Ultrastructure of spermatids and spermatozoa in three polychaetes with modified biology of reproduction: Autolytus sp., Chitinopoma serrula,and Capitella capitata

Unlike the primitive type of spermatozoon found in most polychaetes, the spermatozoon of Autolytus has a bilateral symmetry with elongated nucleus, and the mitochondria surround the posterior part of the nucleus. A rather large disk-shaped acrosome is situated along one side of the anterior part of the nucleus. From the anterior margin of the distal centriole emerge long striated rootlets, which run along the nuclear envelope to the anterior part of the nucleus. The spermatozoon of Chitinopoma serrula has an elongated, slightly bent nucleus, a thimble-like acrosome apically on the anterior surface of the nucleus, and an elongated middle piece containing 4 rod-like mitochondria developed from spherical mitochondria surrounding the basal part of the tail flagellum. In the spermatozoon of Capitella capitata, both nucleus and middle piece are elongated compared to the primitive type. The large and conical acrosome is placed asymmetrically at the nucleus and consists of an acrosomal vesicle and subacrosomal substance. The greater part of the middle piece forms a collar around the initial part of the tail flagellum. The cytoplasm of the collar contains granular material. One or two small mitochondria lie around the 2 centrioles at the base of the nucleus.

These types of spermatozoa represent early steps in the evolution of modified spermatozoa combined with changed biology of reproduction. The modified spermatozoa are larger than the primitive ones.     

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。     

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。     

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.     

Ultrastructure of spermatids and spermatozoa in the cyclostomatous bryozoan Tubulipora (Bryozoa,Cyclostomata)

Summary Differentiation of spermatids to mature spermatozoa in the bryozoan Tubulipora liliacea was studied by transmission electron microscopy. The spermatozoon of Tubulipora is of a filiform, modified type, and has evolved from the primitive type as an adaptation to a specialized biology of fertilization. The head of the spermatozoon consists of a small, conical acrosome capping an elongated, cylindrical, anteriorly tapering nucleus. A basal invagination in the nucleus contains the proximal portion of the axoneme and a dense attachment matrix. The flagellar axoneme has the typical 9+2 structure. Four elongated rodshaped mitochondria with typical cristae surround the axoneme in the cylindrical middle piece. Granular electron-dense material is accumulated in the form of four columns alternating with four long cylindrical mitochondria. The mitochondrial middle piece is separated externally from the tail region by an involution of the plasma membrane. The tail region contains a cytoplasmic sheath with accessory fibers surrounding the axoneme. Nine outer, coarse fibers extend posteriorly paralleling the nine doublets of the axoneme. The coarse fibers develop from electron-dense plate-like structures associated with the doublets of the axoneme. A characteristic feature in spermiogenesis is that spermatozoa develop in tetrads. There seem to be significant differences in spermatozoan ultrastructure between the three bryozoan classes Stenolaemata, Gymnolaemata, and Phylactolaemata. The differences indicate different lines of evolution of fertilization biology in these groups.Abbreviations used in the figures a acrosome - av acrosomal vesicles - ax axoneme - c coarse fiber - d electron dense rod - m mitochondrion - mp middle piece - Scale bars=0.5 m - mt microtubule - n nucleus - ne nuclear envelope - p nuclear protrusion - pm plasma membrane - t tail     

Ultrastructure of the spermatozoon of Lepidogalaxias salamandroides and its phylogenetic significance

The spermatozoon of Lepidogalaxias salamandroides possesses an acrosome (putative), one or two perforatoria (putative) but no nine-triplet centrioles. Two elongated mitochondria (12 μm long) are situated in parallel between the nucleus (20 μm long) and the axoneme (53 μm long). The above features are unique among other teleosts with internal fertilization. The presence of an “acrosome” in this primitive teleost supports the hypothesis that this structure has been secondarily lost in teleosts during evolution. The uncertainty of phylogenetic placement of this fish is reflected by its unique sperm ultrastructure.     

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     

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.     

Acrosome reaction in spermatozoa of the hagfish Eptatretus burgeri (Agnatha)

Fertilization of the hagfish or myxiniformes, a member of the most primitive vertebrate group and an animal of phylogenic interest, is unknown. Here, induction of an acrosome reaction for spermatozoa in the hagfish, Eptatretus burgeri, was successfully achieved by treatment of mature spermatozoa with ionomycin and excess Ca2+. The spermatozoon produced an acrosomal process that elongated from the apex of the long sperm head. The reaction bears resemblance to that of invertebrate spermatozoa rather than that of vertebrate spermatozoa. The result provides insights into the phylogenetical changes that have occurred in this sperm reaction.     

Ultrastructure and morphology of spermatozoa in Chinese sturgeon (Acipenser sinensis Gray 1835) using scanning and transmission electron microscopy

The Chinese sturgeon (Acipenser sinensis Gray 1835) is an endangered anadromous sturgeon inhabiting the Yangtze River in China. In this study, the ultrastructure and morphology of spermatozoa was studied using transmission and scanning electron microscopy with a cryo-holder. The spermatozoon consisted of an elongated head with a distinct acrosome and nucleus region, a midpiece and a flagellum. The mean length of the head and midpiece, the flagellum and total length of spermatozoon were 4.48, 33.3 and 37.8 microm, respectively. The nucleus was an elongated trapezoid shape with anterior (acrosome) end narrower than the posterior. Granular material and an actin filament were observed within the anterior acrosome. Three to five endonuclear canals were present. The midpiece was eudipleural along its longitudinal axis. Compared to other sturgeon species, the data from the present study suggest a more recent evolutionary linkage between Chinese sturgeon and white sturgeon (Acipenser transmontanus Richardson 1836).     

Spermiogenesis and modified sperm morphology in the "seepworm" Methanoaricia dendrobranchiata (Polychaeta: Orbiniidae) from a methane seep environment in the Gulf of Mexico: implications for fertilization biology

Spermatogenesis and mature sperm morphology have been described along with limited observations of the ovary in Methanoaricia dendrobranchiata, an orbiniid polychaete associated with dense populations of the mussel Bathymodiolus childressi at brine pools on the Louisiana slope, Gulf of Mexico. The species is gonochoric with gonads serially repeated in numerous segments and each associated with a nexus of blood vessels at the base of the parapodia. In the female, synchronous, intraovarian egg development occurs with the release from the ovary of large, yolky eggs into the coelom at first meiotic metaphase. Sperm develop in the coelom as free-floating, plasmodial clones interconnected via an anuclear cytophore. At the end of spermiogenesis, mature spermatozoa float freely in the coelom. The mature spermatozoon differs significantly from that of shallow-water orbiniid species by possessing an elongated nucleus and a greatly elongated and curved acrosome reaching 19.5 microm in length. The spermatozoon resembles an ent-aquasperm and may not fertilize the eggs directly in seawater in the classical manner. We hypothesize that the unusual spermatozoon morphology in this species has evolved due to the hypoxic environment in which the adults live and that fertilization biology is likely modified in some way to minimize sperm exposure to high levels of hydrogen sulfide. An analysis of life-history features in shallow-water orbiniids is used to infer reproductive features in M. dendrobranchiata that could not be directly documented.     

北草蜥精子的超微结构--兼评不同类群蜥蜴精子形态的差异

应用透射电镜对北草蜥精子的超微结构研究结果表明,北草蜥精子头部顶体囊始终呈圆形,由皮质和髓质组成;顶体囊单侧脊的皮质与髓质问具电子透亮区;穿孔器1个,无穿孔器基板;具顶体下腔;细胞核长形,核内小管缺,核前电子透亮区缺,核肩圆。尾部颈段具片层结构。中段短,多层膜结构缺;纵切面上具2层线粒体;横切面上每圈线粒体6个;2组致密体,具连续的环状结构;线粒体与环状结构的排列模式：rs1／mi1、rs2／mi2;纤维鞘伸人中段,具终环。主段前面部分具薄的细胞质颗粒区;纤维3和8至主段前端消失;轴丝呈“9＋2”型。蜥蜴科内不同种类的线粒体数目不同,但都具有2组致密体。不同类群蜥蜴的顶体囊、顶体下腔、核前电子透亮区、穿孔器基板、核肩,以及线粒体与致密体的数目和排列方式等精子超微结构特征都为研究蜥蜴的系统发生提供了辅助信息。     

External calcium ions are requisite for fertilization of sea urchin eggs by spermatozoa with reacted acrosomes

That a small amount of external calcium ions is requisite for the fertilization by spermatozoa with reacted acrosomes was found by some simple experiments using jelly-treated sperm of the sea urchin, Hemicentrotus pulcherrimus. When eggs were inseminated with the jelly-treated sperm in artificial seawaters containing calcium at various concentrations, the percentage of fertilization decreased concomitant with the reduction in the amount of external calcium ions, 50% at 40 μM calcium and almost 0% at less than 10 μM. On the other hand, it was observed that both the morphology of the reacted acrosome and the binding capacity of the jelly-treated spermatozoa to eggs were not influenced by the calcium deficiency. These results suggest that external calcium ions are indispensable even for the fertilization processes following sperm binding to eggs after the acrosome reaction, such as penetration of reacted spermatozoa through vitelline layer and/or membrane fusion between egg and spermatozoon.     

Ultrastructure of Sperm in <Emphasis Type="Italic">Mercenaria stimpsoni</Emphasis> and <Emphasis Type="Italic">Mactra chinensis</Emphasis> (Mollusca: Bivalvia) from the Sea of Japan

The ultrastructure of the sperm of the common bivalve species Mercenaria stimpsoni and Mactra chinensis from Peter the Great Bay is described. The sperm structure is typical for animals with external insemination. The sperm consists of a head, middle part, and flagellum. The sperm head of M. stimpsoni has a curved crescent form and includes the nucleus and acrosome; the head length is 9.8 μm. The acrosome is subdivided to the acrosome granule and the periacrosomal material. There are 4 mitochondria of about 0.8 μm in size in the middle part of the spermatozoon. The mitochondria surround the centriolar apparatus, which consists of proximal and distal centrioles located at a right angle. The axoneme originates from the distal centriole. The sperm of M. chinensis is barrel-shaped, with a head length of 3.2 μm. The acrosome is relatively larger, and its height is 1–1.2 μm. There are also 4 mitochondria 0.6–0.8 μm in the middle part of the spermatozoon. The sperm structure of the described species is typical of the families to which the mollusks belong, with insignificant variations.     

An Ultrastructural Study of the Spermatozoa from Prionospio cf. queenslandica and Tripolydora sp.: Two Spionid Polychaetes with Different Reproductive Methods

The fine structure of the spermatozoa of two spionids is described. The spermatozoon of Prionospio cf. queenslandica is typical of an animal utilizing external fertilization, in having a subspheroidal nucleus, a midpiece composed of unmodified rounded mitochondria surrounding two centrioles and a free flagellum. The acrosome is unusual in showing bilateral symmetry. The spermatozoon of Tripolydora sp. resembles that of spionids utilizing spermatophores, in possessing an extremely elongate nucleus and midpiece. The nucleus is penetrated by the 9+2 axoneme for its entire length, linking with a single centriole at the anterior end. Platelets surround the nucleus and intermingle with the mitochondria of the midpiece, which terminates with an annulus. The acrosome shows some internal vesiculation and substructuring. Sperm structure in relation to reproductive methods is discussed and the view of external fertilization as primitive is questioned.     

THE SPERMATOZOON OF ARTHROPODA: ZOOTERMOPSIS NEVADENSIS AND ISOPTERAN SPERM PHYLOGENY

In this paper, the ultrastructure of the spermatozoon of Zootermopsis nevadensis (Isoptera, Hodotermitidae) and of some Rhinotermitidae and Termitidae is described. Zootermopsis sperm is rod like, aflagellate, immotile, and without an acrosome; it is composed of a filiform nucleus encircled by a monolayered microtubular manchette, and a few mitochondria. This spermatozoon was previously thought to be flagellate, and therefore the most primitive in Isoptera: our present study suggests a new phylogenetical position for Hodotermitidae. All the species of Rhinotermitidae and Termitidae studied by us show a similar spheroidal sperm model, devoid of acrosome, flagellum and manchette at spermatid stage, and are made up of only a round nucleus, two mitochondria and a centriole. This widely distributed model seems to be the more evolved in the order. The nature of sperm evolution in the Isoptera is considered.     

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.     

Spermiogenesis and spermatozoa ultrastructure of Aonides oxycephala (Annelida: Spionidae) from the Sea of Japan

Adults of Aonides oxycephala, common inhabitants of shallow boreal waters in the Atlantic and Pacific Oceans, release gametes into the water where fertilization and lecithotrophic larval development occur. During spermiogenesis, the acrosomal vesicle migrates from the posterior to the anterior end of the spermatid and the number of mitochondria reduces from six in early spermatids to four in mature spermatozoa. Each spermatozoon has an ovoid head with the acrosome 1.4?±?0.1?µm long and 1.6?±?0.1?µm wide and the nucleus 1.7?±?0.1?µm long and 2.3?±?0.1?µm in diameter, four spherical mitochondria, two centrioles oriented perpendicular to each other, putative glycogen in the shape of dense granules in the midpiece, and a flagellum with 9?×?2?+?2 organization of microtubules. The acrosome is a complex heterogeneous structure with five ordered layers of different electron densities, lying in a shallow depression on the anterior end of the nucleus. The nucleus is barrel-shaped (truncated ovoid) with the centriolar fossa housing the distal and proximal centrioles. Spermiogenesis and ultrastructure of spermatozoa of A. oxycephala are similar to those of another free spawning spionid, Marenzelleria viridis. Aonides and Marenzelleria have not, however, been considered as closely related taxa; thus, similarity in the morphology of their sperm might result from convergence or parallelism.