Reproductive biology of female big‐bellied seahorses

In this study, ovarian morphology, reproductive condition and sex steroid levels were investigated in the big‐bellied seahorse Hippocampus abdominalis , collected by snorkel and SCUBA diving in Wellington Harbour, New Zealand. Within the ovary, oocytes were contained between an outer muscular wall and an inner layer of luminal epithelium. Two germinal ridges ran along the entire length of the ovary. In cross‐section, oocytes were arranged in sequential order of development beginning at the germinal ridges and ending at the mature edge. Ovarian lamellae were absent. Vitellogenic and advanced cortical alveoli oocytes were elongated in shape, whereas maturing oocytes were distinctively pear‐shaped. Mature oocytes were large (2·6 – 4·4 mm in length) and aligned with the animal pole towards the muscular wall. Reproductively mature females were found throughout the year indicating a protracted reproductive season. The gonado‐somatic index was significantly different between all ovarian stages, but the hepato‐somatic index was not. Females with previtellogenic ovaries had significantly higher plasma concentrations of testosterone than females with vitellogenic or maturing ovaries. There was no significant difference in plasma concentrations of testosterone between females with vitellogenic or maturing ovaries, or in plasma concentrations of 17β‐oestradiol between females in all ovarian stages. This study contributes to the knowledge on the reproductive biology of female syngnathids.     

Proliferation of Oogonia and folliculogenesis in the viviparous teleost Ilyodon whitei (Goodeidae)

Oogonial proliferation in fishes is an essential reproductive strategy to generate new ovarian follicles and is the basis for unlimited oogenesis. The reproductive cycle in viviparous teleosts, besides oogenesis, involves development of embryos inside the ovary, that is, intraovarian gestation. Oogonia are located in the germinal epithelium of the ovary. The germinal epithelium is the surface of ovarian lamellae and, therefore, borders the ovarian lumen. However, activity and seasonality of the germinal epithelium have not been described in any viviparous teleost species regarding oogonial proliferation and folliculogenesis. The goal of this study is to identify the histological features of oogonial proliferation and folliculogenesis during the reproductive cycle of the viviparous goodeid Ilyodon whitei. Ovaries during nongestation and early and late gestation were analyzed. Oogonial proliferation and folliculogenesis in I. whitei, where intraovarian gestation follows the maturation and fertilization of oocytes, do not correspond to the late oogenesis, as was observed in oviparous species, but correspond to late gestation. This observation offers an example of ovarian physiology correlated with viviparous reproduction and provides elements for understanding the regulation of the initiation of processes that ultimately result in the origin of the next generation. These processes include oogonia proliferation and development of the next batch of germ cells into the complex process of intraovarian gestation. J. Morphol. 275:1004–1015, 2014. © 2014 Wiley Periodicals, Inc.     

An ultrastructural study of the ovary cord organization and oogenesis in the amphibian leech Batracobdella algira (Annelida,Clitellata, Hirudinida)

This study reveals the ovary micromorphology and the course of oogenesis in the leech Batracobdella algira (Glossiphoniidae). Using light, fluorescence, and electron microscopies, the paired ovaries were analyzed. At the beginning of the breeding season, the ovaries were small, but as oogenesis progressed, they increased in size significantly, broadened, and elongated. A single convoluted ovary cord was located inside each ovary. The ovary cord was composed of numerous germ cells gathered into syncytial groups, which are called germ-line cysts. During oogenesis, the clustering germ cells differentiated into two functional categories, i.e., nurse cells and oocytes, and therefore, this oogenesis was recognized as being meroistic. As a rule, each clustering germ cell had one connection in the form of a broad cytoplasmic channel (intercellular bridge) that connected it to the cytophore. There was a synchrony in the development of the clustering germ cells in the whole ovary cord. In the immature leeches, the ovary cords contained undifferentiated germ cells exclusively, from which, previtellogenic oocytes and nurse cells differentiated as the breeding season progressed. Only the oocytes grew considerably, gathered nutritive material, and protruded at the ovary cord surface. The vitellogenic oocytes subsequently detached from the cord and filled tightly the ovary sac, while the nurse cells and the cytophore degenerated. Ripe eggs were finally deposited into the cocoons. A comparison of the ovary structure and oogenesis revealed that almost all of the features that are described in the studied species were similar to those that are known from other representatives of Glossiphoniidae, which indicates their evolutionary conservatism within this family.

     

Ovaries of Tubificinae (Clitellata, Naididae) resemble ovary cords found in Hirudinea (Clitellata)

The ultrastructure of the ovaries and oogenesis was studied in three species of three genera of Tubificinae. The paired ovaries are small, conically shaped structures, connected to the intersegmental septum between segments X and XI by their narrow end. The ovaries are composed of syncytial cysts of germ cells interconnected by stable cytoplasmic bridges (ring canals) and surrounded by follicular cells. The architecture of the germ-line cysts is exactly the same as in all clitellate annelids studied to date, i.e. each cell in a cyst has only one ring canal connecting it to the central, anuclear cytoplasmic mass, the cytophore. The ovaries found in all of the species studied seem to be meroistic, i.e. the ultimate fate of germ cells within a cyst is different, and the majority of cells withdraw from meiosis and become nurse cells; the rest continue meiosis, gather macromolecules, cell organelles and storage material, and become oocytes. The ovaries are polarized; their narrow end contains mitotically dividing oogonia and germ cells entering the meiosis prophase; whereas within the middle and basal parts, nurse cells, a prominent cytophore and growing oocytes occur. During late previtellogenesis/early vitellogenesis, the oocytes detach from the cytophore and float in the coelom; they are usually enveloped by the peritoneal epithelium and associated with blood vessels. Generally, the organization of ovaries in all of the Tubificinae species studied resembles the polarized ovary cords found within the ovisacs of some Euhirudinea. The organization of ovaries and the course of oogenesis between the genera studied and other clitellate annelids are compared. Finally, it is suggested that germ-line cysts formation and the meroistic mode of oogenesis may be a primary character for all Clitellata.     

Ovarian germinal epithelium and folliculogenesis in the common snook, Centropomus undecimalis (Teleostei: centropomidae)

The ovarian germinal epithelium in the common snook, Centropomus undecimalis, is described. It consists of epithelial and prefollicle cells that surround germ cells, either oogonia or oocytes, respectively. The germinal epithelium borders a body cavity, the ovarian lumen, and is supported by a basement membrane that also separates the epithelial compartment of the ovarian lamellae from the stromal compartment. During folliculogenesis, the epithelial cells, whose cytoplasmic processes encompass meiotic oocytes, transform into prefollicle cells, which become follicle cells at the completion of folliculogenesis. The follicle is a derivative of the germinal epithelium and is composed of the oocyte and surrounding follicle cells. It is separated from the encompassing theca by a basement membrane. The cells that form the theca interna are derived from prethecal cells within the extravascular space of the ovarian stroma. The theca externa differentiates from undifferentiated cells within the stromal compartment of the ovary, from within the extravascular space. The theca interna and the theca externa are not considered to be part of the follicle and are derived from a different ovarian compartment than the follicle. Meiosis commences while oocytes are still within the germinal epithelium and proceeds as far as arrested diplotene of the first meiotic prophase. The primary growth phase of oocyte development also begins while oocytes are still within the germinal epithelium or attached to it in a cell nest. The definitions used herein are consistent between sexes and with the mammalian literature.     

Morphology and ultrastructure of the somatic cells in Astacus leptodactylus ovary

We defined the somatic environment in which female germinal cells develop, and performed ultrastructural analyses of various somatic cell types, with particular reference to muscle cells and follicle cells, that reside within the ovary at different stages of oogenesis. Our findings show that ovarian wall of the crayfish is composed of long muscle cells, blood cells, blood vessels and hemal sinuses. The follicle and germinal cells lie within a common compartment of ovarian follicles that is defined by a continuous basal matrix. The follicle cells form branching cords and migrate to surround the developing oocytes. A thick basal matrix separates the ovarian interstitium from ovarian follicles compartment. Transmission electron microscopy shows that inner layer of basal matrix invaginates deeply into the ovarian compartment. Our results suggest that before being surrounded by follicle cells to form follicles, oogonia and early previtellogenic oocytes reside within a niche surrounded by a basal matrix that separates them from ovarian interstitium. We found coated pits and coated vesicles in the cortical cytoplasm of previtellogenic and vitellogenic oocytes, suggesting the receptor mediated endocytosis for transfer of material from the outside of the oocytes, via follicle cells. The interstitial compartment between the inner muscular layer of the ovarian wall and the basal matrix of the ovarian follicle compartment contains muscle cells, hemal sinuses, blood vessels and blood cells. Granular hemocytes, within and outside the vessels, were the most abundant cell population in the ovarian interstitium of crayfish after spawning and in the immature ovary. J. Morphol. 277:118–127, 2016. © 2015 Wiley Periodicals, Inc.     

Ultrastructure of the ovary and oogenesis in the polychaete Capitella jonesi (Hartman, 1959)

Ultrastructural features of the ovary and oogenesis in the polychaete Capitella jonesi (Hartman, '59) have been described. The ovaries are paired, sac-like follicles suspended by mesenteries in the ventral coelom throughout the midbody region of the mature worm. Oogenesis is unsynchronized and occurs entirely within the ovary, where developing gametogenic stages are segregated spatially within a germinal and a growth zone. Multiplication of oogonia and differentiation of oocytes into the late stages of vitellogenesis occur in the germinal region of the ovary, whereas late-stage vitellogenic oocytes and mature eggs are located in a growth zone. Follicle cells envelop the oocytes in the germinal zone of the ovary and undergo hypertrophy and ultrastructural changes that correlate with the onset of vitellogenesis. These changes include the development of extensive arrays of rough ER and numerous Golgi complexes, formation of microvilli along the surface of the ovary, and the initiation of extensive endocytotic activity. Oocytes undergo similar, concomitant changes such as the differentiation of surface microvilli, the formation of abundant endocytotic pits and vesicles along the oolemma, and the appearance of numerous Golgi complexes, cisternae of rough ER, and yolk bodies. Yolk synthesis appears to occur by both autosynthetic and heterosynthetic processes involving the conjoined efforts of the Golgi complex and rough ER of the oocyte and the probable addition of extraovarian (heterosynthetic) yolk precursors. Evidence is presented that implicates the follicle cells in the synthesis of yolk precursors for transport to the oocytes. At ovulation, mature oocytes are released from the overy after the overlying follicle cells apparently withdraw. Bundles of microfilaments within the follicle cells may play a role in this withdrawal process.     

Ovary of the pipefish,Syngnathus scovelli

Gross dissection, light microscopy, and transmission electron microscopy were used to generate a detailed understanding of the ovarian anatomy of the pipefish, Syngnathus scovelli. The ovary is a cylindrical tube bounded by an outer layer consisting of a smooth muscle wall and an inner layer of luminal epithelium, with follicles sandwiched between the two layers. A remarkable feature of this ovary is a sequential pattern of follicle development. This pattern begins at the germinal ridge with a gradient of follicles of increasing developmental age extending to the mature edge. The germinal ridge is an outpocketed region of the luminal epithelium containing early germinal cells and somatic prefollicular cells. Therefore, the germinal ridge and luminal epithelium share the same ovarian compartment and follicle formation occurs within this compartment. The mature edge is defined as the site of oocyte maturation and ovulation. The outer ovarian wall contains unmyelinated nerve fibers throughout. Longitudinally oriented unmyelinated nerves are also observed near the smooth muscle bundles associated with the mature edge. Oocytes near the mature edge are polarized such that the germinal vesicle (nucleus) is generally oriented toward the luminal epithelium. The sandwichlike organization of the ovary results in follicles that have a shared theca. An extensive lymphatic network is also interspersed among the follicles. Thus, the exceptional features of the pipefish ovary make it particularly well suited for the examination of early events in oogenesis. Specifically, we characterize pipefish folliculogenesis in detail.     

Germinal vesicle breakdown is not fully dependent on MAPK activation in maturing oocytes of marine nemertean worms

Previously, it has been shown that oocytes of marine nemertean worms resume meiosis and undergo germinal vesicle breakdown (GVBD) following treatment with either natural seawater (NSW), or the neurohormone serotonin (5-hydroxytryptamine or 5-HT). In this investigation of the nemerteans Cerebratulus lacteus and Cerebratulus sp., immunoblots and kinase assays were used to compare the roles of two regulatory kinases: mitogen-activated protein kinase (MAPK) and Cdc2/cyclin B (referred to as maturation promoting factor or MPF). Based on such analyses, an ERK (extracellular signal regulated kinase) type of MAPK was found to be activated concurrently with Cdc2/cyclin B during NSW- and 5-HT-induced maturation. MAPK activation occurred prior to GVBD and seemed to be controlled primarily by phosphorylation rather than de novo protein synthesis. Inhibition of MAPK signaling by U0126 was capable of delaying but not permanently blocking Cdc2/cyclin B activation and GVBD in 5-HT treated oocytes and subsets of NSW-treated oocytes. Collectively such data indicated that GVBD is not fully dependent on MAPK activation, since Cdc2/cyclin B can apparently be activated by MAPK-independent mechanism(s) in maturing nemertean oocytes.     

Ovary organization and oogenesis in two species of Lumbriculida (Annelida,Clitellata)

The aim of the present study is to describe the organization of the ovary and mode of oogenesis at the ultrastructural level in two representatives of Lumbriculida – Lumbriculus variegatus and Stylodrilus heringianus. In both species studied, the ovaries are small and conically shaped structures that are attached to the intersegmental septum via a thin ligament. The ovaries are composed of germline cysts formed by germ cells interconnected by stable cytoplasmic bridges. As a rule, the cyst center is occupied by a poorly developed anuclear cytoplasmic mass, termed a cytophore, whereas the germ cells are located at the periphery of the cyst. Germline cysts are enveloped by somatic cells. The ovaries of the species studied are polarized, i.e., along the long axis of the ovary there is an evident gradient of germ cell development. The data obtained suggest ovary meroism, i.e., two categories of germ cells were found: oocytes, which continue meiosis, gather nutrients, grow and protrude into the body cavity, and nurse cells, which do not grow and are supposed to supply oocytes with cell organelles and macromolecules via the cytophore. The ovary structure and mode of oogenesis in the species studied were compared with those of other clitellate annelids. As a rule, in all clitellates studied to date, the ovaries are composed of germline cysts equipped with a cytophore and associated with somatic cells; however, the ovary morphology differs between taxa regarding several quantitative and qualitative features. The ovary organization and mode of oogenesis in L. variegatus and S. heringianus strongly resemble those found in Tubificinae and Branchiobdellida studied to date. Our results also support a sister-group relationship between Lumbriculida and a clade comprising ectoparasitic clitellates (i.e., Branchiobdellida, Acanthobdellida and Hirudinida) with Branchiobdellida as a plesiomorphic sister group to Acanthobdellida and Hirudinida.     

Oogenesis in the tropical land crab,Gecarcinus lateralis (Freminville)

Summary The ovaries of small and large adult Gecarcinus were studied histologically and histochemically at various stages in the annual cycle. At all seasons of the year, dividing cells are seen within germinal nests in the ovary. Following division, the cells within the germinal nest enlarge and appear to move out into the stroma, forming cords of young oocytes that become encapsulated by follicle cells. Glycogen, not demonstrable in cells within the germinal nests, is present in the perinuclear cytoplasm of both young and mature oocytes. Lipid is distributed peripherally in the cytoplasm of the oocytes. Deoxyribonucleoprotein is demonstrable within the nuclei of germinal nest cells and of the young oocytes; it is not detectable within the nuclei of the large oocytes. The histological observations suggest that oogenesis occurs throughout the reproductive life of Gecarcinus.Dedicated to Professor Berta Scharrer on her 60th birthday in love, respect and admiration. — This work was supported in part by U.S.P.H.S. Training Grant GM-102.I express my thanks to the late Dr. Helen W. Deane and Dr. Dorothy E. Bliss for their help and encouragement.     

Germinal epithelium, folliculogenesis, and postovulatory follicles in ovaries of rainbow trout, Oncorhynchus mykiss (Walbaum, 1792) (Teleostei, protacanthopterygii, salmoniformes)

The rainbow trout, Oncorhynchus mykiss (Walbaum, 1792), is a salmoniform fish that spawns once per year. Ripe females that had ovulated naturally, and those induced to ovulate using salmon gonadotropin-releasing hormone, were studied to determine whether follicles were forming at the time of spawning and to describe the process of folliculogenesis. After ovulation, the ovaries of postspawned rainbow trout were examined histologically, using the periodic acid-Schiff procedure, to stain basement membranes that subtend the germinal epithelium and to interpret and define the activity of the germinal epithelium. After spawning, the ovary contained a few ripe oocytes that did not ovulate, numerous primary growth oocytes including oocytes with cortical alveoli, and postovulatory follicles. The germinal epithelium was active in postspawned rainbow trout, as determined by the presence of numerous cell nests, composed of oogonia, mitotic oogonia, early diplotene oocytes, and prefollicle cells. Cell nests were separated from the stroma by a basement membrane continuous with that subtending the germinal epithelium. Furthermore, follicles containing primary growth oocytes were connected to the germinal epithelium; the basement membrane surrounding the follicle joined that of the germinal epithelium. After ovulation, the basement membrane of the postovulatory follicle was continuous with that of the germinal epithelium. We observed consistent separation of the follicle, composed of an oocyte and surrounding follicle cells, from the ovarian stroma by a basement membrane. The follicle is derived from the germinal epithelium. As with the germinal epithelium, follicle cells derived from it never contact those of the connective tissue stroma. As with epithelia, they are always separated from connective tissue by a basement membrane.     

Developmental morphology of the neonatal alligator (Alligator mississippiensis) ovary

American alligator (Alligator mississippiensis) ovary development is incomplete at hatching. During the months following hatching, the cortical processes of oogenesis started in ovo continues and folliculogenesis is initiated. Additionally, the medullary region of the gonad undergoes dramatic restructuring. We describe alligator ovarian histology at hatching, 1 week, 1 month, and 3 months of age in order to characterize the timing of morphological development and compare these findings to chicken ovary development. At hatching, the ovarian cortex presents a germinal epithelium containing oogonia and a few primary oocytes irregularly scattered between somatic epithelial cells. The hatchling medulla shows fragmentation indicative of the formation of lacunae. By 1 week of age, oocytes form growing nests and show increased interactions with somatic cells, indicative of the initiation of folliculogenesis. Medullary lacunae increase in diameter and contain secretory material in their lumen. At 1 month, nest sizes and lacunar diameters continue to enlarge. Pachytene oocytes surrounded by somatic cells are more frequent. Trabeculae composed of dense irregular connective tissue divide cortical nests. Three months after hatching oocytes in meiotic stages of prophase I up to diplotene are present. The ovary displays many enlarged follicles with oocytes in diplotene arrest, thecal layers, lampbrush chromosomes, and complete layers of follicular cells. The medulla is an elaborated complex of vascularized lacunae underlying the cortex and often containing discrete lymphoid aggregates. While the general morphology of the alligator ovary is similar to that of the chicken ovary, the progression of oogenesis and folliculogenesis around hatching is notably slower in alligators. Diplotene oocytes are observed at hatching in chickens, but not until 3 months in alligators. Folliculogenesis is completed at 3 weeks in chickens whereas it is still progressing at 3 months in alligators.     

Localization of a carbohydrate antigen associated with growing oocytes and ovarian surface epithelium.

We used a monoclonal antibody (PS1) to a carbohydrate antigen to study the development of the oocyte and follicle during early stages of differentiation in several mammalian species. This antigen has been shown to localize within the cytoplasm of oocytes in primordial follicles as well as in growing oocytes. It is also localized within distinct layers of the zona pellucida (ZP) of developing follicles. Although this antibody was made against a specific ZP glycoprotein, the antigen also appears to be abundant in cells of the ovarian surface epithelium (OSE). The localization of this carbohydrate moiety has been observed in ovaries of rabbits of different ages as well as in the ovarian surface epithelium of other mammalian species including cat, cynomolgus monkey, baboon, and human. These studies demonstrate that there is an abundant carbohydrate antigenic determinant which is associated with both the mammalian oocyte and the ovarian surface epithelium but which is not apparent in other ovarian cell types or in non-ovarian secretory epithelium. This antibody probe should provide a valuable tool for studying the development and differentiation of the ovary, since this antigen is associated with two highly differentiated but distinct cell types.     

Ecological studies of the nemertean fauna in an estuarine system of the northwestern Gulf of Mexico

The distribution and abundance of nemerteans in the brackish-water lakes of Sea Rim State Park, Texas, near the Louisiana border, were studied and compared with other macrobenthos during one year. Six of 93 macrobenthic species collected were nemerteans (0.9% of the total number of specimens). Only one species of nemertean, Carinoma sp., was consistently present. This species is the most ubiquitous and, probably, the most abundant nemertean in the estuarine systems of the Texas coast. Carinoma sp. was collected at Sea Rim from a salinity range of 0–21 ppt and at other Texas estuaries from 2–26 ppt. Preliminary experiments with Carinoma sp. as predator and as prey indicated that it feeds on polychaete worms and in turn is fed upon by white (Penaeus setiferus) and brown (P. aztecus) shrimp.     

Ultrastructural investigations of the ovary and oogenesis in the pycnogonids Cilunculus armatus and Ammothella biunguiculata (Pycnogonida, Ammotheidae)

Abstract. Ovarian ultrastructure and oogenesis in two pycnogonid species, Cilunculus armatus and Ammothella biunguiculata , were investigated. The ovary is morphologically and functionally divided into trunk and pedal parts. The former represents the germarium and contains very young germ cells in a pachytene or postpachytene phase, whereas the latter houses developing previtellogenic and vitellogenic oocytes and represents the vitellarium. Intercellular bridges were occasionally found between young (trunk) germ cells. This indicates that in pycnogonids, as in other animal groups, at the onset of oogenesis clusters of germ cells are generated. As nurse cells are absent in the ovaries of investigated species, the clusters must secondarily split into individual oocytes. In the vitellarium, the oocytes are located outside the ovary. Each oocyte is connected to the ovarian tissue by a stalk composed of several somatic cells. The stalk cells directly associated with the oocyte are equipped with irregular projections that reach the oocyte plasma membrane. This observation suggests that the stalk cells may play a nutritive role. The ooplasm of vitellogenic oocytes comprises mitochondria, free ribosomes, stacks of annulate lamellae, active Golgi complexes, and vesicles derived from these complexes. Within the latter, numerous electron-dense bodies are present. We suggest that these bodies contribute to yolk formation.     

Ovary architecture of two branchiobdellid species and Acanthobdella peledina (Annelida, Clitellata)

The aim of this study was to present data about ovary organization and oogenesis in two small groups of clitellate annelids, i.e. in representatives of Acanthobdellida (Acanthobdella peledina) and Branchiobdellida (Branchiobdella pentodonta and Branchiobdella parasitica), and to compare them to ovaries known from true leeches and oligochaetous clitellates. In A. peledina, the ovaries have the form of elongated cords, termed ovary cords, and are enveloped by coelomic sacs, the so-called ovisacs. The ovisacs are paired and each one contains only one ovary cord. The morphology and structure of the ovary cords depend on the maturity level of the animal. In young specimens the ovary cords are short and contain mainly oogonial cells and germ cells entering meiosis. Oogonia divide mitotically without full cytokineses, and as a result germ-line cysts are formed. As the animals grow, the cords become more elongated and the germ cells within the cords differentiate into nurse cells and oocytes. Oocytes gather cell organelles and, finally, detach from the ovary cord and float freely in the ovisac lumen.In both examined branchiobdellidans the ovaries are also paired. They are short and conical and are not enclosed within ovisacs. The narrow end of each ovary is connected to the intersegmental septum via a ligament, whereas the outermost (broad) end of the ovary extends freely into the coelom. The ovaries are polarized. Their narrow ends contain oogonia, whereas nurse cells and growing oocytes, gradually projecting from the ovary, can be found in their middle and outermost parts. Early vitellogenic oocytes detach from the ovary and float freely in the coelom.In all of the species studied, the ovaries are made up of germ-line cysts associated with somatic (follicular) cells. The architecture of a germ-line cyst is exactly the same as in other clitellate annelids that have been studied to date. Each germ cell in a cyst has one stable cytoplasmic bridge connecting it with a central anuclear cytoplasmic mass, a cytophore. The fate of germ cells constituting cysts is diverse. The majority of the cells withdraw from meiosis and become nurse cells; only a few continue meiosis, grow and become oocytes. The meroistic mode of oogenesis is suggested. We suggest also that the formation of germ-line cysts and ovary meroism should be regarded as basal conditions for all Clitellata. The occurrence of ovisacs enveloping the ovaries in A. peledina and Hirudinida is regarded as a synapomorphy of both groups, whereas ovaries found in B. pentodonta and B. parasitica have no ovisacs and resemble ovaries described in Oligochaeta sensu stricto.     

Ovary of the seahorse,Hippocampus erectus

The ovary of the seahorse, Hippocampus erectus, is a cylindrical tube bounded by an outer layer consisting of a mesothelium and muscular wall and by an inner luminal epithelium, with a single row of developing follicles sandwiched between the two layers. Follicles are produced by a germinal ridge, which contains oogonia, early oocytes, and prefollicle cells, and which runs along the length of the ovary. The germinal ridge is an outpocketing of the luminal epithelium, as indicated by a continuous underlying basal lamina. Prefollicle cells invest diplotene oocytes and the complex eventually pinches off the germinal ridge as a primordial follicle surrounded by a basal lamina derived from the germinal ridge. Subsequent investment of the primordial follicle by elements of the theca complete the process of folliculogenesis. H. erectus has two ovaries and each ovary has two dorsally located germinal ridges. Thus, in each ovary the derived follicular lamina is bilaterally symmetrical: two temporally and spatially arranged sequences of developing follicles are produced, with the largest follicles found along the ventral midline of the ovary. The advantages of developmental, kinetic, and systemic analyses of these unusual ovaries are indicated.     

Phylogenetic significance of the structure of adult ovary and oogenesis in a primitive chilognathan diplopod,Hyleoglomeris japonica Verhoeff (Glomerida,Diplopoda)

Some histological details of the adult ovary of Hyleoglomeris japonica are described for the first time in the glomerid diplopods. The ovary is a single, long sac-like organ extending from the 4th to the 12th body segment along the median body axis, lying between the alimentary canal and the ventral nerve cord. The ovarian wall consists of a layer of thin ovarian epithelium which surrounds a wide ovarian lumen. A pair of longitudinal “germ zones,” including female germ cells, runs in the lateral ovarian wall. Each germ zone consists of two types of oogenetic areas: 1) 8–12 narrow patch-shaped areas for oogonial proliferation, arranged metamerically in a row along each of the dorsal and ventral peripheries, and 2) the remaining wide area for oocyte growth. Oogonial proliferation areas include oogonia, very early previtellogenic oocytes, and young somatic interstitial cells, among the ovarian epithelial cells. The larger early previtellogenic oocytes in the oogonial proliferation areas are located nearer to the oocyte growth area, and migrate to the oocyte growth area. They are surrounded by a layer of follicle cells and are connected with the ovarian epithelium of the oocyte growth area by a portion of their follicles. They grow into the ovarian lumen, but their follicles are still connected with the oocyte growth area. Various sizes of the previtellogenic and vitellogenic oocytes in the ovarian lumen are connected with the oocyte growth area; the smaller oocytes are connected nearer to the dorsal and ventral oogonial proliferation areas, while the larger ones are connected nearer to the longitudinal middle line of the oocyte growth area. Following the completion of vitellogenesis and egg membrane formation in the largest primary oocytes, the germinal vesicles break down. Ripe oocytes are released from their follicles directly into the ovarian lumen to be transported into the oviducts. Ovarian structure and oogenesis of H. japonica are very similar to those of other chilognathan diplopods. At the same time, however, some characteristic features of the ovary of H. japonica are helpful for understanding the structure and evolution of the diplopod ovaries. Some aspects of the phylogenetic significance in the paired germ zones of H. japonica are discussed. J. Morphol 231:277–285, 1997. © 1997 Wiley-Liss, Inc.     

Oogenesis and oocytes

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.