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 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 spermiogenesis and the spermatozoon of Macracanthorhynchus hirudinaceus (Pallas, 1781) (Acanthocephala: Archiacanthocephala), a parasite of the wild boar Sus scrofa

The present paper describes the ultrastructure of spermiogenesis and the spermatozoon of Macracanthorhynchus hirudinaceus, an acanthocephalan parasite of the wild boar Sus scrofa. At the beginning of spermatogenesis, spermatocytes exhibit synaptonemal complexes and 2 centrioles. In the spermatid, only 1 centriole remains, generating a flagellum with a 9+2 pattern. Another ultrastructural feature observed during the spermiogenesis of M. hirudinaceus is the condensation of the chromatin, forming a "honeycomb" structure in the old spermatid and a homogeneous, electron-dense structure in the spermatozoon. The mature spermatozoon of M. hirudinaceus presents a reversed anatomy, as has been described previously in other species of the Acanthocephala. The spermatozoon is divided into 2 parts: an axoneme, and a nucleocytoplasmic derivative. The spermatozoon flagellum exhibits a 9+2 or 9+0 pattern. The process of spermiogenesis and the ultrastructural organization of the spermatozoon of M. hirudinaceus are compared with available data regarding other acanthocephalan species.     

三角帆蚌精子的形态及超微结构

运用电子显微镜技术对三角帆蚌精子的形态和超微结构进行研究。结果发现,三角帆蚌精子为原生型,分为头部、中段和尾部,头部呈子弹头形,电子致密且均匀,主要是核所在的区域。核前端由3－4个小的电子致密颗粒组成一个浅弧形的囊泡,为顶体结构,中段具有5个球形线粒体,环绕着两个相互垂直的中心粒。中段末端具有的鞭毛质领结构（flagellar collar)为一电子致密环,与远端中心粒之间由9个分叉的电子致密小片连接。尾部为典型的9＋2结构。     

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

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

Ultrastructure of spermiogenesis and the mature spermatozoon of Tetrabothrius erostris loennberg, 1896 (Cestoda, Tetrabothriidae)

The spermiogenesis of Tetrabothrius erostris is characterized by the following events: formation of a differentiation zone containing 2 basal bodies and a pair of rootlets; one of the basal bodies gives rise to a free flagellum, the other induces formation of a flagellar bud; rotation at 90° of the flagellum prior to its fusion with the middle cytoplasmic process of the differentiation zone and partial rotation of the flagellar bud; penetration of the nucleus between the rootlets and appearance of a spur-like protrusion in the differentiation zone; elongation and twisting of the differentiation zone, resulting in twisting of the peripheral microtubules and migration of the nucleus; formation of a crested body; proximal densification of the spermatozoon prior to its detachment from the spermatid rosette. The mature spermatozoon has a single axoneme of 9+“1” type and twisted peripheral microtubules. It consists of 3 portions: a proximal part with a crested body, a middle region rich in β-glycogen, and a distal part containing the nucleus. The pattern of spermiogenesis resembles most closely that in phyllobothriid tetraphyllideans, and probably reflects a relationship of the family Tetrabothriidae with this group.     

The ultrastructure of spermiogenesis and spermatozoa in Diplomystes mesembrinus

Spermiogenesis in Diplomystes mesembrinus , one of the most primitive species from the Siluriformes, occurs in cysts. Differentiation of spermatids is characterized by chromatin compaction, flagellum development, nuclear fossa formation, rotation of the nucleus, and excess cytoplasm elimination. The spermatozoon head is round, the nucleus contains highly condensed chromatin clusters, the midpiece is short, the axoneme shows a 9 +2 pattern with two discrete lateral projections, and the acrosome is absent. The nuclear fossa penetrates deeply into the nucleus, including the centriolar complex and the start of the axoneme. The single large C-shaped mitochondrion surrounds the initial segment of the axoneme. The structural features of D. mesembrinus spermatozoon are similar to the Clupeiformes.     

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.     

The cytology and ultrastructure of zoospores of Hydrurus foetidus (Chrysophyceae)

The unusual tetrahedral shape of Hydrurus foetidus (Vill.) Trev. zoospores is associated with a complex skeletal system of microtubules extending from a broad flagellar root (up to 19 microtubules) into each of three, pointed anterior processes. The posterior end, also pointed and supported by a separate set of microtubules, contains a single large chloroplast with a prominent posterior furrow containing mitochondrial elements. A large immersed pyrenoid is penetrated by paired thylakoids. There is no eyespot. Numerous large Golgi bodies occur immediately anterior to the nucleus and up to 5–6 contractile vacuoles lie near the cell surface at the anterior end. Two terminally inserted flagella extend from the cell surface, a long one serving for cell locomotion, and the other vestigial with an axonemal pattern of 9+0. The flagellar root system consists of: (1) a thin, striated rhizoplast extending from the basal body of the long flagellum and ramifying over the surface of a conspicuous, anteriorly directed, conical projection of the nucleus; (2) a broad microtubular root which emanates from near the basal body of the long flagellum and appears to function as a MTOC; (3) a compound root, consisting of a striated fiber and two associated microtubules, which runs alongside the basal body of the stubby flagellum before terminating at the cell surface; and (4) a short two-membered microtubular root, also associated with the basal body of the stubby flagellum. Other components of the flagellar apparatus include a large dense body near the proximal end of the basal body of the short flagellum, and a small, dense, core-like structure closely associated with one of its triplet fibers. The flagellar apparatus of H. foetidus is remarkably similar in ultrastructure to that of Chrysonebula holmesii Lund.     

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

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

Spermiogenesis in two paramphistomes from Nile fish in Egypt: an ultrastructural study

The process of spermiogenesis in two paramphistomes, Sandonia sudanensis and Basidiodiscus ectorchis from the Nile fish Synodontis schall in Egypt was studied by transmission electron microscopy. Spermiogenesis is characterized by the outgrowth of the zone of differentiation, presenting two basal bodies separated by a microtubule organizing centre, each basal body developing into a flagellum. Proximodistal fusion of these flagella with a median cytoplasmic extension gives rise to the spermatozoon. The mature spermatozoon possesses two axonemes of the 9+'1' pattern typical of parasitic helminths. There are few ultrastructural studies on spermiogenesis in paramphistomes, which are considered the most primitive digenetic trematodes. The present study provides new and more detailed information on this process, including the presence of a lateral flange and external ornamentation of the cell membrane. The value of sperm ultrastructure as a taxonomic tool in phylogeny is also discussed.     

Exceptions to the prevailing pattern of tubules (9 + 9 + 2) in the sperm flagella of certain insect species

Various deviations from classical 9 + 2 flagellar structure are found in sperm of insect species. In mature spermatozoa of a psocid, Psocus, the outer flagellar tubules are not straight, but are disposed in a long-pitched helix such that they form an angle of about 8° with a single dense rod located in the position usually occupied by the central pair. In young spermatids of Psocus the outer tubules are straight; thus, spiraling of the flagellar tubules occurs during the course of spermiogenesis. Spiraling of flagella also occurs in the cat flea Ctenocephalides felis. Variations in the number and morphology of the central element or elements occur in other insect species besides Psocus. Among the observed deviations from a central pair of tubules are a 9 + 0 tubule pattern in the sperm of three species of mayflies, a 9 + 1 tubule pattern in the sperm of two species of mosquitoes, and 9 + 7 tubules in sperm of two species of caddis flies. Spermatozoa of treehoppers vary in yet another respect from the typical 9 + 9 + 2 insect flagellum. These sperm tails branch into four long tails, three of which each contain two doublet and two singlet tubules while the fourth branch contains three doublet and three singlet tubules. The wide distribution of insects with aberrant flagella suggests that the variant forms have evolved independently.     

Structural diversity of bacterial flagellar motors

The bacterial flagellum is one of nature's most amazing and well-studied nanomachines. Its cell-wall-anchored motor uses chemical energy to rotate a microns-long filament and propel the bacterium towards nutrients and away from toxins. While much is known about flagellar motors from certain model organisms, their diversity across the bacterial kingdom is less well characterized, allowing the occasional misrepresentation of the motor as an invariant, ideal machine. Here, we present an electron cryotomographical survey of flagellar motor architectures throughout the Bacteria. While a conserved structural core was observed in all 11 bacteria imaged, surprisingly novel and divergent structures as well as different symmetries were observed surrounding the core. Correlating the motor structures with the presence and absence of particular motor genes in each organism suggested the locations of five proteins involved in the export apparatus including FliI, whose position below the C-ring was confirmed by imaging a deletion strain. The combination of conserved and specially-adapted structures seen here sheds light on how this complex protein nanomachine has evolved to meet the needs of different species.     

The sperm of Hexagenia (Pseudeatonica) albivitta Walker (Ephemeroptera: Fossoriae: Ephemeridae)

This study describes the sperm morphology of the mayfly Hexagenia (Pseudeatonica) albivitta (Ephemeroptera). Its spermatozoon measures approximately 30 μm of which 9 μm corresponds to the head. The head is composed of an approximately round acrosomal vesicle and a cylindrical nucleus. The nucleus has two concavities, one in the anterior tip, where the acrosomal vesicle is inserted and a deeper one at its base, where the flagellum components are inserted. The flagellum is composed of an axoneme, a mitochondrion and a dense rod adjacent to the mitochondrion. A centriolar adjunct is also observed surrounding the axoneme in the initial portion of the flagellum and extends along the flagellum for at least 2 μm, surrounding the axoneme in a half‐moon shape. The axoneme is the longest component of the flagellum, and it follows the 9+9+0 pattern, with no central pair of microtubules. At the posterior region of the flagellum, the mitochondrion has a dumb‐bell shape in cross sections that, together with the rectangular mitochondrial‐associated rod, is responsible for the flattened shape of the flagellum. An internal membrane is observed surrounding both mitochondrion and its associated structure.     

Cryopreservation of Atlantic croaker spermatozoa: evaluation of morphological changes.

The spermatozoon of the Atlantic croaker (Micropogonias undulatus) is a primitive type in that it lacks an acrosome. The kidney-shaped head has a diameter of about 1.5 microns and is occupied by a granular and electron-dense nucleus. The short midpiece contains 3 spherical mitochondria and encircles the basal body of the flagellum but is separated from it. The flagellum consists of the typical 9 + 2 axoneme and surrounding plasma membrane but lacks a lateral ridge. Spermatozoa of Atlantic croaker diluted in either NaCl or sodium citrate solutions with or without DMSO were examined with the electron microscope before freezing in liquid nitrogen and after thawing. Damage following cryopreservation appeared to be greater to the mitochondria, plasma membrane, and 9 + 2 axoneme than to the nucleus. The incidence of postthaw damage in spermatozoa diluted with NaCl solutions containing DMSO was remarkably lower than that with either pure NaCl solutions, pure sodium citrate solutions, or sodium citrate solutions containing DMSO.     

Spermatozoal ultrastructure of four Sparidae fishes: Acanthopagrus berda, Acanthopagrus australis, Lagodon rhomboids and Archosargus probatocephus

The mature spermatozoa of two Taiwan protandrous hermaphrodite Sparidae Acanthopagrus berda and Acanthopagrus australis are investigated and compared with those of other two Sparidae (Lagodon rhomboids and Archosargus probatocephus) from the Western hemisphere. Ultrastructurally the spermatozoon of these four species has a spherical, homogeneously electron-dense nucleus with an axial nuclear fossa. The midpiece contains one to four spherical mitochondria and encircles the basal body of the flagellum. The mature spermatozoa of the four species are of the primitive or ect-aquasperm form and conform to the teleostean type I spermatozoon with the flagellar axis inserts perpendicular and medial to the nuclear fossa. Variation in the depths of the nuclear fossa and mitochondria number is substantial in these four Sparidae species. This study provide useful systematic characters to the existing knowledge of comparative spermatology of Sparidae.     

The ultrastructure of the trophic cells of the protostelidPlanoprotθstelium aurantium

Summary The protostelidPlanoprotostelium aurantium Olive andStoianovitch has trophic cells which are either amoebae or flagellates. The general morphology and ultrastructure are consistent with what has been reported for otherEumycetozoa (protostelids, myxomycetes, and dictyostelids). The flagellar apparatus structure has the same basic pattern as that of other flagellate eumycetozoans. It shares with all these an anteriorly directed flagellum and centriole and microtubule arrays (MTA) 2–4. Unlike more primitive species which have two centrioles per flagellar apparatus,P. aurantium has only one. Also, the flagellar apparatus is independent of the nucleus inP. aurantium, not linked to it as in the primitive species. These features are useful in explaining the differences in swimming behavior betweenP. aurantium and biflagellate species. Evidence is presented to show thatP. aurantium is closely related to the non-flagellateProtostelium mycophaga Olive andStoianovitch.This research represents part of a Ph.D. dissertation presented to the University of North Carolina.     

DEVELOPMENT OF THE FLAGELLAR APPARATUS OF NAEGLERIA

Flagellates of Naegleria gruberi have an interconnected flagellar apparatus consisting of nucleus, rhizoplast and accessory filaments, basal bodies, and flagella. The structures of these components have been found to be similar to those in other flagellates. The development of methods for obtaining the relatively synchronous transformation of populations of Naegleria amebae into flagellates has permitted a study of the development of the flagellar apparatus. No indications of rhizoplast, basal body, or flagellum structures could be detected in amebae. A basal body appears and assumes a position at the cell surface with its filaments perpendicular to the cell membrane. Axoneme filaments extend from the basal body filaments into a progressive evagination of the cell membrane which becomes the flagellum sheath. Continued elongation of the axoneme filaments leads to differentiation of a fully formed flagellum with a typical "9 + 2" organization, within 10 min after the appearance of basal bodies.     

Fine structure of spermatozoa in the blackspot sea bream Pagellus bogaraveo (Brünnich, 1768) with some considerations about the centriolar complex

Scanning and transmission electron microscopy were used to investigate the fine structure of the sperm of the sparid fish Pagellus bogaraveo.The spermatozoon of P. bogaraveo belongs, like that of the other sparid fish, to the teleostean “type I” spermatozoon with the flagellar axis insert perpendicular to the nuclear fossa. It has an ovoidal head, a short, cylindrically shaped midpiece and a long tail region. The nucleus reveals a deep invagination (nuclear fossa), in which the centriolar complex is located, and a satellite nuclear notch shaped like a golf club. The two centrioles are perpendicular to each other and show a conventional “9+0” pattern. The distal centriole is attached to the nuclear envelope by means of basal feet and radial fibers made of electron-dense material. Below the basal plate, plasma membrane pinches in, and the necklace, a specialized connection joining axonemal doublets to the plasma membrane, is visible. The short midpiece houses one mitochondrion. The flagellum is perpendicularly and eccentrically with respect to the nucleus and contains the conventional “9+2” axoneme.     

Structure and significance of cruciate flagellar root systems in green algae: Female gametes ofBryopsis lyngbyei (Bryopsidales)

The ultrastructure of the flagellar apparatus in the biflagellate female gametes of the green algaBryopsis lyngbyei has been studied in detail. In the flagellum and basal body, microtubule septations occur in some of the B-tubules. The transition region of the flagellum is extremely long (260–290 nm), exhibits a stellate pattern in cross section but lacks the transverse diaphragm. The two basal bodies form an angle of 180° and overlap at their proximal ends. They are connected by a compound non-striated capping plate. Terminal caps associated with the capping plate partially close the proximal end of each basal body. A cruciate flagellar root system with three different types of microtubular roots is present, i. e. the flagellar apparatus does not show 180° rotational symmetry. One root type contains 2 microtubules which are connected to an elaborate cylindrical structure, presumably a mating structure. The opposite root exhibits 3 microtubules over its entire length and is not associated with a cylindrical structure. In their proximal parts both roots are linked to an underlying crescent body. The other two microtubular roots are probably identical and consist of 4 (or 5) microtubules which show configurational changes. These two identical roots insert into the capping plate and link to the inner side (i. e. the side adjacent to the other basal body) of each basal body, whereas the other two roots attach to the outer sides of each basal body. System I striated fibres are probably associated with each of the four roots, while system II fibres have not been observed. The flagellar apparatus of female gametes ofB. lyngbyei shows many unique features but in some aspects resembles that of ulvalean algae. Functional and phylogenetic aspects of cruciate flagellar root systems in green algae are discussed.