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.     

Studies on the annual reproductive cycle of the female cobra,Naja naja. IV. OVARIAN HISTOLOGY

Morphological changes of the ovary of the Chinese cobra, Naja naja, throughout the annual reproductive cycle are described. A single clutch of between 6 and 22 eggs is produced in late June. From July to the following April the ovary remains quiescent and contains small previtellogenic, hydration stage follicles. The growth of an ovarian follicle from a primary oocyte to maturation and ovulation is estimated to take three years. The histology of the germinal epithelium and the follicular granulosa shows seasonal changes correlated with the growth of the oocyte. During the quiescent period, the germinal epithelium lacks mitotic activity, but during April, when yolk deposition and rapid growth of the preovulatory follicles take place, the germinal epithelium shows intense mitotic activity. The growth of the smallest hydration stage follicles, and the occurrence of cytoplasmic bridges between the pyriform cells of the granulosa and the developing oocyte, also appear to increase during this period. The possible function of the pyriform cell is discussed and the literature on the origin and fate of these cells in the squamate ovary is reviewed. Postovulatory follicles (corpora lutea) and two types of atresia are described and compared with what is known of these structures in other reptiles.     

Development and fate of the postovulatory follicle complex,postovulatory follicle,and observations on folliculogenesis and oocyte atresia in ovulated common snook,Centropomus undecimalis (Bloch, 1792)

The common snook, Centropomus undecimalis , was induced to ovulate using a time‐release, GnRH analogue. Ovulation occurred the afternoon or evening the day after hormone administration. The time of ovulation was established within half an hour. At ovulation, three fish per time‐group were divided into 0, 6, 12, 18 hr and one thru five days post‐ovulation to study changes in the postovulatory follicle complex (POC). Histology of the ovaries revealed changes in the POC, postovulatory follicle (POF) and oocyte atresia through five days post‐ovulation. Within 24 hr, nuclei of the POF cells lost their initial spherical or oval configuration, and by four days the basement membrane within the POC had fragmented. There was a temporal separation between ovulation and post‐ovulation folliculogenesis; that is, in that the formation of new follicles commenced within the germinal epithelium between 12–48 hrs after ovulation. Morphology of the POC was best revealed with the reticulin stain; it is composed of the POF and postovulatory theca (POT). These are separated by a basement membrane, reflecting the origin of a follicle from a germinal epithelium while the theca is derived from stroma. The POF is composed of the former follicle cells that surrounded and contacted the oocyte during its development; the follicle is composed of the oocyte and its surrounding follicle cells. The POC is composed of a prominent basement membrane separating the POT from the POF. The reticulin stain clearly defines compartmentation in the ovary and supports redefinition of the POF as the follicle cells that formerly surrounded the oocyte prior to ovulation. J. Morphol. 278:547–562, 2017. © 2017 Wiley Periodicals, Inc.     

Development of the germinal epithelium and early folliculogenesis during ovarian morphogenesis in the cichlid fish Cichlasoma dimerus (Teleostei,Perciformes)

Formation of the germinal epithelium and folliculogenesis during ovarian development in Cichlasoma dimerus were described at the light‐ and electron‐microscopic levels. Prior to gonadal differentiation, germ cells and enveloping support cells reside within an inpocketing of the coelomic epithelium. Separation of the germinal and interstitial compartments of the gonad by a basement membrane is apparent from early gonadal development. Upon ovarian differentiation, oogonia undergo cyst‐forming divisions leading to the formation of clusters of interconnected cystocytes that synchronously enter meiosis, becoming oocytes. At the pachytene step, each oocyte becomes individualized by cytoplasmic extensions of prefollicle cells, thereby developing as an ovarian follicle. Subsequent somatic reorganization leads to the formation of the ovarian lumen in a cephalo‐caudal gradient. As a result, the germinal epithelium becomes internalized and lines the ovarian lumen. As defined by its origin from the germinal epithelium, the ovarian follicle is composed of an oocyte and the surrounding follicle cells. Thecal cells derived from the stroma encompass the basement membrane outside the follicle, thus forming a follicle complex. A common basement membrane is shared by the germinal epithelium and the follicle complex along a small portion of its surface. This point of attachment represents the site at which the oocyte would be released to the ovarian lumen during ovulation.     

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.     

Oogenesis: From Oogonia to Ovulation in the Flagfish,Jordanella floridae Goode and Bean, 1879 (Teleostei: Cyprinodontidae)

We provide histological details of the development of oocytes in the cyprinodontid flagfish, Jordanella floridae. There are six stages of oogenesis: Oogonial proliferation, chromatin nucleolus, primary growth (previtellogenesis [PG]), secondary growth (vitellogenesis), oocyte maturation and ovulation. The ovarian lamellae are lined by a germinal epithelium composed of epithelial cells and scattered oogonia. During primary growth, the development of cortical alveoli and oil droplets, are initiated simultaneously. During secondary growth, yolk globules coalesce into a fluid mass. The full‐grown oocyte contains a large globule of fluid yolk. The germinal vesicle is at the animal pole, and the cortical alveoli and oil droplets are located at the periphery. The disposition of oil droplets at the vegetal pole of the germinal vesicle during late secondary growth stage is a unique characteristic. The follicular cell layer is composed initially of a single layer of squamous cells during early PG which become columnar during early vitellogenesis. During primary and secondary growth stages, filaments develop among the follicular cells and also around the micropyle. The filaments are seen extending from the zona pellucida after ovulation. During ovulation, a space is evident between the oocyte and the zona pellucida. Asynchronous spawning activity is confirmed by the observation that, after ovulation, the ovarian lamellae contain follicles in both primary and secondary growth stages; in contrast, when the seasonal activity of oogenesis and spawning ends, after ovulation, the ovarian lamellae contain only follicles in the primary growth stage. J. Morphol. 277:1339–1354, 2016. © 2016 Wiley Periodicals, Inc.     

Ovarian structure and oogenesis of the extremophile viviparous teleost Poecilia mexicana (Poeciliidae) from an active sulfur spring cave in Southern Mexico

The structure of the ovary and oogenesis of Poecilia mexicana from an active sulfur spring cave is documented. Poecilia mexicana is the only poeciliid adapted to a subterranean environment with high hydrogen sulfide levels and extreme hypoxic conditions. Twenty females were captured throughout one year at Cueva del Azufre, located in the State of Tabasco in Southern Mexico. Ovaries were processed with histological techniques. P. mexicana has a single, ovoid ovary with ovigerous lamella that project to the ovarian lumen. The ovarian wall presents abundant loose connective tissue, numerous melanomacrophage centers and large blood vessels, possibly associated with hypoxic conditions. The germinal epithelium bordering the ovarian lumen contains somatic and germ cells forming cell nests projecting into the stroma. P. mexicana stores sperm in ovarian folds associated with follicles at different developmental phases. Oogenesis in P. mexicana consisted of the following stages: (i) oogonial proliferation, (ii) chromatin nucleolus, (iii) primary growth, subdivided into: (a) one nucleolus, (b) multiple nucleoli, (c) droplet oils‐cortical alveoli steps; (iv) secondary growth, subdivided in: (a) early secondary growth, (b) late secondary growth, and (c) full grown. Follicular atresia was present in all stages of follicular development; it was characterized by oocyte degeneration, where follicle cells hypertrophy and differentiate in phagocytes. The ovary and oogenesis are similar to these seen in other poeciliids, but we found frequent atretic follicles, melanomacrophage centers, reduced fecundity and increased of offspring size.     

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.     

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.     

Developmental profiles of activin betaA, betaB, and follistatin expression in the zebrafish ovary: evidence for their differential roles during sexual maturation and ovulatory cycle

Our recent experiments showed that gonadotropin(s) stimulated activin betaA and follistatin expression through the cAMP-PKA pathway but suppressed betaB via a cAMP-dependent but PKA-independent pathway in cultured zebrafish follicle cells. Given that pituitary gonadotropins are the major hormones controlling the development and function of the ovary, the differential expression of activin betaA and betaB as well as follistatin in response to gonadotropin(s) raises an interesting question about the temporal expression patterns of these molecules in vivo during sexual maturation and ovulatory cycle. Three experiments were performed in the present study. In the first experiment using sexually immature zebrafish, we followed the expression of activin betaA, betaB, and follistatin at the whole ovary level during a 10-day period in which the ovary developed from the primary growth stage to the one with nearly full-grown follicles. Activin betaA expression was very low at the primary growth stage but significantly increased with the growth of the ovary, and its rise was accompanied by an increase in follistatin expression. In contrast, the expression of activin betaB could be easily detected in the ovary of all stages; however, it did not exhibit an obvious trend of variation during the development. The second experiment examined the stage-dependent expression of activin betaA, betaB, and follistatin at the follicle level in the adult mature zebrafish. The expression of activin betaA was again low in the follicles during the primary growth stage, but exhibited a phenomenal increase after the follicles entered vitellogenesis with the peak level reached at midvitellogenic stage; in contrast, activin betaB mRNA could be easily detected at all stages with a slight increase during follicle growth. The expression of follistatin, on the other hand, also increased significantly during vitellogenesis; however, its level dropped sharply after reaching the peak at the midvitellogenic stage. In the third experiment, we investigated the dynamic changes of the ovarian activin betaA, betaB, and follistatin expression during the daily ovulatory cycle. The expression of activin betaA and follistatin gradually increased from 1800 h onward and reached the peak level around 0400 h when the germinal vesicles had migrated to the periphery in the full-grown oocytes. In contrast, activin betaB expression steadily declined, although not statistically significant, during the same period, but increased sharply at 0700 h when mature oocytes started to appear in most of the ovaries collected. In conclusion, activin betaA and betaB exhibit distinct expression patterns during the development of the ovary and the daily ovarian cycle of the zebrafish. It seems that activin betaA is involved in promoting ovary and follicle growth, whereas activin betaB may have a tonic role throughout follicle development but becomes critical at the late stage of oocyte maturation and/or ovulation.     

Mechanism of internal fertilization in Pegea socia (Tunicata,Thaliacea), a salp with a solid oviduct

The ovary of the salp Pegea socia (Bosc, 1802) is located at the end of an atrial diverticulum. The ovary consists of a single oocyte encased in a layer of follicle cells and is connected to the atrial epithelium by an oviduct. Transmission electron microscopy shows that the oocyte lacks a vitelline layer, cortical granules, and yolk granules and that the oviduct lacks a continuous lumen. What previous authors thought was a lumen is a line of dense intercellular junctions running down the center of the oviduct. The sperm nucleus in this species, as in other salps, is elongate. The tubular mitochondrion spirals about the sperm nucleus giving it a corkscrew-shape appearance. Sperm reach the ovary when the oocyte is still at the germinal vesicle stage. Many sperm swim up the atrial diverticulum and burrow through the cells of the atrial epithelium, oviduct, and follicular epithelium. Thus oviduct shortening, which occurs when the oocyte is in the meiotic divisions, is evidently unrelated to sperm moving up the oviduct. All previous authors, who argued either that a continuous lumen is necessary for sperm to move up the oviduct or that sperm bypass the oviduct, were incorrect. © 1994 Wiley-Liss, Inc.     

Ultrastructure and melatonin 1a receptor distribution in the ovaries of African ostrich chicks

Healthy 90-day-old ostrich chicks were used in the present study. The ultrastructure and melatonin 1a receptor (MT1) distribution in the ovaries of ostrich chicks was observed by transmission electron microscope and light microscope. The results showed that the ostrich chick ovary contained primordial follicles, primary follicles and secondary follicles, but no mature follicles. There are some unique ultrastructural characteristics observed in the secondary follicle, such as the cortical granule, which was located in cytoplasm beside the nucleus and appeared first in the oocyte. The zona radiata appeared in the secondary follicle, and there was an obvious vitelline membrane. There were intraovarian rete, connecting rete, and extraovarian rete in the ovaries of ostrich chicks. This is the first study that provides immunohistochemical evidence for the localization of the melatonin MT1 in the ostrich chick ovary. The germinal epithelium, follicular cell layer of every grade of follicle, cytoplasm of the oocyte and interstitial cells all expressed MT1. The expression of positive immunoreactivity materials was the strongest in the follicular cell layer of the primordial follicle and germinal epithelium, was weaker in the follicular cell layer of the primary follicle and secondary follicle, and was weakest in the oocytes of all grades of follicle. In addition, the extraovarian rete displayed strong positive expression of MT1, while there was no positive expression in the intraovarian rete or connecting rete. The positive expression of MT1 immunoreactivity in the ovary was very strong, implying that the ovary is an important organ for synthesizing MT1.     

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.     

扬子鳄卵巢性类固醇激素受体的免疫细胞化学研究

为探讨扬子鳄卵巢内不同性类固醇激素受体在卵泡发育中的调控作用,研究采用组织学和免疫细胞化学方法,运用激光共聚焦显微镜,对扬子鳄不同发育时期卵泡中的雌激素受体、雄激素受体和孕激素受体进行了检测。结果发现,3种类固醇激素受体在卵巢各期滤泡细胞中均有表达,在4月Ⅱ-Ⅳ期卵泡的滤泡细胞中阳性反应最强;9月卵巢的滤泡细胞中阳性反应最弱;ER和AR不仅在各期滤泡细胞中存在阳性位点,在6月卵泡的卵母细胞胞质中也有表达。结果说明,在扬子鳄卵母细胞生长发育和成熟过程中,3种激素受体通过与其对应的激素结合对滤泡细胞的发育、卵黄的合成与积累以及排卵起着重要的调控作用。       

九孔鲍卵子发生及卵巢发育的组织学观察

采用组织学方法研究了九孔鲍(Haliotis diversicolor supertexta)的卵子发生、卵巢结构及其发育.根据卵细胞的大小、形状,核仁的形态,卵黄颗粒的积累情况,滤泡的结构等.将九孔鲍卵子的发生分为卵原细胞、卵黄发生前的卵母细胞和卵黄发生期的卵母细胞3个时期;卵巢壁由外膜及内生殖上皮构成,生殖上皮分化产生卵原细胞和滤泡细胞;卵巢的结构单位是滤泡.根据卵巢的外部形态和内部组织结构,将九孔鲍的卵巢发育分为休止期、增殖期、生长期、成熟期和排放期共5期.     

Ultrasound image characteristics of ovarian follicles in relation to oocyte competence and follicular status in cattle

Assessment of the quality of the female gamete has become paramount for in vitro procedures. There is a need to identify reliable indicators of oocyte competence and develop a simple, non-invasive method to assess competence. The aim of this study was to investigate the relationships among ultrasonographic attributes of a follicle, its stage of development and the competence of the oocyte that it contains. We tested the hypotheses that follicular echotexture characteristics are related to: (1) the phase of development of the follicle, (2) the presence of the corpus luteum (CL) and/or the dominant follicle in the ovary, and (3) developmental competence of cumulus oocyte complexes (COC) from the same ovary. Crossbred beef cows (n=143), age 4-14 years, were given a luteolytic dose of dinoprost to cause ovulation. Ultrasound-guided ablation of all follicles > or = 4mm was done 8 days later to induce new follicular wave emergence during a luteal phase. Ultrasonographic images of dominant follicles and the three largest subordinate follicles (n=402 follicles; 84 cows) were acquired on Days 2, 3, 5 or 7 of the follicular wave (Day 0: wave emergence), i.e. growing, early-static, late static, and regressing phases of subordinate follicle development, respectively. From a subset of these animals (n=33), ovaries were collected within 30 min of slaughter and COC from subordinate follicles > or = 3mm underwent in vitro maturation, fertilization and culture to the blastocyst stage.Image analysis revealed differences in echotexture between dominant and subordinate follicles among Days 2-7 of the follicular wave. Images of dominant and subordinate follicles at Day 7 of the wave displayed consistently lower grey-scale values (P<0.05) in the peripheral antrum, follicular wall and perifollicular stroma than all other days. Follicle images displayed a consistent pattern of variation in echotexture among follicular phases. Data did not support the hypothesis of a local effect of the CL or dominant follicle on follicular echotexture. Echotexture values of the perifollicular stroma were lower in ovaries that did not produce embryos compared to ovaries that produced embryos. Our results showed that the changes in follicular image attributes are consistent with changes in follicular status. The sensitivity of the technique is not yet sufficient for use in a diagnostic setting, but results provide rationale for further development of image analysis as a tool for evaluating oocyte competence in situ.     

Localization of vascular endothelial growth factor in the zona pellucida of developing ovarian follicles in the rat: a possible role in destiny of follicles

There is increasing evidence that in many species angiogenic factors, such as vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF), may have important roles in folliculogenesis. The aim of this study is to determine the localization of VEGF and its receptors, Flt-1 and KDR, and bFGF expression in the rat ovary and to evaluate their distributions throughout the different follicular stages. Out of 20 virginal female rats, 10 were studied during the natural ovarian cycle without any ovulation induction. The other 10 were superovulated and their ovaries were studied by western analysis and immunohistochemistry. Granulosa cells (GC) and oocytes of primordial follicles were negative for VEGF. In early primary follicles, VEGF was present in the oocyte but its immunoreactivity was weak, while newly developing zona pellucida (ZP) of primary follicles was negative for VEGF. Subsequently, with the commencement of antral spaces between GC of the secondary follicle, ZP of some secondary follicles became strongly positive for VEGF, forming a continuous ring around the oocyte. In preovulatory mature follicles granulosa and theca interna (TI) cells showed a weak immunoreactivity for VEGF. Western blot analyses have also demonstrated that VEGF, a 26-kDa protein, was present in follicles. Moreover, in ovulated cumulus–oocyte complex we observed a halo-like immunoreactivity of VEGF around the fully mature oocyte. The immunoreactivity for Flt-1 and KDR receptors in growing follicles was mostly limited to GC and TI cells. Anti-bFGF did not exhibit any immunoreactivity in ZP of follicles at any stage. Its expression was weak in GC of the follicles at different stages, whereas, it could be localized to some extent in the blood capillaries of TI of antral follicles and in blood vessels localized in the stroma. Interestingly, VEGF immunoreactivity in the ZP of some secondary follicles is very striking. Accordingly, the possibility that VEGF may be an important regulatory molecule for the dominant follicle selection or atresia should be considered.     

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.     

Activity of the ovarian germinal epithelium in the freshwater catfish,Pimelodus maculatus (Teleostei: Ostariophysi: Siluriformes): Germline cysts,follicle formation and oocyte development

Distinct types of oogonia are found in the germinal epithelium that borders the ovarian lamellae of Pimelodus maculatus: A‐undifferentiated, A‐differentiated and B‐oogonia. This is similar to the situation observed for spermatogonia in the vertebrate testis. The single A‐undifferentiated oogonia divide by mitosis giving rise to A‐groups of single differentiated oogonia, each enclosed by epithelial cells that are prefollicle cells. Subsequently, the single A‐differentiated oogonia proliferate to generate B‐oogonia that are interconnected by cytoplasmic bridges, hence, forming germline cysts. The prefollicle cells associated with them also divide. Within the germline cysts, B‐oogonia enter meiosis becoming oocytes. Meiotic prophase and early folliculogenesis occur within the germline cysts. During folliculogenesis, prefollicle cells grow between the oocytes, encompassing and individualizing each of them. The intercellular bridges disappear, and the germline cysts are broken down. Next, a basement membrane begins to form around the nascent follicle, separating an oocyte and its associated prefollicle cells from the cell nest. Folliculogenesis is completed when the oocyte and the now follicle cells are totally encompassed by a basement membrane. Cells derived from the ovarian stroma encompass the newly‐formed ovarian follicle, and become the theca, thereby completing the formation of the follicle complex. Follicle complexes remain attached to the germinal epithelium as they share a portion of basement membrane. This attachment site is where the oocyte is released during ovulation. The postovulatory follicle complex is continuous with the germinal epithelium as both are supported by a continuous basement membrane. The findings in P. maculatus reinforce the hypothesis that ovarian follicle formation represents a conserved process throughout vertebrate evolution. J. Morphol. 2011. © 2011 Wiley‐Liss, Inc.