Fine structure of the spermatozoon of the viviparous teleost, Cymatogaster aggregata

The approximately 50 μm long sperm of Cymatoguster aggregata is composed of an elongate head (4 μm), an elongate mitochondria1 midpiece (3.5 μm) and a tail flagellum (roughly 40 μm). The sperm lacks an acrosome. Contained within depressions on one surface of the compressed head are a proximal centriole and a distal centriole separated by an electron dense, intercentriolar body. The anterior portion of the tail flagellum originates at the basal body (distal centriole) and is contained within an extracellular, flagellar tunnel within the mitochondria1 midpiece. The morphological similarity of C. uggregutu sperm to sperm of other internally fertilizing fishes supports the hypothesis that spermatozoan morphology is related to the mode of fertilization and that an elongate head and midpiece are specializations for internal fertilization.     

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.     

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.     

Surface activity at the egg plasma membrane during sperm incorporation and its cytochalasin B sensitivity: Scanning electron microscopy and time-lapse video microscopy during fertilization of the sea urchin Lytechinus variegatus

Sperm incorporation and the formation of the fertilization cone with its associated microvilli were investigated by scanning electron microscopy of eggs denuded of their vitelline layers with dithiothreitol or stripped of their elevating fertilization coats by physical methods. The activity of the elongating microvilli which appear to engulf the entering spermatozoon was recorded in living untreated eggs with time-lapse video microscopy. Following the acrosome reaction, the elongated acrosomal process connects the sperm head to the egg surface. About 15 microvilli adjacent to the attached sperm elongate at a rate of 2.6 μm/min and appear to engulf the sperm head, midpiece, and sperm tail. These elongate microvilli swell to form the fertilization cone (average height, 6.7 ± 2.0 μm) and are resorbed as the sperm tail enters the egg cytoplasm 10 min after insemination. Cytochalasin B, an inhibitor of microfilament motility, completely inhibits the observed egg plasma membrane surface activity in both control and denuded eggs. These results argue for a role of the microfilaments found in the egg cortex and microvilli as necessary for the engulfment of the sperm during incorporation and indicate that cytochalasin interferes with the fertilization process at this site.     

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.     

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.     

An ultrastructural analysis of the mature spermatozoon of Anguispira alternata (say) (pulmonata,stylommatophora)

Mature spermatozoa from the hermaphroditic duct of adult snails were examined using various techniques of light microscopy as well as scanning and transmission electron microscopy. The sperm are approximately 557 μm in length including a dextrally spiral head approximately 13 μm long. The head consists of an electron-dense nucleus sculptured into a double-ridged spiral and an acrosome projecting approximately 0.45 μm beyond the apex of the nucleus. The acrosome consists of a membrane-bound vesicle approximately 0.1 μm in diameter and a column of homogeneous material which extends along one side of the terminal spiral of the nucleus. This material is separated from the nucleus by the nuclear envelope. The neck region, though similar to that found in other pulmonates, possesses a unique coiled structure surrounding the central doublet of the axoneme. The midpiece axoneme possesses a 9+9+2 configuration anteriorly grading into a 9+2 pattern for the majority of its length. There are three mitochondrial helices – one primary and two secondary – in the midpiece. Only the primary helix persists throughout the midpiece.     

Sperm incorporation,the pronuclear migrations,and their relation to the establishment of the first embryonic axis: Time-lapse video microscopy of the movements during fertilization of the sea urchin Lytechinus variegatus

The movements during fertilization have been investigated with differential interference optics and recorded by time-lapse video microscopy of the clear egg of the sea urchin Lytechinus variegatus. Sperm-egg binding occurs rapidly, and following a time when the sperm gyrates on the egg surface, gamete fusion occurs. A rapid cortical contraction radiates from the fusion site and is succeeded by the elevation of the fertilization coat. Sperm incorporation occurs in two stages: the fertilization cone enlarges around and above the erect and immotile sperm and then the sperm head, midpiece, and tail are displaced along the subsurface region of the egg at an average rate of 3.5 μm/min. The formation of the sperm aster moves the male pronucleus from the subsurface region of the egg toward the egg center at a rate of 4.9 μm/min. When the rays of the radial sperm aster appear to contact the female pronucleus, the female pronucleus migrates at a rate of 14.6 μm/min to the center of the sperm aster. The now adjacent pronuclei are moved to the egg center by the continuing enlargement of the sperm aster at a rate of 2.6 μm/min. Syngamy is usually preceded by the disassembly of the sperm aster. The centripetal migration of the pronuclei appears involved in the establishment of the first embryonic axis; cleavage occurs within 8° of the direction of this centering motion.     

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.     

Four lines of spermatid development and dimorphic spermatozoa in the sea urchin Anthocidaris crassispina (Echinodermata, Echinoida)

 The process of sperm development in the sea urchin Anthocidaris crassispina was studied by light and electron microscopy. Similar to other echinoids studied, a single flagellum, striated rootlet and nuage-like materials were present in spermatogonia of A. crassispina. Spermatocytes near the diplotene stage showed intracellular localization of the axoneme which appeared to be a retracted flagellum prior to cell division. Fibrous filaments were associated with a proximal centriole in spermatocytes and spermatids and might be involved in movement of the proximal centriole. An acrosomal vesicle was developed and a residual body was formed in spermatids. The special development patterns in A. crassispina attributed to the presence of two patterns of tail development and two patterns of mitochondrial development during spermiogenesis. These four lines of spermiogenesis resulted in the formation of four morphological types of sperm cell, i.e. sperms with: (1) a symmetrical midpiece and posterior tail, (2) an asymmetrical midpiece and posterior tail, (3) a symmetrical midpiece and bent tail and (4) an asymmetrical midpiece and bent tail. Sperm cells with bent tails (type 3+4) were probably still at the late spermatid stage because results of scanning electron microscopy demonstrated gradual detachment and eventual straightening of the bent tail, and their percentage occurrence in the sperm population decreased significantly (P<0.05) towards the spawning season of A. crassispina. Spermatozoa with a symmetrical midpiece were dominant (averaging 70% occurrence in the sperm population) over those with an asymmetrical midpiece. The dimorphic spermatozoa in A. crassispina (types 1, 2) are both considered to be euspermatozoa as their morphology is typical for Echinoida. Accepted: 4 May 1998     

Localized surface antigens of guinea pig sperm migrate to new regions prior to fertilization

We have previously defined distinct localizations of antigens on the surface of the guinea pig sperm using monoclonal antibodies. In the present study we have demonstrated that these antigen localizations are dynamic and can be altered during changes in the functional state of the sperm. Before the sperm is capable of fertilizing the egg, it must undergo capacitation and an exocytic event, the acrosome reaction. Prior to capacitation, the antigen recognized by the monoclonal antibody, PT-1, was restricted to the posterior tail region (principle piece and end piece). After incubation in capacitating media at 37 degrees C for 1 h, 100% of the sperm population showed migration of the PT-1 antigen onto the anterior tail. This redistribution of surface antigen resulted from a migration of the surface molecules originally present on the posterior tail. It did not occur in the presence of metabolic poisons or when tail-beating was prevented. It was temperature-dependent, and did not require exogenous Ca2+. Since the PT-1 antigen is freely diffusing on the posterior tail before migration, the mechanism of redistribution could involve the alteration of a presumptive membrane barrier. In addition, we observed the redistribution of a second surface antigen after the acrosome reaction. The antigen recognized by the monoclonal antibody, PH-20, was localized exclusively in the posterior head region of acrosome-intact sperm. Within 7-10 min of induction of the acrosome reaction with Ca2+ and A23187, 90-100% of the acrosome-reacted sperm population no longer demonstrated binding of the PH-20 antibody on the posterior head, but showed binding instead on the inner acrosomal membrane. This redistribution of the PH-20 antigen also resulted from the migration of pre-existing surface molecules, but did not appear to require energy. The migration of PH-20 antigen was a selective process; other antigens localized to the posterior head region did not leave the posterior head after the acrosome reaction. These rearrangements of cell surface molecules may act to regulate cell surface function during fertilization.     

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).     

Vergleichende Ultrastrukturuntersuchungen der Eu- und Paraspermien von 13Protodrilus-Arten (Polychaeta,Annelida) und ihre taxonomische und phylogenetische Bedeutung

The morphology of the slender, filiform spermatozoa of 13Protodrilus species of 22 different populations is investigated by light and transmission electron microscopy. All species have two types of spermatoza: fertile euspermatozoa, and paraspermatozoa, which are probably infertile and may comprise up to 20% of the total number of mature gametes. This is the first record of sperm dimorphism in polychaetes. The general construction pattern of the euspermatozoa is very complex. It shows a longish tapering acrosomal vesicle with an internal acrosomal rod, a rod-like conical nucleus, and a midpiece with numerous very complex supporting elements and two thin mitochondrial derivatives. Further, it has a ‘peribasal body’ surrounding the basal body of the axoneme, an anulus region with an ‘anchoring apparatus’ and an anulus cuff. Posteriorly, the tail region proper contains in some species 2 to 9 supporting rods. In several species the euspermatozoon shows very distinct and species-specific alternations of this ‘general pattern’ relating to e.g. size of sperm elements, structure of acrosome and nucleus, presence or absence of axial rod, and number, shape and size of supporting elements in midpiece and tail. In a number of species some sections of the euspermatozoon overlap with each other more or less strongly. The paraspermatozoon has a comparatively simple construction pattern and possesses no supporting structures in midpiece and tail region. The midpiece is very short and, in some species, entirely surrounded by its two thin and elongate mitochondrial derivatives. An axial rod is often missing or reduced; different sperm sections never overlap each other. In contrast to the euspermatozoa, the paraspermatozoa of the different species have a very similar ultrastructure. Their possible function in spermatophore transfer and histolytical opening of the female epidermis is discussed. A comparison of the different forms of euspermatozoa inProtodrilus elucidates possible plesiomorphous and apomorphous sperm traits. Very likely, the hypothetical plesiomorphous type of spermatozoa inProtodrilus has a very similar morphology to that of the paraspermatozoa, which for this reason are considered to be a sort of persisting representatives of the ancientProtodrilus sperm type. InProtodrilus, the different traits of the euspermatozoa represent excellent taxonomic characters for distinguishing species (e.g. ‘sibling species’). They can also be used well for phylogenetics within the genus, whereas the relations ofProtodrilus to other polychaete groups cannot be clarified solely on the basis of sperm characters, since in all groups the sperm structure is primarily an adaptation to a specific mode of reproduction. Generally, the value of sperm characters in phylogenetic considerations at higher taxonomic levels seems to be very limited due to the surprisingly wide range of different sperm structures within a single genus as is demonstrated in the present paper.      

Morphology and fine structure of Acipenser persicus (Acipenseridae, Chondrostei) spermatozoon: Inter-species comparison in Acipenseriformes

This study describes morphology and fine structure of the Persian sturgeon (Acipenser persicus) (Acipenseridae, Chondrostei) spermatozoon. The results show that the spermatozoon of A. persicus is differentiated into an elongated head (length: mean±SD: 7.1±0.5μm) with an acrosome (length: 1.2±0.2μm), a cylindrical midpiece (length: 1.8±0.5μm), a flagellum (length: 50.3±5.9μm) and a total length of 59.2±6.2μm. Ten posterolateral projections (PLPs) arise from the posterior edge of the acrosome and there were 3 endonuclear canals that traversed the nucleus from the acrosomal end to the basal nuclear fossa region. Three to six mitochondria were in peripheral midpiece and the proximal and distal centrioles were located near to "implantation fossa" and basement of the flagellum. The axoneme has a typical eukaryotic structure composed of 9 peripheral microtubules and a central pair of single microtubule surrounded by the plasma membrane. Lateral fins were observed along the flagellum. The fins started and ended at 0.5-1μm from midpiece and at 4-6μm from the end of flagellum. There were significant differences in the size of almost all measured morphological parameters between males and flagellar, midpiece and nucleus characters were more isolated parameters that can be considered for detecting inter-individual variations. This study showed that sperm morphology and fine structure are similar among sturgeon species, but the dimensions of the parameters may differ.     

Morphology and ultrastructure of Siberian sturgeon (Acipenser baerii) spermatozoa using scanning and transmission electron microscopy

BACKGROUND INFORMATION: Available data concerning the sperm morphology of teleost fishes demonstrate wide variation. In the present study, the spermatozoa of Siberian sturgeon (Acipenser baerii Brandt, 1869), a chondrostean fish, was investigated. In contrast with teleost fish, chondrostean spermatozoa have a head with a distinct acrosome, whereas other structures, such as a midpiece and a single flagellum, are present in spermatozoa of most species. RESULTS: The average length of the head including the acrosome and the midpiece was 7.01+/-0.83 microm. Ten posterolateral projections derived from the acrosome were present on a subacrosomal region, with mean lengths of 0.94+/-0.15 microm and widths of 0.93+/-0.11 microm. The nucleus consisted of electrodense homogeneous nuclear chromatin. Three intertwining endonuclear canals, bound by membranes, traversed the nucleus longitudinally from the acrosomal end to the basal nuclear fossa region. There were between three and six mitochondria, two types of centrioles (proximal and distal) in the midpiece and two vacuoles composed of lipid droplets. The flagellum (44.75+/-4.93 microm in length), originating from the centriolar apparatus, had a typical 9+2 eukaryotic flagellar organization. In addition, there was an extracellular cytoplasm canal between the cytoplasmic sheath and the flagellum. CONCLUSIONS: A principal components analysis explained the individual morphological variation fairly well. Of the total accumulated variance, 41.45% was accounted for by parameters related to the head and midpiece of the sperm and the length of the flagellum. Comparing the present study with previous studies of morphology of sturgeon spermatozoa, there were large inter- or intra-specific differences that could be valuable taxonomically.     

Sperm morphology,spermatogenesis and spermiogenesis of three species of chitons (Mollusca,Polyplacophora)

Summary Mature sperm of the three species, Onithochiton quercinus, Chiton pelliserpentis and Plaxiphora paeteliana are eupyrene and basically of the primitive type. The sperm are small, with a distinct head, midpiece with a few spherical to oval mitochondria and a long tail with a (2×9)+2 axoneme. They are unusual among primitive sperm in being bilaterally symmetrical, with a long anterior filament containing an extension of the nucleus and lacking an acrosome. Spermatogenesis occurs synchronously throughout the testis in inwardly folded tissue plates. Spermatogonia arise adjacent to the central blood sinus in each tissue plate. Cells in successive stages of spermatogenesis are displaced towards the luminal surface. The cytoplasm of all stages contains ribosomes, rough endoplasmic reticulum, lysosomes and mitochondria. A Golgi complex is present in secondary spermatocytes and spermatids but does not form an acrosome. During spermiogenesis Golgi complexes are confined to the posterior region of developing sperm and are eventually shed in the residual cytoplasm behind the midpiece. Preacrosomal vesicles are not formed. The long anterior filament of the sperm and lack of an acrosome are features associated with the fertilization of eggs surrounded by a chorion which may have pores or a micropyle. The exact method of fertilization in chitons remains to be elucidated.Abbreviations af anterior filament - bh body of the head - bn body of the nucleus - bs blood sinus - c collar - dc distal centriole - esg early spermatogonium - fc fibrous chromatin - gc granular chromatin - if implantation fossa - lsg later spermatogonium - m mitochondrion - mc muscle cell in blood sinus - mm midpiece mitochondrion - mt microtubule - mI primary spermatocyte undergoing first meiotic division - mII secondary spermatocyte undergoing second meiotic division - n nucleus - ncc nuclear condensing chromatin - ne nuclear envelope - pc proximal centriole - rc resorbing cell - s spermatozoon - 1°sc primary spermatocyte - 2°sc secondary spermatocyte - st spermatid - t tail - tc thinning cytoplasm - tf tail flagellum - tpec tissue plate epithelial cell