are no longer present in the head region of the mature spermatozoon. The diameter of the chromatin fibrils increases to about 10 nm and they become oriented parallel to the long axis of the cell. With the decrease in the nuclear volume the fibrils unite laterally to form longitudinal sheets, and these finally merge in the mature spermatozoon into a mass of very dense chromatin without perceptible internal structure.     

Fine structure of the germinating cells during the spermiogenesis of Arion rufus (Mollusca,Pulmonate Gastropoda)

Changes in spermatozoan ultrastructure have been studied during spermiogenesis of the slug Arion rufus (Gastropoda, Pulmonata, Stylommatophora). The ovotestis was investigated during the male stage, definite by the presence of spermatozoa. Some peculiar characteristics are shown by early spermatids: Around the nucleus, the nuclear envelope presents two thick layers located on opposite sides, the apical and basal plates, that will determine the antero-posterior axis of the spermatid. The chromatin, first dispersed throughout the nucleoplasm gives later on thick filaments which become attached over the inner surface of these plates. The chromatin filaments are then arranged parallel to the antero-posterior axis as the nucleus elongates. The position of the plates determines the antero-posterior axis of the spermatid. In the mature spermatozoa, the chromatin is more condensed and the nucleus presents an helical organization. The acrosome and flagellum are respectively attached externally to the center of the apical and basal plates. The acrosome consists of a membrane-bound vesicle and forms a column of homogeneous material. In the middle piece, the mitochondria have been transformed into a mitochondrial derivate by the way of a complicated metamorphosis. The axoneme is surrounded by three mitochondrial helices but only one of them contains glycogene granules. © 1996 Wiley-Liss, Inc.     

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.     

Transition of basic protein during spermatogenesis of <Emphasis Type="Italic">Fenneropenaeus chinensis</Emphasis> (Osbeck, 1765)

According to the ultrastructural characteristic observation of the developing male germ cells, spermatogenesis of the crustacean shrimp, Fenneropenaeus chinensis, is classified into spermatogonia, primary spermatocytes, secondary spermatocyte, four stages of spermatids, and mature sperm. The basic protein transition during its spermatogenesis is studied by transmission electron microscopy of ammoniacal silver reaction and immunoelectron microscopical distribution of acetylated histone H4. The results show that basic protein synthesized in cytoplasm of spermatogonia is transferred into the nucleus with deposition on new duplicated DNA. In the spermatocyte stage, some nuclear basic protein combined with RNP is transferred into the cytoplasm and is involved in forming the cytoplasmic vesicle clumps. In the early spermatid, most of the basic protein synthesized in the new spermatid cytoplasm is transferred into the nucleus, and the chromatin condensed gradually, and the rest is shifted into the pre-acrosomal vacuole. In the middle spermatid, the nuclear basic protein linked with DNA is acetylated and transferred into the proacrosomal vacuole and assembled into the acrosomal blastema. At the late spermatid, almost all of the basic protein in the nucleus has been removed into the acrosome. During the stage from late spermatid to mature sperm, some de novo basic proteins synthesized in the cytoplasm belt transfer into the nucleus without a membrane and almost all deposit in the periphery to form a supercoating. The remnant histone H4 accompanied by chromatin fibers is acetylated in the center of the nucleus, leading to relaxed DNA and activated genes making the nucleus non-condensed.     

Sperm morphology of the Eurasian beaver, Castor fiber: an example of a species of rodent with highly derived and pleiomorphic sperm populations

The structural organization of the spermatozoon from the Eurasian beaver, Castor fiber (Family: Castoridae), was determined and compared to that of other sciuromorph rodents. The beaver spermatozoon has a head, which is variable in form but usually paddle-shaped, with a small nucleus and very large acrosome, and a tail that is relatively short compared to that of most other rodents. Transmission electron microscopy indicates that in most testicular spermatozoa the acrosome projects apically, although in a few it becomes partly flexed. During the final stages of maturation, however, the acrosome becomes highly folded so that the apical segment comes to lie alongside part of the acrosome that occurs lateral to the nucleus, with, in some cases, fusion taking place between the outer acrosomal membranes. The sperm nucleus is wedge-shaped, being broader basally and narrowing apically with an occasional large nuclear vacuole occurring. This spermatozoon structure is markedly different from that found in the other species of Geomyoidea, which is the sister group of the Castoridae. The findings thus emphasize the highly divergent nature of the beaver spermatozoon and demonstrate that, within the proposed Infraorder Castorimorpha, very large differences in sperm structure have evolved.