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Sperm structure of the Xenacoelomorpha (Acoelomorpha plus Xenoturbellida) is updated in the light of new discoveries or new interpretations of existing data. Nemertodermatida and Acoela (Acoelomorpha) have introsperm with certain basic features in common, but all acoels lack acrosomes and usually have two flagella with unusual combinations of microtubules, whereas all nemertodermatids have small, simple acrosomes and a typical 9 + 2 flagellum. Xenoturbellida is currently considered as the sister taxon to Acoelomorpha. Xenoturbella bocki has an aquasperm that has almost nothing in common with the sperm of Acoelomorpha. We argue that the aquasperm ultrastructure of X. bocki has much in common with sperm of hemichordates and to some extent echinoderms, which was previously disputed. Molecular analyses have on the one hand supported a connection with deuterostomes but on the other hand have negated it, suggesting that the closest common ancestor of Xenacoelomorpha is either the Nephrozoa, Deuterostomia or Protostomia. Sperm structure is highly diverse among Xenacoelomorpha, with protostome-like traits in Acoelomorpha and deuterostome-like traits in Xenoturbella. Assuming Xenacoelomorph monophyly and ancestral introsperm in this taxon, however, suggests that the re-expression of the aquasperm form of Xenoturbella, involving some key changes in sperm morphology, is a secondarily derived state that could have occurred through "progenetic spermiogenesis" with the precocious development of round spermatids to maturity. [ABSTRACT FROM AUTHOR]

The functional morphology of the mammiliform penial glands of Littorina saxatilis has been investigated with both light and electron microscopy. These penial glands line the ventral edge of the penis and orient with the female mantle during copulation. Secretions are released from the penial glands to this interface where they probably function in adhesion. The penial gland secretions comprise heterogeneous granules as well as apocrine and mucous secretions. The heterogeneous granules are produced in separate multicellular glands arranged in a series of lobes that lie outside a thick smooth muscle layer enclosing the lumen. Each glandular lobe is surrounded by a thin layer of smooth muscle. Secretions are transported in individual cellular processes that pass through the thick smooth muscle layer and empty into the lumen. Surrounding the lumen is an epithelium containing apocrine secretory cells as well as occasional goblet-type, mucous cells. The combined action of the muscles forces secretions out of the lumen through the penial papilla, onto the external surface of the mammiliform penial gland. Longitudinal muscles extend into the penial papilla enabling its protrusion or retraction. Retraction of the penial papilla following secretion release is thought to create negative pressure beneath the penial gland producing suction adhesion. The visco-elastic properties of the penial gland secretion are qualitatively different from foot mucus and may represent specialization to an adhesive function. [ABSTRACT FROM AUTHOR]

The present study examines spermiogenesis, and in particular the formation of the acrosome, in ten species of chitons belonging to four families. This study emphasizes the formation of the acrosome but brings to light several other structures that have received little or no mention in previous studies. The process of spermiogenesis is essentially similar in each species, although Chaetopleura exhibits some significant differences. In early spermiogenesis the Golgi body secretes numerous small pro-acrosomal vesicles that gradually migrate into the apical cytoplasm. The chromatin condenses from granules into fibres which become twisted within the nucleus. A small bundle of chromatin fibres projects from the main nuclear mass into the anterior filament; this coincides with the appearance of a developing manchette of microtubules around the nucleus that originates from the two centrioles. Radiating from the distal centriole is the centriolar satellite complex, which is attached to the plasma membrane by the annulus. The distal centriole produces the flagellum posteriorly and it exits eccentrically through a ring of folded membrane that houses the annulus. Extending from the annulus on one side of the flagellum, in all but one species, is a dense fibrous body that has not been previously reported. The proximal centriole lies perpendicular to the end of the distal centriole and is attached to it by fibro-granular material. Pro-acrosomal vesicles migrate anteriorly through the cytoplasm and move into the anterior filament to one side of the expanding nucleus. Eventually these vesicles migrate all the way to the tip of the sperm, where they fuse to form one of two granules in the acrosome. In mature sperm the nucleus is bullet-shaped with a long anterior filament and contains dense chromatin with occasional lacunae. The mitochondria vary in both number and position in the mature sperm of different species. Both centrioles are housed eccentrically in a posterior indentation of the nucleus, where the membranes are modified. The elongate flagellum tapers to a long filamentous end-piece that roughly corresponds to the anterior filament and may be important in sperm locomotion for hydrodynamic reasons. An acrosome is present in all ten species and stained positively for acid phosphatase in three species that were tested. [ABSTRACT FROM AUTHOR]

The fine structure of Sertoli cells in three marine prosobranch molluscs has been studied with light- and electron microscopy. Sertoli cells of prosobranchs are modified columnar epithelial cells that maintain continuous contact with the basal lamina and extend from it to the lumen of a testicular tubule. Spermatogenesis takes place between adjacent Sertoli cells, but a continuous layer of cytoplasm separates the spermatogonia from the basal lamina, thus restricting the basal compartment to spermatogonium mother cells. Substances traversing the basal lamina from the interstitial space must pass either through or between the Sertoli cells. However, between the cells, a permeability barrier composed of septate and desmosome-like junctions blocks the passage of substances, such as the tracer lanthanum nitrate. The basally-located nucleus is irregularly shaped with fine granular euchromatin and some peripheral heterochromatin; satellite karyosomes border the nucleolus. There is an extensive intracellular digestive system that is used effectively to phagocytize waste sperm and residual cytoplasm. Cytoplasmic processes of Sertoli cells penetrate throughout the germinal epithelium. In some prosobranchs that exhibit sperm polymorphism these processes must coordinate to bring together a clone of eupyrene sperm and a carrier sperm at a particular time in development. The only cytoskeletal elements available within the processes to generate such movements are microtubules. We propose that the term 'nurse cell', which has been used in the past to describe at least three different cell types, including Sertoli cells and apyrene sperm, be restricted to abortive oogonia that contribute to development of an oocyte. [ABSTRACT FROM AUTHOR]

The prosobranch Fusitriton oregonensis exhibits an unusual form of sperm polymorphism. The viable, eupyrene sperm are attached in groups of about fifty to worm-shaped, apyrene, carrier sperm. There is a second apyrene sperm, which is lancet-shaped and has a different internal organization than the carrier, but does not transport eupyrene sperm. The eupyrene sperm are filiform (185 μm long), with a conical acrosome, elongate nucleus and midpiece. They contain large stores of glycogen in the principal piece, together with an unusually high proportion of protein. The latter is due to a complex interconnecting system of fibres that supports the tail internally. A distinct annulus is located, characteristically, at the junction between midpiece and principal piece. The carrier sperm has a core of about 112 axonemes that arise from basal bodies in the anterior end and extend through its entire length of 36 μm. The basal bodies have unstriated rootlets that are embedded in a granular cap. Large membrane-bound 'yolk bodies' are arranged along the length of the carrier sperm, on either side of the median axonemal core. Dense bodies, which may be indigestible residues formed from the degeneration of the nucleus, are excreted by exocytosis. Individual carrier sperm are capable of 'corkscrew' propulsion, resembling that of spirochaetes. The lancet sperm is three times as long as the carrier. The sixteen or so axonemes, which are arranged peripherally like a cage enclosing the cytoplasm, originate from a dense centriolar plate in the anterior end. The cytoplasm is filled with secretions including small yolk granules, dense bodies (also excreted), clear vesicles, and a membranated granular secretion that resembles mucus. The possible functions of the lancet and carrier sperm are discussed. [ABSTRACT FROM AUTHOR]

The filiform sperm of Littorina sitkana swims effectively in sea water and more viscous fluids, overcoming the problems of a non-uniform flagellar beat with an unusual mechanism, which involves three main events: (1) the sperm rotates anti-clockwise (looking from tail to head); then (2) stops rotating and stiffens itself to form a screw-shape, with the tail being held perpendicular to the middle piece, and finally; (3) reverses its rotation and propels itself forward in a clockwise spiral. The average velocity of sperm is approximately 185 μms with a rotational frequency of 24 revs. The mechanism of propulsion may involve two kinetic centers at opposite ends of the sperm, which coordinate their movements to produce anti-clockwise rotation, stationary twisting, or clockwise rotation, in a manner reminiscent of spirochaetes. Littorina sperm also exhibit slower methods of propulsion including swimming backwards (tail first) at 18 μm, and 'gliding' at about 30 μm. The adaptive significance of the rapid propulsion is not obvious, because Littorina copulate and fertilize internally and at each stage in the transfer there are external aids to sperm transport, such as ciliary action (oviduct) and muscular expulsion (bursa and seminal receptacle). The filiform shape, however, is well-adapted for long-term storage in the female body. These points are discussed. [ABSTRACT FROM AUTHOR]

Nurse cells develop from diploid cells in the testis. Each cell undergoes a reduction division which leaves the nucleus with half the volume of a normal diploid cell. They send out pseudopodia which form desmosomelike junctions with developing spermatids. The nurse cells detach from the testicular wall, their nuclei degenerate and secretion droplets form in the cytoplasm. The pseudopodia are drawn in as the cytoplasmic secretions swell and the nurse cell becomes spherical. The eupyrene sperm become grouped unilaterally and at this stage are attached to the nurse cell by only the tips of their acrosomes. At maturity the nurse cells with their clumps of attached eupyrene sperm (spermatozeugmata) are released from the testis via ducts into the seminal vesicles, where they are stored prior to copulation. Nurse cells serve similar functions to those of apyrene sperm which are common among the Molluscs. We believe that the nurse cell and apyrene sperm are homologous. [ABSTRACT FROM AUTHOR]

Waste sperm and spermatozeugmata in the seminal vesicle of Littorina scutulata are phagocytised either by cell buds (large vesicles given off from the epithelial cells) or by the epithelial cells themselves. Cell buds containing sperm, are in turn engulfed by epithelial cells. In both cases, heterophagic vacuoles are formed inside the cell and subsequently the vacuoles are fused with primary lysosomes or lysosomal derivatives to become secondary lysosomes. Throughout this process the sperm are being digested. The second lysosome transforms further to telolysosome and finally to residual body when the sperm is completely digested. [ABSTRACT FROM AUTHOR]

The fine structure of the spermatogonium, spermatocyte and spermatid of a marine snail, Littorina sitkana is described. The ring centriole (annulus) is formed from the distal centriole and it migrates to the base of the mitochondrial region where it lies in a joint-like structure which is formed by an area of invaginated plasma membrane. The distal and proximal centrioles are at first perpendicular to each other but the proximal centriole rotates to a position coaxial with the distal centriole and fuses with it. The peripheral doublet fibers are continuous between the two centrioles but the central fibers originate only in the distal centriole. The acrosome differentiates from the proacrosomal granule which is derived from a Golgi body. Microtubules, present at this stage, may assist acrosomal formation. Chromatin condensation begins with the formation of fibrous strands, then to lamellar plates which become folded and later twisted around the flagellar shaft. In the final stages the lamellae appear in cross section as concentric rings which eventually fuse to form a homogeneously dense nuclear tube. [ABSTRACT FROM AUTHOR]

The filiform spermatozoon of Littorina contains a conical acrosome positioned anteriorly to the nucleus. The nucleus is essentially a tube of condensed chromatin surrounding the flagellar shaft and only the apical third is covered by a nuclear envelope. A structure resembling a basal body, which is either a modified proximal centriole, or a derivative of the distal centriole, terminates in a basal plate at the tip of the nucleus. Distally it gives rise to the nine outer flagellar doublets, which are continuous with the outer fibres of the distal centriole located 0.26 μ behind the 'basal body'. Thus the 'basal body' and distal centriole are lined up with each other and the axis of the flagellum. A supernumerary tubule connects the outer arm of the A subfibre of doublet 5 with the 'basal body'. The flagellar shaft extends from the apex of the nucleus to the tip of the tail. In the mitochondrial region the plasmalemma fuses with the outer mitochondrial sheath to form a series of pores that connect the mitochondria directly with the external environment. A distinct joint separates the mitochondrial and tail regions enabling the tail to beat perpendicularly to the main flagellar axis. The last section of this paper is given to a discussion of sperm motility in the light of these structural observations. [ABSTRACT FROM AUTHOR]