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.     

Ultrastructure of Spermatozoa and Spermatids in Bonellia viridis and Hamingia arctica (Echiura) With Some Phylogenetic Considerations

Spermatozoa of the echiurans Bonellia viridis and Hamingia arctica show a similar ultrastructure. They are of a modified type. The head consists of a roughly cylindrical nucleus, which has a cover of electron-dense material. The acrosome is very large and consists of an acrosomal vesicle and a rod-shaped perforatorium or acrosomal rod. In close association with the nucleus, one or two mitochondria are found forming an irregular ring around the posterior tip of the nucleus and the centriolar apparatus. There are two centrioles, the proximal one with the conventional triplet microtubular structure. The tail flagellum is about 50 μm long and has the 9+2 axonemal structure. The oblique attachment of the acrosome to the anterior part of the nucleus gives the spermatozoon a bilateral symmetry. However, in the nuclear morphology, the arrangement of electron-dense material around the nucleus, in the mitochondria, and in the attachment of the tail flagellum, the spermatozoon shows asymmetric organization. The sperm structure in bonelliids is unique but its genesis and the morphology of the mitochondrial midpiece support the theory that the echiurans are related to the annelids. The main results of the study are summarized in Fig. 11.     

Interspecific Variation in Structural Organisation of the Spermatozoon in the Asian Bandicoot Rats,Bandicota Species (family Muridae)

The structural organisation of the spermatozoon from two species of bandicoot rats Bandicota bengalensis and Bandicota indica was investigated by light and electron microscopy together with the effect of incubation in Triton-X 100 and sodium dodecyl sulphate. The sperm head of B. bengalensis is invariably falciform, has a uniform electron-dense nucleus capped by an acrosome with a posteriolateral equatorial segment, a subacrosomal cytoskeleton with a large rostral perforatorium, and a sperm tail, attached to the lower concave surface of the sperm head, with typical coarse fibres and fibrous sheath. By contrast, the sperm head shapes of B. indica are generally conical or bulbous, the nucleus contains a few large vacuoles, the acrosome lacks an equatorial segment, no recognisable perforatorium occurs, and the sperm tail, which is attached basally, is very short with only modest development of coarse fibres and fibrous sheath. These results indicate that, within the genus Bandicota, huge interspecific differences in morphology of the spermatozoon have evolved. The spermatozoa of B. bengalensis are similar to those of Rattus and many other murids and thus presumably represent the ancestral condition, whereas those of B. indica (and B. savilei) are unlike spermatozoa from any other eutherian mammal so far described. © 1998 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd. All rights reserved     

A comparative study on the fine structure of developing spermatozoa in the isopod,Oniscus asellus,and the amphipod,Orchestoidea sp.

Summary Developing spermatids and mature spermatozoa from the isopod, Oniscus asellus and the amphipod, Orchestoidea sp. have been examined with the light microscope and the electron microscope and have been found to have similar morphologies. As spermiogenesis proceeds the nucleus migrates to one pole of the spermatid at which point an acrosome, contiguous rod, and cross-striated tail develop. The acrosomal vesicle elongates to a cone-shaped, mature acrosome lying at the apex of a cross-striated tail and nucleus which are situated at approximate forty-five degrees to each other. The cross-striated tail originates as an evagination of the spermatid plasma membrane near the acrosomal vesicle. The tail eventually grows to lengths of four to five hundred microns. The mature, tail-like appendage is cross-striated at major 750 to 800 Å, and minor 125 to 150 Å, periodicities. When observed in vitro, mature sperm of both species appear non-motile.Possible homologies of this unusual spermatozoon with other types of spermatozoa are made and it is concluded that: 1) isopod and amphipod spermatozoa should be classified as non-flagellate; 2) the cross-striated tail, previously thought to be a flagellum, is a non-motile structure associated in development and possible function with the acrosome; and 3) the rodlike structure contiguous with the acrosome is similar to perforatoria described in some vertebrate sperm.Supported by U.S.P.H.S. Grant No. NB-06285 and Training Grant No. 5-Tl-GM-202. — The author wishes to express his grateful appreciation for the technical assistance given by Miss Ann Barnett during the course of this investigation.     

Ultrastructure of the spermatozoon of Megaselia scalaris Loew (Diptera:Brachycera:Cyclorrhapha:Phoridea:Phoridae)

When viewed by scanning electron microscopy (SEM), the spermatozoon of the phorid dipteran Megaselia scalaris appears threadlike, lacking distinct head and tail areas. These areas can be observed, however, in appropriately stained material. Measurements of Feulgen-stained material reveal average lengths of the head, tail, and total cell of 18.7, 128.7, and 147.4 μm, respectively. When tested for sulfhydryl and disulfide groups, the head displays only disulfide groups. Transmission electron microscopy (TEM) reveals 12 different regions: three (1–3) in the head, four (9–12) in the tail, and five (4–8) in a short zone of overlap between the head and tail. Most of the cell lies in regions 9 and 11 of the tail and 3 of the head, accounting for, respectively, 37.3%, 45.7%, and 11.2% of the total length. A tubelike acrosome indents the anterior end of the nucleus. The tail originates asymmetrically in relation to the long axis of the cell as a peglike structure associated with the dorsolateral region of the nucleus. No centriole is visible, and the nucleus has a notched appearance in longitudinal sections. Two mitochondrial derivatives and an axoneme displaying a 9+9+2 microtubule configuration and ATPase activity extend throughout most of the tail. In regions 9 and 10, an asymmetrically arranged accessory body is also present. Features having possible taxonomic utility include the asymmetrically arranged accessory body, the size and shape of the acrosome, and the notched appearance of the nucleus. The present report is apparently the first to describe the spermatozoon of a cyclorrhaphous dipteran which is not a member of the Schizophora.     

Die Spermatozoen der Zecke Ornithodorus moubata (Murr)

Zusammenfassung Die reifen, den Weibchen entnommenen Spermatozoen von Ornithodorus moubata bestehen aus einem kolbig verdickten Vorderende und einem längeren, dünneren schwanzartigen Hinterende, in dem der Kern und das Akrosom sich befinden. Den Kern durchbohrt ein Stab, der mit einem Konus an der Akrosomplatte befestigt ist. Die Akrosomplatte ist eine Verdickung der Akrosomlamelle. Diese ist größtenteils als Akrosomkanal in das schwanzartige Ende eingestülpt. Zwischen Kern und Akrosomplatte bzw. -lamelle liegen 2 Zentriolen. Die Oberfläche ist mit Fortsätzen bestanden, die im Bereich der Spitze (Apex) pilzförmig, auf dem übrigen Spermatozoon leistenartig sind. Die Struktur der Fortsätze wird analysiert. Sie sind durch feine Füße mit dem Spermatozoon in Verbindung. Diese Strukturen stellen zytologische Differenzierungen dar, für die es bisher in der vergleichenden Zytologie keine Entsprechung gibt.

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The spermatozoa of the tick Ornithodorus moubata (Murr)

Summary Mature spermatozoa of Ornithodorus moubata removed from females consist of a thickened club shaped anterior part and a longer and thinner posterior part formed like a tail. The latter contains the nucleus and the acrosome. A rod perforates the nucleus and is attached to the acrosomal plate by a conical structure. The acrosomal plate is a thickening of the acrosomal lamella, which is mostly invaginated into the posterior part of the spermatozoon as an acrosomal channel. Between nucleus and acrosomal plate or acrosomal lamella two centrioles are located. The surface is occupied with processes which are fungiform at the apex and formed like strips at the main part of the spermatozoon. The structure of these processes has been analysed. They are connected to the spermatozoon by fine feet. These processes are cytological differentiations, which so far are unknown in comparative cytology.

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Herrn Prof. Dr. K. Goerttler zum 70. Geburtstag in Verehrung gewidmet.

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Mit Unterstützung durch die Deutsche Forschungsgemeinschaft.     

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.     

MORPHOLOGICAL OBSERVATIONS ON THE SPERMATOZOA OF ECHINOTHURID SEA URCHINS*

The morphology of the spermatozoa of three species of echinothurid sea urchins, Asthenosoma ijimai, Araeosoma owstoni, Hapalosoma gemmiferum, was investigated by means of transmission and scanning electron microscopy. The spermatozoa of these three species of echinothurid sea urchins have similar fine structure, but they differ in several features from the more familiar regular sea urchins. 1) The external anatomy of the head region of the echinothurid spermatozoon is diagnostic in that it has a highly elongated head. 2) The spermatozoon of echinothurid sea urchins has a very long slender nucleus, protruding on its proximal end, so that the shape of the nucleus resembles a sperhead. 3) The acrosomal granule in the acrosomal vesicle of the echinothurid spermatozoon is not a mass of homogenous particulate material but an electron opaque rod condensed in the central part of the acrosomal vesicle. Scanning electron microscopic examination revealed that echinothurid spermatozoa form acrosomal processes similar to those of other regular sea urchins. 4) The basal body is situated just beneath the middle of the posterior protrusion of the nucleus. The distal centriole is located beside the basal body almost in contact with it. The axis of the distal centriole is almost but not quite parallel to that of the basal body. A satellite complex can be recognized around the posterior part of the proximal centriole.     

黄姑鱼（ Nibea albiflora） 与大黄鱼（ Pseudosciaena crocea） 精子超微结构的观察与比较

应用扫描电镜（SEM）与透射电镜（TEM）观察了黄姑鱼和大黄鱼精子的超微结构。结果显示,黄姑鱼和大黄鱼精子无论在形态、大小还是超微结构上都十分相似。黄姑鱼和大黄鱼精子均由头部、中段和尾部（鞭毛）3部分组成。精子头部形状近似椭圆形,无顶体,细胞核呈肾形。中心粒复合体位于细胞核背侧,近、远端中心粒相互垂直,远端中心粒分化成基体并形成轴丝。中段的袖套呈筒状,4～5个圆形的线粒体围绕轴丝呈环形排列。精子尾部为单鞭毛,轴丝为典型“9＋2”结构,鞭毛表面质膜形成不规则侧鳍。     

Interspecific variation of sperm morphology in the Australian rodent genus Zyzomys

The sperm head morphology and tail length of two species of Australian rock rats, Zyzomys argurus and Zyzomys pedunculatus, are presented. In Z. argurus the sperm head has an apical hook together with two ventral processes extending from the upper concave surface that are largely composed of cytoskeletal material, and the sperm tail is about 135 µm in length. By contrast, in Z. pedunculatus the sperm head is paddle‐shaped with the nucleus capped by an acrosome that has a large apical segment and is surrounded by a thin layer of cytoskeletal material, and the sperm tail is only around 85 µm in length. Since the structure of the spermatozoon of Z. argurus is similar to that of most of the old endemic Australian rodents it is presumed to be the ancestral condition within the Zyzomys genus with that of Z. pedunculatus being highly derived and showing convergence with the sperm structure in some other orders of mammals.     

Spermiogenesis and ultrastructure of spermatozoa in Saxipendium coronatum (Hemichordata,Enteropneusta), with consideration of their relation to reproduction and dispersal

Summary Sperm ultrastructure and spermiogenesis of the enteropneust hemichordate Saxipendium coronatum conforms to the general pattern of the prototype spermatozoon found in many phyla. The sperm is about 29 m long, including head, middle piece, and tail. The Saxipendium spermatozoon has some unique features. The head is pyramidal in shape and the nucleus has four frontal ridges radiating from the base of the acrosomal region. The acrosome is composed of a large acrosomal vesicle surrounded by periacrosomal material. The acrosomal region projects about 1 m in front of the nucleus and has a width at the base of 1.5 m. The middle piece is dish-shaped and contains a large mitochondrial mass surrounding the centriolar region. The centriolar region is partially located in a centriolar fossa at the basal part of the nucleus. In spermatids, an anchoring fiber apparatus is observed surrounding the centriolar region. The distal ends of the fibers are attached to the plasmalemma by electron-dense thickenings. The tail is a simple flagellum. The sperm of Saxipendium and the small eggs found in the female suggest non-specialized external fertilization and embryogeny leading to a planktotrophic larva. The main results of the fine structure of the spermatozoon in Saxipendium are summarized in Fig. 12.Abbreviations used in the figures an antrum - av acrosomal vesicle - ax axoneme - d distal centriole - ep epidermis - f flagellum - gp gonopore - m mitochondrion - mp middle piece - n nucleus - p proximal centriole - per periacrosomal material - sp sperm - te testis - vac vacuolated cells     

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.     

Ultrastructural investigation of spermiogenesis in Peripatopsis capensis (Onychophora)

Early spermatids of the onychophoran Peripatopsis capensis are spherical cells with a centrally located nucleus, numerous mitochondria, Golgi complexes, microtubules and two centrioles. During spermiogenesis, Golgi vesicles migrate to one side of the cell where they form a tight aggregate, which is later shed. The mature spermatozoon has no acrosome. Several mitochondria fuse to form a middle piece containing three large mitochondria. Nucleus and middle-piece elongate, presumably under the influence of helically twisted microtubules. Outside this set of microtubules a continuous layer of endoplasmic reticulum cisternae is formed which separates the interior portion of the cell from an external cytoplasmic rim, which is later shed. Outside the 9 + 2 complex, the tail presents nine accessory microtubules, and a peripheral layer of microtubules beneath the plasma membrane. The enforcement of the tail structure may be related to the fertilization biology of this animal, which is by “hypodermal” impregnation.     

Ultrastructure of spermatozoa of Peregrinus maidis (Homoptera,Delphacidae)

Summary The spermatozoa of Peregrinus maidis Ashm. are thread-like, approximately 650 long and 1 wide including the head (approximately 28 ).The main part of the spermatozoa consists of two mitochondria derivatives, a central body between them, the axial filament complex, and a newly found element consisting of two wing-shaped bodies. Each mitochondrion derivative shows a peripheral and an inner part. The peripheral part is formed by cristae arranged perpendicularly to the long axis of the spermatozoon. The cristae are approximately 70 Å wide. The dense layers between them measure approximately 280 Å. The inner part of the mitochondrion derivative shows a crystalline array, formed by sub-units of approximately 100 Å diameter. The wing-shaped bodies consist of tubular elements.The head has an elongated nucleus with an electron transparent space inside. At the anterior end of the nucleus lies a tapered acrosome. This appears fibrous and parts of the acrosome fibers seem to run along the nucleus. Acknowledgements. The authors wish to thank Dr. G. H. Bergold for suggestions and support, Drs. J. André, D. W. Fawcett, P. Maillet and G. F. Meyer for very helpful discussion. They are also grateful to Mr. O. Suárez for assistance in the preparation of the organs of P. maidis and to Mrs. M. de Pingarrón for technical assistance.     

The spermatozoon of a 'living fossil': Tubiluchus troglodytes (Priapulida)

The spermatozoon of Tubiluchus troglodytes, the first priapulid formally described from the Mediterranean Sea has a head composed of an acrosome and a nucleus. The acrosome is divided in two branches coiled around the nucleus. The nucleus is basally columnar, but apically generates two rods helically coiled one around the other. The midpiece is formed by an axoneme with 27 accessory microtubules, surrounded by three mitochondria. An annulus separates the midpiece from the tail that contains a 9 + 2 axoneme surrounded by nine accessory microtubules. The spermatozoon of T. troglodytes is similar to that of the other two species known from the genus, and completely different from the 'primitive' one of the other priapulids. Since Tubiluchus is considered the most basal of the extant priapulids, and the only genus with an internal fertilization, it may be that in priapulids the external fertilization is a derived character.     

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     

Structural features of the spermatozoon of a passeridan bird,the Carib grackle,Quiscalus lugubris

The spermatozoon of the Carib grackle, Quiscalus lugubris, a member of the family Icteridae, is generally similar in organization to the passerine-type of spermatozoon, in being highly elongated and displaying a helical structure of the acrosome, nucleus and principal piece of the tail. There are subtle variations in acrosomal structural features between this organelle in the grackle and that in some of the very few passerine species of birds in which the spermatozoon has been studied. The proximal centriole is present, and, thus, the Carib grackle is the third passeridan bird in which this organelle, hitherto regarded as absent in passerine birds, has been described in the spermatozoon. The spermatozoon of this bird also possesses a granular helix, which feature has been found variably even in the scanty available reports on passerine spermatozoa. It is advocated that the spermatozoon be studied in many more species of this large clade of birds. This report provides a basis for the study of spermiogenesis in the Carib grackle, with the aim of exposing, inter alia, a number of developmental features and processes of certain organelles that have received attention, recently, in the spermatozoa of passerine birds.     

Semicystic spermatogenesis and biflagellate spermatozoon ultrastructure in the Nile electric catfish Malapterurus electricus (Teleostei: Siluriformes: Malapteruridae)

Spermatogenesis and spermatozoon ultrastructure in the Nile electric catfish Malapterurus electricus are described using scanning and transmission electron microscopy. Although the testis organization conforms to the ‘unrestricted’ spermatogonial type, the species has a rare type of spermatogenesis not previously described among catfishes, ‘semicystic’, in which the cyst ruptures before the spermatozoon stage. Spermiogenesis also involves some peculiar features such as condensation of the chromatin in the posterior part of the nucleus to form a compact electron‐dense mass with some irregular electron‐lucent lacunae, while the uppermost part of the nucleus is a loose electron‐lucent area, absence of the nuclear rotation and, as a consequence, the centriolar complex and the initial segment of each flagellum arise directly in a position perpendicular to the basal pole of the nucleus, and occurrence of numerous vesicles in the midpiece. In addition, spermiogenesis includes migration of the diplosome and mitochondria to the basal pole of the nucleus, formation of two moderate nuclear fossae, each of which contains the centriolar complex, development of two independent flagella and elimination of the excess cytoplasm. The mature spermatozoon has a more or less round head with no acrosome or acrosomal vesicle, a long midpiece with numerous mitochondria and vesicles and two long tails or flagella having the classical axoneme structure of 9 + 2 microtubular doublet pattern and with no lateral fins and membranous compartment. These findings suggest that the ultrastructural features of spermiogenesis and spermatozoa of M. electricus are synapomorphies of types I and II spermiogenesis and spermiogenesis is closely similar to the type described in the Nile catfish Chrysichthys auratus.     

Spermiogenesis and sperm in Mesostoma viaregginum (Plathelminthes, Rhabdocoela): an ultrastructural study to infer sperm orientation

Spermiogenesis in Mesostoma viaregginum begins with the formation of a zone of differentiation containing striated rootlets, two centrioles, and an intercentriolar body in-between. These centrioles generate two parallel free-flagella with the 9+“1” pattern of the Trepaxonemata growing out in opposite directions. Spermatid differentiation is characterised by a 90° latero-ventral rotation of flagella and a subsequent disto-proximal centriolar rotation, with a distal cytoplasmic projection. The former rotation involves the compression of a row of cortical microtubules and allows recognising a flagellar side and an aflagellar side in the late spermatid and in the mature spermatozoon. At the end of the differentiation, centrioles and microtubules lie parallel to the spermatid axis. The disto-proximal centriolar rotation is proposed as a synapomorphy for the Rhabdocoela. The modifications of the intercentriolar body during spermiogenesis and the migration of the nucleus and the centrioles towards the cytoplasmic distal projection are also described. The mature spermatozoon of M. viaregginum is filiform and tapered at both ends and presents many features found in the Rhabdocoela gametes. The nucleus disappears before the flagellar insertion and a density gradient of mitochondria is observed along the sperm axis. The anterior end of the spermatozoon of M. viaregginum is characterised by a tapering capped by a membrane expansion. This study has enabled us to describe precisely the orientation of spermatozoa in the Rhabdocoela in general: the centriolar extremity is proposed as the anterior one for the Rhabdocoela.     

Ultrastructural study of spermiogenesis and the testicular and spermathecal spermatozoon of the Gonochoristic Tardigrade Xerobiotus pseudohufelandi (Eutardigrada,Macrobiotidae)

This paper investigates by scanning and transmission electron microscopy spermiogenesis and spermatozoon morphology of the gonochoristic eutardigrade Xerobiotus pseudohufelandi (Macrobiotidae). During spermiogenesis clusters of spermatids are connected by cytoplasmic bridges that persist up to an advanced stage of maturation. Spermiogenesis is characterized by distinctive modifications of the nucleus and by the differentiation of an acrosome, tail and substantial midpiece. Testicular spermatozoa are folded with the hinge located between the head and midpiece, thus resembling a nut-cracker. The head includes a rod-shaped, bilayered acrosome and an elongated, helicoidal nucleus with condensed chromatin. The large kidney-shaped midpiece has hemispherical swellings/ovoid elements surrounding the centriole and an incomplete mitochondrial sleeve. The flagellum contains a ‘9+2’ axoneme and terminates in a tuft of microtubules. Spermathecal spermatozoa always have linear profiles. The acrosome and nucleus have the same morphological pattern as in testicular spermatozoa, whereas the midpiece is thin and lacks the hemispherical swellings, and the tail is reduced to a short stub. Functional considerations are presented, based upon this different morphology. Moreover, phyletic comparisons are made on the basis of sperm morphology, both for the family Macrobiotidae and the class Eutardigrada. J. Morphol. 234:11–24, 1997. © 1997 Wiley-Liss, Inc.