Kevin J. Eckelbarger

Ultrastructure of the gonad and gametogenesis in the eastern oyster,Crassostrea virginica. I. Ovary and oogenesis

The ultrastructural features of the ovary and oogenesis are described in the eastern oyster,Crassostrea virginica (Gmelin, 1791). The ovary is a diffuse organ consisting of highly branching acini in which oocytes develop. The acini are surrounded by a matrix of vesicular connective tissue (VCT tells) which serves a nutrient storage function. Each acinus is bathed by fluid within a surrounding connective tissue compartment, the hemocoel, which likely serves as a means of transporting nutrients to the oocytes. Oocytes begin growth while positioned near to the inner acinus wall. As differentiation proceeds and they enter the late stages of vitellogenesis, they become stalle-shaped and project into the acinus lumen. Follicle tells are closely associated with oocytes during the early and middle stages of vitellogenesis but they are largely confined to the basal, stalked region of late-stage oocytes. Vitellogenesis occurs through a process of autosynthesis, involving the combined activity of the Golgi complex and rough endoplasmic reticulum, and heterosynthesis in which extraovarian precursors are incorporated into oocytes via receptor-mediated endocytosis involving the basal surface of the oocytes. It is suggested that the follicle tells play some important role during oogenesis but probably are not the major source of yolk precursors. The VCT celas are probably the main source of nutrients for vitellogenesis.

Ultrastructure of Sperm Development and Mature Sperm Morphology in Three Species of Commensal Bivalves (Mollusca: Galeommatoidea)

Ultrastructural features of the ovotestes, spermatogenesis, and the mature sperm are described for three galeommatid bivalves, Divariscintilla yoyo, Divariscintilla troglodytes, and Scintilla sp., from stomatopod burrows in eastern Florida. All three species yielded similar results except with respect to mature sperm dimensions. The ovotestis contains three types of somatic cells within the testicular portion: flattened myoepithelial cells defining the outer acinal wall; underlying pleomorphic follicle cells containing abundant glycogen deposits; and scattered, amoeboid cells containing lysosomal‐like inclusions which are closely associated with developing sperm. Early spermatogenesis is typical of that reported from other bivalves. In contrast, the late stages of spermiogenesis involve the migration and gradual rotation of the acrosomal vesicle, resulting in a mature acrosome tilted about 70° from the long axis of the cell. The mature sperm possesses an elongated, slightly curved nucleus; a subterminal, concave acrosome with a nipple‐like central projection; five spherical mitochondria and two centnoles in the middlepiece; and a long flagellum. The rotational asymmetry and the presence of perimitochondrial glycogen deposits in these sperm are unusual in the Bivalvia and may be associated with fertilization specializations and larval brooding common among galeommatoideans.

Ultrastructure of the gonad and gametogenesis in the eastern oyster,Crassostrea virginica. II. Testis and spermatogenesis

The ultrastructural features of the testis and spermatogenesis have been described from the eastern oyster,Crassostrea virginica (Gmelin, 1791). The testis is a diffuse organ consisting of branching acini containing differentiating sperm in a variety of stages. Spermatogonia are located nearest the outer wall of the acinus, while spermatocytes and spermatids are positioned nearer the lumen. Mature spermatozoa are all confined to the central region of the acinus. The acinus is surrounded by an intermittent layer of myoepithelial cells and is bathed by a

ULTRASTRUCTURAL DIFFERENCES IN THE EGGS AND OVARIAN FOLLICLE CELLS OF CAPITELLA (POLYCHAETA) SIBLING SPECIES

uid-filled hemocoel. Vesicular connective tissue (VCT cells) fills the region between adjacent acini, and the cells contain glycogen granules and lipid droplets. Each acinus is divided radially into subcompartments that are partially separated by pleomorphic accessory cells which remain in close contact with sperm until late stages of development. Sperm are similar to those described in other oysters, except that five midpiece mitochondria were observed in some sperm rather than the usual four, and the acrosomal vesicle lacked the “whorled” substructure described in some other oyster sperm. We suggest that the neutral term “accessory cells” be applied to bivalve testicular somatic cells until more detailed studies are available to justify the use of “Sertoli cell” and other descriptive terms which have previously been adopted from other taxa only distantly related to bivalves.

An ultrastructural study of spermatogenesis in the nudibranch mollusc Spurilla neapolitana

Ultrastructural studies of ovarian follicle cells and mature eggs in four sibling species in the polychaete genus Capitella have revealed distinct and consistent morphological differences that parallel in some respects the differences between the species in egg size, and embryonic and larval development. Capitella spp. I and II are extremely similar in all respects: the follicle cells lack lipid and contain a modest amount of glycogen; the mature eggs are rich in lipid and glycogen and contain very similar proteid yolk granules. In both species mature eggs have a characteristic electron-dense band and a zone of mitochondria in the cortical ooplasm. These sympatric species have eggs that are similar in size and lecithotrophic larvae that are planktonic for only a short time. Capitella sp. III (Capitella jonesi) has ovarian follicle cells containing a small amount of lipid and no glycogen, while the mature eggs have a small amount of lipid, abundant glycogen, and large proteid yolk granules. These small eggs show no evidence of an electron-dense band or any concentration of mitochondria in the cortical ooplasm. This species has planktotrophic larvae that remain in the plankton for many weeks. Capitella sp. IIIa has ovarian follicle cells rich in both lipid and glycogen. The large mature eggs are rich in lipid, have relatively little glycogen, and have abundant proteid yolk granules. The cortical ooplasm contains electron-dense material similar to that observed in the eggs of species I and II but it is distributed in a discontinuous band. This species has direct development, and juvenile worms emerge from the parental brood tube after metamorphosis. The egg envelopes and microvilli of the eggs of all four sibling species undergo substantial morphological changes following release from the ovary into the coelom. The significance of these morphological and biochemical differences between the species is not known, but the lack of intraspecific variation in these characters suggests that their presence or absence reflects specific differences in the processes of yolk formation and utilization.

Oogenesis and oocytes

The ultrastructural features of spermatogenesis were investigated in the nudibranch mollusc Spurilla neapolitana. Sperm develop in the proximal half of numerous sac‐like acini which are radially arranged within about ten ovotestis lobes. Accessory cells line the inner wall of the testicular portion of each acinus and are connected to developing sperm by numerous desmosomes. Stages of spermatid development have been divided into precup, cup, postcup, and elogate stages depending on the general shape of the nucleus. Nuclear differentiation includes the formation of anterior and posterior nuclear plaques, condensation of chromatin fibrils into nuclear lamellae, the insertion of the developing flagellar axoneme into a shallow, nuclear implantation fossa, and eventual formation of an elongated sperm head with a terminal twist. Spermiogenesis also includes the differentiation of an anterior, perinuclear structure having the characteristics of an acrosome, the appearance of peculiar arrays of ER cisternae, and the fusion of mitochondria into a large mitochondrial derivative which eventually encircles the axoneme, forming a crystalline‐like periaxonemal sheath.

Ultrastructure of the ovary and oogenesis in the jellyfish Linuche unguiculata and Stomolophus meleagris, with a review of ovarian structure in the Scyphozoa

The morphological features of polychaete ovarian morphology and oogenesis are reviewed. Some basic information on ovarian structure and/or oogenesis is known for slightly more than half of recognized polychaete families although comprehensive studies of oogenesis have been conducted on ∼0.1% of described species. Relative to other major metazoan groups, ovarian morphology is highly variable in the Polychaeta. While some species appear to lack a defined ovary, most have paired organs that are segmentally repeated to varying degrees depending on the family. Ovaries vary widely in their location but are most frequently associated with the coelomic peritoneum, parapodial connective tissue, or elements of the circulatory system. The structural complexity of the ovary is correlated with the type of oogenesis expressed by the species. In some polychaetes, extraovarian oogenesis occurs in which previtellogenic oocytes are released into the coelom from a simple ovary where differentiation occurs in a solitary fashion or in association with nurse cells or follicle cells. In other species, intraovarian oogenesis occurs in which oocytes undergo vitellogenesis within the ovary, often in association with follicle cells that may provide nutrition. Vitellogenesis probably includes both autosynthetic and heterosynthetic processes; autosynthesis involves the manufacture of yolk bodies via the proteosynthetic organelles of the oocyte whereas heterosynthesis involves the extraovarian production of female-specific yolk proteins that are incorporated into the oocyte through a receptor-mediated process of endocytosis. Variation in the speed of egg production varies widely and appears to be correlated with the vitellogenic mechanism employed. Mature ova display a wide range of egg envelope morphologies that often show some intrafamilial similarities.

Ovarian ultrastructure and vitellogenesis in ten species of shallow-water and bathyal sea cucumbers (Echinodermata : holothuroidea)

Ovarian structure and oogenesis has been examined in six scyphozoan species including the semaeostome Diplumularis antarctica Maas, 1908 (collected in 1987 in McMurdo Sound, Antarctic), the rhizostomes Cassiopea xamachana Bigelow, 1892 (collected in Belize in 1988), and Stomolophus meleagris L. Agassiz, 1862 (collected in Ft. Pierce Inlet in 1988), and the coronates Periphylla periphylla (Peron and Lesueur, 1810), Nausithoe atlantica Broch, 1914 (both collected in the Bahamas in 1988), and Linuche unguiculata (Schwartz, 1788) (collected in Nassau Harbor, Bahama Islands in 1989). Based on these findings and information on five other scyphozoan species from additional literature sources, at least two fundamentally different types of ovaries exist in the Scyphozoa. In semaeosotome and rhizostome species, oocytes develop in close association with specialized gastrodermal cells called trophocytes which may serve a nutritive function. However, coronate species lack trophocytes and oocytes develop freely in the mesoglea. The ovaries of S. meleagris and L. unguiculata are used as models to represent the ultrastructural events occurring during oogenesis in species having trophocytes and those lacking them, respectively. In both L. unguiculata and S. meleagris, the ovaries arise as evaginations of the gastrodermis in the floor of interradial pouches. Germ cells appear to originate from endodermally-derived gastrodermal cells and migrate into the mesoglea prior to vitellogenesis. In L. unguiculata, the oocytes develop freely within the mesoglea throughout vitellogenesis, while in S. meleagris each oocyte maintains contact with specialized gastrodermal cells called trophocytes. In the vitellogenic oocytes of both species, numerous invaginations of the oolemma result in the formation of intraooplasmic channels throughout the ooplasm. These channels are intimately associated with cisternae of rough endoplasmic reticulum and may play some role in yolk precursor uptake by substantially increasing the surface area of the oocyte. Vitellogenesis is similar in both species and involves the autosynthetic activity of the Golge complex and rough endoplasmic reticulum, and the heterosynthetic incorporation of yolk precursors through receptor-mediated endocytosis. However, in the oocytes of S. meleagris, the trophocytes probably play a role in the transfer of nutrients from the gastrovascular cavity to the oocyte. The present study suggests that scyphozoans were among the first metazoans to develop ovarian accessory cells during their reproductive evolution. The trophocyte-oocyte association observed in some scyphozoans is similar to but structurally less complex than the trophonema-oocyte association described from anthozoans. Scyphozoan ovarian morphology helps support the view that the Scyphozoa share a closer phylogenetic relationship with the Anthozoa than with the Hydrozoa.

Ultrastructure and Development of Dimorphic Sperm in the Abyssal Echinoid Phrissocystis multispina (Echinodermata:Echinoidea): Implications for Deep Sea Reproductive Biology

Comparative ultrastructural features of the ovary and vitellogenesis have been described for six shallow water and four bathyal species of sea cucumbers representing four major holothuroid orders. Ovarian structure is similar in all ten species except for features of the peritoneal cells of the outer layer and the follicular inner epithelial cells surrounding the developing oocytes. The peritoneal cells vary from monociliated squamous or cuboidal cells to large columnar cells. Ultrastructural evidence suggests that these cells might be capable of incorporating materials from the perivisceral coelom. The follicular inner epithelial cells of two deep-sea species resemble podocytes, a feature previously unre-ported in holothuroid ovaries. It is suggested that these cells function to increase nutrient exchange between the genital haemal sinus and the oocyte during vitellogenesis. In all ten species, the oocytes appear to participate in yolk synthesis through the interaction of the Golgi complex and rough endoplasmic reticulum. The similarity in the ultrastructural features of vitellogenesis suggests that the process of yolk synthesis has been highly conserved in holothuroids. Endocytotic activity was detected in seven of ten species but it is uncertain if this is directly related to vitellogenesis. Cilia and intracellular structures resembling striated ciliary rootlets were observed in the oocytes of four of the ten species studied. The significance of this finding is unclear but could indicate that germ cells have a somatic cell origin.

Modified Sperm Ultrastructure in Four Species of Soft- Bodied Echinoids (Echinodermata: Echinothuriidae) From the Bathyal Zone of the Deep Sea

Mature males of the abyssal echinoid Phrissocystis multispina produce two types of sperm including a euspermatozoon typical of echinoids, and a paraspermatozoon, which is bipolar-tailed. The structure of the testis and most features of spermatogenesis are similar to that of other echinoids. Development of both sperm types is identical until the spermatid stage when the nucleus of the paraspermatozoon undergoes chromatin reduction. Both sperm types have acrosomes typical of other echinoid sperm. However, we never observed a Golgi complex during any stage of sperm differentiation so the origin of the acrosome is unclear. Both the distal and proximal centrioles are involved in the formation of an anteriorly and posteriorly directed flagellum in the paraspermatozoon. Mixtures of both sperm types tend to clump due to the entanglement of sperm axonemes in the paraspermatozoon flagellum. Although the function of the paraspermatozoa is unknown, they may play a role in facilitating fertilization through the reduction of euspermatozoon diffusion during spawning. This study reports only one of several recently discovered reproductive adaptations associated with deep-sea habitats.