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 Spermatozoon of an Opisthobranch, Tornatina sp. (Mollusca, Gastropoda, Retusidae)

The spermatozoon of Tornatina sp. has been studied with phase-contrast light microscopy and transmission electron microscopy. The head of the spermatozoon consists of an elongate acrosome which caps the apex of an unusually complex, helical nucleus. This elaborate nuclear morphology has not been previously reported, but possibly is found in other opisthobranch gastropod spermatozoa. An axoneme is inserted deeply into the base of the nucleus whilst posterior from the nucleus, the axoneme is ensheathed successively by the mitochondrial derivative (midpiece) and 'glycogen' granules (glycogen piece). The midpiece exhibits fine structure similar to that observed in other euthyneuran spermatozoa (paracrystalline and matrix materials) and possesses a single helical compartment filled with what are probably glycogen granules. A dense ring structure occurs at the junction of the midpiece and glycogen piece. The spermatozoon of Tornatina and other gastropods (prosobranch and euthyneuran) are compared.     

An Electron Microscopic Study of Spermatogenesis in Marenzelleria viridis (Verrill, 1873) (Polychaeta; Spionidae)

Abstract Spermatogenesis in Marenzelleria viridis was studied by ultrastructural investigation. The testes are formed on the greatly ramified nephridial blood vessel and are enveloped by a thin layer of peritoneal cells. The spermatogonia vary in shape, are about 10 μm in diameter and are not linked by intercellular bridges. Pairs or tetrads of spermatocytes connected by intercellular bridges float freely in the coelomic cavity. A complex acrosome is produced by a Golgi complex. The acrosome consists of four to five different structures, forms cisternae and, in the mature spermatozoon, lies deep in an invagination of the nucleus. Two centrioles are also situated in a deep centriolar fossa, the proximal centriole being perpendicular to the distal one. The mature spermatozoon is an ect-aquasperm measuring about 5 μm in length and 2.5 μm in width. The midpiece consists of five spherical mitochondria arranged around the axoneme behind the nucleus. The axoneme is connected to the plasma membrane by a satellite complex. The microtubules of the flagellum are arranged in a typical 9 × 2 + 2 configuration. The spermatogenesis and the sperm morphology of M. viridis were compared with those of other members of the family Spionidae. Copyright © 1996 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd.     

Ultrastructure of Euspermiogenesis in the Mesogastropod Stenothyra sp. (Prosobranchia, Rissoacea, Stenothyridae)

The structure of mature and developing euspermatozoa of the rissoacean gastropod Stenothyra sp. has been studied using transmission electron microscopy. During cuspermiogenesis nuclei pass through fibrillar and lamellar phases of condensation. A Golgi-derived acrosome attaches to the nucleus during the fibrillar phase. Spherical mitochondria of early euspermatids fuse to form the mitochondrial sheath which undergoes metamorphosis to form helical midpiece elements, paracrystalline material and helical midpiece compartments. Mature euspermatozoa consist of a flat acrosome (acrosomal cone, axial rod, basal plate), short curved nucleus (2.5–2.8 μm) and elongate midpiece and glycogen piece. Coarse fibres associated with the axoneme emerge from a posterior invagination of the nucleus and continue into the initial portion of the midpiece. In the proximal portion of the midpiece, two helical compartments (filled with membranous material) are present—only one of which persists further posteriorly. No compartments occur in the distal region of the midpiece. Posterior to the midpiece, the axoneme is surrounded by tightly-packed (glycogen) granules and terminates within this region. The distal end of the euspermatozoon consists solely of glycogen granules surrounded by the plasma membrane. Although coarse fibres (associated with the axoneme), midpiece paracrystalline material and helical compartments are commonly reported in sperm of euthyneuran gastropods, this represents the first report of all three features in any prosobranch euspermatozoon.     

Structure and ultrastructure of the spermatozoa of Bephratelloides pomorum (Fabricius) (Hymenoptera: Eurytomidae)

The spermatozoa of Bephratelloides pomorum are very long and fine. Each spermatozoon measures about 620 μm in length by 0.38 μm in diameter and, when seen under the light microscope, appears to be wavy along its entire length. The head, which is approximately 105 μm, comprises a small acrosome and a nucleus. The acrosome is made up of a cone-shaped acrosomal vesicle surrounding the perforatorium and the anterior end of the nucleus. Innumerable filaments radiate from it. The perforatorium has a diameter equal to that of the nucleus at their junction, where it fits with a concave base onto the rounded nuclear tip. The nucleus is helicoidal and completely filled with homogeneous compact chromatin. It is attached to the tail by a very long and quite electron-dense centriolar adjunct that extends anteriorly from the centriole in a spiral around the nucleus for approximately 8.5 μm. The tail consists of an axoneme with the 9+9+2 microtubule arrangement pitched in a long helix, as well as a pair of spiraling mitochondrial derivatives (with regularly arranged cristae) that coil around the axoneme, and two small accessory bodies. As well as the spiraling of the nucleus, mitochondrial derivatives and axonemal microtubules, the sperm of B. pomorum present other very different morphological features. These features include the acrosome and centriolar adjunct, both of which differentiate the spermatozoa from the majority of sperm found in other Hymenoptera. In addition these structural variations demonstrate that the sperm of chalcidoids provide characteristics that can certainly prove useful for future phylogenetic analysis at the subfamily level and, possibly, the genus too.     

Structure and formation of the unusual sperm of Patelloida latistrigata (Mollusca : Patellogastropoda): implications for fertilization biology

The structure of the spermatozoa and spermatogenesis of the lottiid limpet Patelloida latistrigata is described by transmission electron microscopy. Although the lengths of the spermatozoa (about 60 μm) and their head region (about 12 μm) are similar to those of other patellogastropods, the structure of the sperm head and midpiece are very different. The head consists of an unusually large acrosome (about 11-μm long) with a broad posterior invagination that houses the relatively small nucleus. The midpiece mitochondria, which are rather elongate with large folded tubular cristae, are housed in a cytoplasmic sheath posterior to the nucleus. The proximal centriole is unusually elongate (about 2-μm long). The axoneme that emerges from the distal centriole is surrounded anteriorly by the cytoplasmic sheath in which the cytoplasmic side of the plasma membrane has electron-dense material. The flagellum is enlarged at its terminal end. Spermatogenesis is similar to that described for other patellogastropods. Patelloida latistrigata, therefore, has spermatozoa that seem to meet the morphological criteria of ent-aquasperm, which raises the question of whether fertilization is truly external in this limpet. However, it is also possible that the modifications to the sperm are linked to unknown specializations of the egg or egg envelope.     

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.     

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.     

Fine structure of spermatozoa in the common pandora (Pagellus erythrinus Linnaeus, 1758) (Perciformes, Sparidae)

Scanning and transmission electron microscopy were used to investigate the fine structure of the sperm of the Sparid fish Pagellus erythrinus L. The spermatozoon of pandora has a spherical head lacking an acrosome, a cone-shaped midpiece and a long tail. The midpiece houses a single mitochondrion. The centriolar complex lies inside the nuclear fossa and is composed of a proximal and a distal centriole which are arranged at right angles to each other. The flagellum is inserted medio-laterally into the head, contains the conventional 9+2 axoneme and possesses one pair of lateral fins. On the basis of its ultrastructural organization, the pandora sperm can be regarded as an evolved form of the primitive spermatozoon found in Teleosts. According to the morphological classification proposed by Mattei (1970), the sperm of pandora belongs to a "type I" designation, like that of the other Sparid fish.     

Structure and ultrastructure of spermatozoa and spermiogenesis in three species of Lucilia Robineau‐desvoidy, 1830 (Diptera: Calliphoridae)

Morphology of male internal reproductive organs, spermatozoa, and spermiogenesis of the blow‐flies Lucilia cuprina, Lucilia eximia, and Lucilia peruviana is first described here, using light and transmission electron microscopy. Spermiogenesis follows the characteristics described for others insect species. The spermatozoa of L. cuprina are similar to those described for other Brachycera. However, in L. eximia and L. peruviana, some differences were found. In L. cuprina and L. eximia species, the spermatozoa are long and thin, measuring about 211 μm and 146 μm in length, of which the head region measures approximately 19 μm and 17 μm, respectively. A polymorphism was observed in L. cuprina and L. eximia spermatozoa. In all three species, the head includes a monolayered acrosome with electron‐lucent material. The shape of the nucleus, in cross sections, varies from circular to oval with completely condensed chromatin. Implantation of the axoneme was observed in the middle region of the nucleus, known as the “peg” region. In the next region, the beginning of two mitochondrial derivatives of similar diameter and different lengths in L. cuprina and only one in L. eximia and L. peruviana was observed. In the overlap region, the following structures were observed: nucleus, centriolar adjunct, mitochondrial derivatives, and axoneme. The axoneme is of a conventional insectan type with a 9 + 9 + 2 microtubular arrangement. The male internal reproductive tract consists of testis, deferent ducts, a strongly developed seminal vesicle, accessory glands, and ejaculatory duct. These features are consistent with the structural diversity of the dipteran reproductive tract and spermatozoa, comprising an essential tool for understanding the complex variations found in the Diptera. J. Morphol. 2011. © 2011 Wiley Periodicals, Inc.     

Spermiogenesis in the Oligochaetoid Polychaete Questa (Annelida, Questidae)

The spermatids are connected to a central cytophore by cytoplasmic bridges and are polarized in the sequence: "empty cytoplasm"; uncondensed nucleus; mitochondria which surround the distal region of the nucleus and the centrioles; axoneme; posterolateral to the base of the axoneme, the Golgi apparatus and (when secreted) the acrosomal rudiment. The dome-shaped acrosome vesicle elongates progressively as it migrates to the tip of the elongating and condensing nucleus; subacrosomal material gives rise to an almost equally long, tubular, thick-walled perforatorium. After the acrosome has greatly elongated, the mitochondria are reduced to two, which lose their rounded form and invest the growing axoneme to give a very elongate midpiece. Transfer of materials from nucleus to mitochondria is discussed. Microtubules surrounding the acrosome and nucleus disappear by maturity, but those internal to the mitochrondria apparently persist as the accessory microtubules, unique in the Annelida, which surround the 9 + 2 axoneme. Microvilli of the egg envelope, which have tetrads of terminal branches (epivitelline projections) resembling epicuticular projections, are less than 1 μm long, whereas the mature acrosome exceeds 5 μm. This suggests that the correlation seen in oligochaetes is absent.     

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]。     

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.     

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

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

Ultrastructure of spermiogenesis in <Emphasis Type="Italic">Cristatella mucedo</Emphasis> Cuvier (Bryozoa: Phylactolaemata: Cristatellidae)

In Cristatella mucedo spermiogenesis occurs in a morula consisting of a large number of spermatids connected with a central cytophore. The mature sperm cell is filiform and consists of a head, a midpiece and a tail region, the latter two separated by a deep circular constriction. The comparatively short head contains a drop-shaped, bilaterally symmetrical and pointed nucleus capped by a minute acrosome. The single centriole is placed in a deep posterior invagination of the nucleus followed by the axoneme with the typical 9 + 2 pattern. The elongated midpiece is 0.9–1.1 μm thick and contains several helices of mitochondria surrounding the axoneme. The tail is thicker (1.3 μm) and richer in cytoplasm with many compact accumulations of an electron-dense substance lying peripherally and another less dense material wrapped around the axoneme. The course of the spermiogenesis and the fine structure of the sperm are very similar to that of Plumatella fungosa. Comparison with other species shows that the same sperm type is recognizable in four of the five families of Phylactolaemata and, provided it occurs also in the fifth family, the Stephanellidae, is a synapomorphy of the entire class.     

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.     

Morphology of testicular and post-testicular spermatozoa in Microstigmus arlei Richards, 1972 and M. nigrophthalmus Melo, 1992 (Hymenoptera: Apoidea: Pemphredoninae) with phylogenetic consideration

The sperm of Microstigmus arlei and Microstigmus nigrophthalmus are twisted in a spiral and consist of two regions: the head, formed by an acrosome and a nucleus, and the flagellum, formed by two asymmetric mitochondrial derivatives, a long centriolar adjunct, an axoneme (9+9+2) and two accessory bodies. The head shows a characteristic morphology. The acrosome is very long and is basically made up of a paracrystalline structure. In the central head region, the acrosome is inserted into the nucleus, which is observed coiling laterally around the paracrystalline structure. In the subsequent part of the spermatozoon the nucleus appears round in transverse sections, and over some length it is still penetrated by the acrosome until shortly distal to the flagellar insertion. At this point the nucleus forms an inverted cone-shaped projection. These morphological characteristics of acrosome and nucleus of the Microstigmus wasp have not been previously described in Apoidea and are useful for phylogenetic evaluation of this superfamily.     

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).     

The fine structure of the spermatozoa of Siphonaria algesirae (Gastropoda,Pulmonata)

The sperm of Siphonaria algesirae (Gastropoda, Pulmonata), a species with internal fertilization, was studied by light and electron microscopy. The spermatozoon is a very long, uniflagellate cell composed of a conical head with an apical acrosome, a midpiece with a helically coiled external sheath containing a complex mitochondrial derivative with a wavelength of ～ 5.5 μm, and an endpiece. There are no axonemal microtubules. Instead, nine homogeneous coarse fibers with transverse striations in the apical zone project toward the anterior section of the midpiece. In the posterior zone of the midpiece the coarse fibers are differentiated in a common microtubular axoneme. The complex mitochondrial derivative of the midpiece shows an organized group of 100 Å diameter spherical particles. Externally the midpiece is surrounded along its length by a cylinder formed by two membranes. A complex structure separates the transitional zone between the midpiece and the endpiece.