Tracking nucleolar dynamics with GFP-Nopp140 during Drosophila oogenesis and embryogenesis

We expressed two green fluorescent protein (GFP)-tagged Nopp140 isoforms in transgenic Drosophila melanogaster to study nucleolar dynamics during oogenesis and early embryogenesis. Specifically, we wanted to test whether the quiescent oocyte nucleus stored maternal Nopp140 and then to determine precisely when nucleoli formed during embryogenesis. During oogenesis nurse cell nucleoli accumulated GFP-Nopp140 gradually such that posterior nurse cell nucleoli in egg chambers at stage 10 were usually brighter than the more anterior nurse cell nucleoli. Nucleoli within apoptotic nurse cells disassembled in stages 12 and 13, but not all GFP-Nopp140 entered the oocyte through inter-connecting cytoplasmic bridges. Oocytes, on the other hand, lost their nucleoli by stage 3, but GFP-Nopp140 gradually accumulated in oocyte nuclei during stages 8–13. Most oocyte nuclei at stage 10 stored GFP-Nopp140 uniformly, but many stage 10 oocytes accumulated GFP-Nopp140 in presumed endobodies or in multiple smaller spheres. All oocyte nuclei at stages 11-12 were uniformly labeled, and GFP-Nopp140 diffused to the cytoplasm upon nuclear disassembly in stage 13. GFP-Nopp140 reappeared during embryogenesis; initial nucleologenesis occurred in peripheral somatic nuclei during embryonic stage 13, one stage earlier than reported previously. These GFP-Nopp140-containing foci disassembled at the 13th syncytial mitosis, and a second nucleologenesis occurred in early stage 14. The resulting nucleoli occupied nuclear regions closest to the periphery of the embryos. Pole cells contained GFP-Nopp140 during the syncytial embryonic stages, but their nucleologenesis started at gastrulation. This work was supported by the National Science Foundation (grant MCB-0234245). O'Keith Dellafosse was supported by the Louisiana Alliance for Minority Participation (LAMP).     

Ovarian nests in cultured females of the Siberian sturgeon Acipenser baerii (Chondrostei,Acipenseriformes)

Ovaries of Acipenser baerii are of an alimentary type and probably are meroistic. They contain ovarian nests, individual follicles, inner germinal ovarian epithelium, and fat tissue. Nests comprise cystoblasts, germline cysts, numerous early previtellogenic oocytes, and somatic cells. Cysts are composed of cystocytes, which are connected by intercellular bridges and are in the pachytene stage of the first meiotic prophase. They contain bivalents, finely granular, medium electron dense material, and nucleoli in the nucleoplasm. Many cystocytes degenerate. Oocytes differ in size and structure. Most oocytes are in the pachytene and early diplotene stages and are referred to as the PACH oocytes. Oocytes in more advanced diplotene stage are referred to as the DIP oocytes. Nuclei in the PACH oocytes contain bivalents and irregularly shaped accumulation of DNA (DNA‐body), most probably corresponding to the rDNA‐body. The DNA‐body is composed of loose, fine granular material, and comprises multiple nucleoli. At peripheries, it is fragmented into blocks that remain in contact with the inner nuclear membrane. In the ooplasm, there is the rough endoplasmic reticulum, Golgi complexes, free ribosomes, complexes of mitochondria with cement, fine fibrillar material containing granules, and lipid droplets. The organelles and material of nuclear origin form a distinct accumulation (a granular ooplasm) in the vicinity of the nucleus. Some of the PACH oocytes are surrounded by flat somatic cells. There are lampbrush chromosomes and multiple nucleoli present (early diplotene stage) in the nucleoplasm. These PACH oocytes and neighboring somatic cells have initiated the formation of ovarian follicles. The remaining PACH oocytes transform to the DIP oocytes. The DIP oocytes contain lampbrush chromosomes and a DNA‐body is absent in nuclei. Multiple nucleoli are numerous in the nucleoplasm and granular ooplasm is present at the vegetal region of the oocyte.     

Elimination of nurse cell nuclei that shuttle into oocytes during oogenesis

Drosophila oocytes develop together with 15 sister germline nurse cells (NCs), which pass products to the oocyte through intercellular bridges. The NCs are completely eliminated during stages 12–14, but we discovered that at stage 10B, two specific NCs fuse with the oocyte and extrude their nuclei through a channel that opens in the anterior face of the oocyte. These nuclei extinguish in the ooplasm, leaving 2 enucleated and 13 nucleated NCs. At stage 11, the cell boundaries of the oocyte are mostly restored. Oocytes in egg chambers that fail to eliminate NC nuclei at stage 10B develop with abnormal morphology. These findings show that stage 10B NCs are distinguished by position and identity, and that NC elimination proceeds in two stages: first at stage 10B and later at stages 12–14.     

Gonads in Histiostoma mites (Acariformes: Astigmata): Structure and development

The development of male and female gonads in arrhenotokous and thelytokous species of Histiostoma was studied using transmission electron microscopy (TEM). All instars were examined: larvae, protonymphs, facultative heteromorphic deutonymphs (=hypopi), tritonymphs, and adults. In testis primordium, spermatogonia surrounding a testicular central cell (TCC) with a gradually enlarging, branched nucleus are present already at the larval stage. Spermatogonia and the TCC are connected via narrow, tubular intercellular bridges revealing that the TCC is a germline cell. Spermatocytes appear at the protonymphal stage. At the heteromorphic deutonymph stage, the testis primordium is similar to that of the protonymph, but in the tritonymph it is much larger and composed as in the adult: spermatids as well as sperm cells are present. The latter are congregated ventrally in the testis at the entrance of the deferent duct.In the larval ovary, an eccentrically located ovarian nutritive cell (ONC) is surrounded by oogonia which are connected with the ONC via tubular intercellular bridges. In later stages, the ovary grows and oocytes appear in the protonymph. Meiotic synaptonemal complexes in oocytes occur from the tritonymph stage. At about the time of the final molting, tubular intercellular bridges transform into peculiar diaphragm-crossed bridges known only in Histiostoma mites. In the adult female, growing oocytes at the end of previtellogenesis lose intercellular bridges and move ventro-laterally to the ovarian periphery towards the oviduct entrance. Vitellogenesis occurs in oviducts.Germinal cells in both the testis and ovary are embedded in a few somatic stroma cells which may be well discernible already in the larval ovary; in the testis, somatic stroma cells are evident not earlier than the end of the tritonymphal stage. The ovary has a thin wall of flat somatic cells, whereas the testis is covered by a basal lamina only.The obtained results suggest that gonads in Histiostoma and other Astigmata originate from two primordial cells only.     

Postembryonic development of the ovary in the penicillate diplopod,Eudigraphis nigricans (Miyosi) (Diplopoda,Penicillata)

Postembryonic development of the ovary through the larval stages was studied in a penicillate diplopod, Eudigraphis nigricans. In the first instar larva a single young cell cluster, consisting of about 20 spherical gonial cells and some smaller interstitial cells, exists beneath the alimentary canal in the third body segment. The gonadal epithelium encompasses the upper surface of this young cell cluster by the end of the first instar. The epithelium then extends forward and backward to form a single long sac-like gonad, leaving the young cell cluster on the center of the gonadal floor as a mound-shaped germarium. In an early second-instar larva, very early previtellogenic oocytes accompanied by some interstitial cells appear in the front and rear surfaces of the ovarian germarium. During the period from the third through the seventh (the last) larval instar, some cell clusters containing several previtellogenic oocytes and interstitial cells successively separate forward and backward from the germarium to form a series of paired patch-shaped vitellarial areas on the extending ventral ovarian epithelium. In each vitellarial area, some of the interstitial cells surround the oocytes to form the follicles. In the seventh instar, the ovarian lumen is extremely expanded, and the late previtellogenic oocytes in the vitellarial areas encroach upward into the ovarian lumen. These oocytes floating in the ovarian lumen are still connected with their own vitellarial areas by partial extensions of their follicles. Some phylogenetic implications of the basic characteristics in structure and postembryonic development of the ovary are discussed. © 1995 Wiley-Liss, Inc.     

Ultrahistology of oogenesis and vitellogenesis in the spider mite Tetranychus urticae

Histology of the ovary of the spider mite Tetranychus urticae is described light and electron microscopically with special reference to oogenesis and vitellogenesis of this mite. Morphology of the ovary is comparable to the typical sac-like chelicerate ovary with oocytes protruding from the ovarian surface, thus resulting in a grape-like appearance. According to different oogenetic stages, a germ, pre-vitellogenic and vitellogenic region can be observed. Oogonia and primary oocytes characterized by extranuclear material or 'yolk nuclei' are situated in the germ region. Primary oocytes develop into three-nucleated nurse cells situated in the periphery of the pre-vitellogenic region, and into pre-vitellogenic oocytes protruding from the ovarian surface. Growth of oocytes is performed while they are in ovarian pouches by uptake of nurse cell cytoplasm and following extraovarian yolk precursors. Intraoocyte yolk synthesis interpreted from altered cytoplasmic organelles also occurs. Processes taking place during oogenesis and vitellogenesis in T. urticae are compared to published information on yolk synthesis of other animal species.     

Ovary cord structure and oogenesis in Hirudo medicinalis and Haemopis sanguisuga (Clitellata, Annelida): remarks on different ovaries organization in Hirudinea

In Hirudo medicinalis and Haemopis sanguisuga, two convoluted ovary cords are found within each ovary. Each ovary cord is a polarized structure composed of germ cells (oogonia, developing oocytes, nurse cells) and somatic cells (apical cell, follicular cells). One end of the ovary cord is club-shaped and comprises one huge apical cell, numerous oogonia, and small cysts (clusters) of interconnected germ cells. The main part of the cord contains fully developed cysts composed of numerous nurse cells connected via intercellular bridges with the cytophore, which in turn is connected by a cytoplasmic bridge with the growing oocyte. The opposite end of the cord degenerates. Cord integrity is ensured by flattened follicular cells enveloping the cord; moreover, inside the cord, some follicular cells (internal follicular cells) are distributed among germ cells. As oogenesis progresses, the growing oocytes gradually protrude into the ovary lumen; as a result, fully developed oocytes arrested in meiotic metaphase I float freely in the ovary lumen. This paper describes the successive stages of oogenesis of H. medicinalis in detail. Ovary organization in Hirudinea was classified within four different types: non-polarized ovary cords were found in glossiphoniids, egg follicles were described in piscicolids, ovarian bodies were found characteristic for erpobdellids, and polarized ovary cords in hirudiniforms. Ovaries with polarized structures equipped with apical cell (i.e. polarized ovary cords and ovarian bodies) (as found in arhynchobdellids) are considered as primary for Hirudinea while non-polarized ovary cords and the occurrence of egg follicles (rhynchobdellids) represent derived condition.     

Nucleus transfer in mammals: how the oocyte cytoplasm modifies the transferred nucleus

Successful development of clones depends on the reprogramming of transferred nuclei in enucleated oocytes. Thus far, oocytes are the only cells that can convert nuclei, which are already differentiated, into undifferentiated stages resembling pronuclei in freshly fertilized zygotes and that can then complete development of the reconstructed embryos. However, we still don't know exactly how the enucleated oocyte (cytoplast) secures this reprogramming. Oocytes exhibit a number of cytoplasmic activities that may be involved reprogramming. We discuss how these activities may be involved in reprogramming of transferred nuclei.     

Ultrastructural study of vitellogenesis and oogenesis of Metadena depressa (Stossich, 1883) Linton, 1910 (Digenea,Cryptogonimidae), intestinal parasite of Dentex dentex (Pisces,Teleostei)

The ultrastructural organization of the female reproductive system of Metadena depressa, digenean intestinal parasite of Sparidae (Dentex dentex), was investigated by electron microscopy. The vitellogenesis is divided into four stages: stage I, vitellocytes have a cytoplasm mainly filled with ribosomes and few mitochondria; stage II, beginning of the synthetic activity; stage III, active shell globule clusters synthesis; stage IV, mature vitellocytes are filled with shell globule clusters and generally contain several large lipid droplets. Glycogen granules are grouped at the periphery of the cell. The three stages of the oogenesis process take place in the ovary: stage I, oogonia are undifferentiated small cells located at the periphery of the organ; stage II, primary oocytes possess a higher nucleo-cytoplasmic ratio and a nucleus with a nucleolus and synaptonemal complexes indicating the zygotene-pachytene stage of the first meiotic division; stage III, mature oocytes are located in the proximal region of the organ and possess a cytoplasmic chromatoid body and cortical granules in a monolayer close to the periphery of the cell.     

Germ-line cysts are formed during oogenesis in Erpobdella octoculata (Annelida,Clitellata, Erpobdellidae)

Abstract

Erpobdella octoculata (Clitellata, Hirudinea, Erpobdellidae) has paired ovarian sacs, each containing several rod-shaped structures termed ovarian bodies. Oogenesis takes place within the ovarian bodies. We show that in the apical part of the bodies the germ-line cells form syncytial cysts of cells interconnected by stable intercellular bridges. Germ-line cyst architecture is broadly similar to that of other clitellate annelids; that is, each germ cell has only one intercellular bridge connecting it to the anuclear cytoplasmic mass, the cytophore. Unlike germ-line cysts described in other leech species, the cytophore in cysts of E. octoculata is poorly developed, taking the form of thin cytoplasmic strands. Oogenesis in E. octoculata is meroistic because the germ cells forming the cysts (cystocytes) have diverse fates, i.e., nurse cells and oocytes appear. One large ramified cell (apical cell) occurs within the apical part of the ovarian body. We compare the ultrastructure of the apical cell found in E. octoculata with that of apical cells described recently in some hirudiniform leeches. The germ-line cysts as well as the oocytes are enveloped by somatic follicular cells. As in other leeches, the follicular cells surrounding the growing oocytes have cytoplasm perforated by intracellular canals. In view of the many similarities between E. octoculata ovarian bodies and the ovary cords described in glossiphoniids and especially in hirudiniform leeches, we suggest that the ovarian bodies found in E. octoculata are in fact modified ovary cords.     

Vasa protein is localized in the germ cells and in the oocyte-associated pyriform follicle cells during early oogenesis in the lizard <Emphasis Type="Italic">Podarcis sicula</Emphasis>

The vasa gene, first identified in Drosophila, is a key determinant for germline formation in eukaryotes. Homologs of vasa have been identified and linked to germline development, in many invertebrates and vertebrates. Here, we analyze the distribution of Vasa in early germ cells (oogonia and oocytes) and previtellogenic ovarian follicles of the lizard Podarcis sicula. During most of its previtellogenic growth, the oocyte in this lizard species is structurally and functionally integrated through intercellular bridges with special follicle cells called pyriform cells. The pyriform cells function similarly to Drosophila nurse cells, but are somatic in origin. In the oogenesis of P. sicula, Vasa is initially highly detected in the oogonia, but its levels decrease in early stage oocytes before the onset of pyriform cell differentiation. In the later stages of oogenesis, the high level of Vasa is related with the nurse function of the pyriform follicle cells. These observations suggest that cells of somatic origin are engaged in the synthesis of Vasa in the oogenesis of this lizard.     

Structure of the ovary and "nurse cells" in a freshwater ostracod, Cyprinotus uenoi Brehm (Podocopida: Cypridoidea)

The adult female of the freshwater ostracod Cyprinotus uenoi Brehm, 1936 (Podocopida: Cypridoidea) has a pair of long, sac-like ovaries separately lying in the posterior part of the left and the right carapace valves. Oogonia and very early previtellogenic oocytes are located in the terminal germarium of each ovary. In the germarium, the oogonia occur in the most terminal region, and the very early previtellogenic oocytes are located in the remainder, arranged in order of size, the larger ones nearer the ovarian lumen. Most of the growing oocytes, previtellogenic and vitellogenic, are found in the ovarian lumen, the larger ones farther from the germarium. In the germarium, a cytoplasmic bridge connects a pair of adjoining germ cells, resulting from an incomplete cytokinesis of oogonial division. Among the previtellogenic and early vitellogenic oocytes in the ovarian lumen, "nurse cells" are found as small, spherical cells in mostly the same number as these oocytes. A cytoplasmic bridge connects each "nurse cell" to an adjoining oocyte. Based on the manner of connection and some morphological features, we consider that each "nurse cell" originates from one of each pair of adjoining germ cells connected by a cytoplasmic bridge in the germarium, as in the true nurse cells of several branchiopod crustaceans and insects with meroistic ovarioles.     

The involvement of nurse cells in oogenesis and embryonic development in the marine sponge,Haliclona ecbasis

Oogenesis and embryonic development in the marine sponge, Haliclona ecbasis, were studied using standard histological procedures. When the oocytes reach a diameter of about 30 μ, nurse cells begin to aggregate around them. Then when the oocytes are about 36 μ in diameter, they begin to engulf the associated nurse cells. Whole nurse cells are engulfed; and although the nucleus of the nurse cells disappears either as or soon after the cells are engulfed, the cytoplasm remains essentially unchanged. The accumulation of these cells within the oocytes most of the cytoplasm is nurse cell cytoplasm. During cleavage of the egg, the engulfed nurse cells are gradually fragmented, but otherwise appear unchanged. At the same time the cytoplasm of the nurse cells is progressively incorporated into that of the blastomeres by what appears to be fusion process. When the latter process is complete, the embryo develops into a typical parenchymula larva.     

The development of oocytes in the ovary of a parthenogenetic tick, Haemaphysalis longicornis

Haemaphysalis longicornis is an important vector of various pathogens in domestic animals and humans. The tick is a unique species with bisexual and parthenogenetic races. Although mating induces oocyte development, it is possible in the parthenogenetic race to complete oogenesis without copulation. Here we examined the developmental process of oocytes from unfed to the oviposition period in parthenogenetic H. longicornis. We classified the developmental stages of oocytes into five stages: stage I, germinal vesicle occupies more than half of the cytoplasm; stage II, germinal vesicle occupies less than half of the cytoplasm; stage III, germinal vesicle migrates from the center in the oocyte to the vicinity of the pedicel cells; stage IV, the cytoplasm is filled with yolk granules of various sizes; stage V, the cytoplasm is occupied by large yolk granules. Oocytes at the unfed period were undeveloped and classified as stage I. Stage I and II oocytes were observed at the rapid feeding period, indicating that oocyte development began after the initiation of blood feeding. All developmental stages of oocytes were observed at the pre-oviposition period. At 10?days after the beginning of the oviposition period, the ratios of stage I and II oocytes were higher than those of the previous period, suggesting that the ovarian development and activity may be continuing. Based on these findings, we propose classification criteria for the oocyte development in the parthenogenetic H. longicornis. The criteria will be useful for understanding the mechanisms of tick reproduction and transovarial transmission of pathogens.     

The chorion defect of the ocelliless mutation caused by abnormal follicle cell function

Homozygous Drosophila females bearing the ocelliless mutation are sterile and produce oocytes with abnormal chorions. It has been possible to determine in which tissues these defects reside by generating ovarian chimeras. Pole cells from ocelliless female embryos can give rise to functional oocytes surrounded by normal chorions when placed in a wild-type environment. Conversely, when wild-type pole cells are placed in homozygous ocelliless females, the oocytes that form from them have abnormal chorions and never give rise to progeny. Thus the chorion defect and sterility of the ocelliless mutation are not germ-line autonomous. Homozygous ocelliless ovaries will attach to the uterus when placed in a wild-type third instar larva, but few eggs are ever laid, and the chorions of stage 14 oocytes remain ocelliless in morphology. Wild-type ovaries continue to produce oocytes with normal chorion morphology when placed into ocelliless hosts, indicating that the ocelliless chorion defect is ovary autonomous. Thus the chorion defect of the ocelliless mutation resides in the ovarian somatic tissue, presumably the follicle cells.     

The ultrastructure of germinal beds in the ovary of Gerrhonotus coeruleus (Reptilia: Anguidae)

A study of ovarian structure in adult Alligator Lizards (Gerrhonotus coeruleus) was conducted by light microscopy and transmission electron microscopy. Particular attention was directed to characterizing the ultrastructure of germ-line cells, prior to follicle formation. General ovarian structure in this lizard is similar to that of other lizards. The paired organs are hollow, thin-walled sacs containing follicles in roughly 3 to 4 size classes. Ovarian germinal tissue consists of oogonia (diploid cells which divide mitotically) and oocytes (meiotic cells), intermixed with ovarian surface epithelial cells. Germ cells reside in two dorsal patches of epithelium per ovary (germinal beds), as is common in lizards. Oogonia in interphase show a highly dispersed chromatin pattern. Within oogonia cytoplasm, Golgi complexes are scarce, rough endoplasmic reticulum is absent, and lipid droplets are rare. Ribosomes are scattered in small clusters. Small, round vesicles are common in all oogonia; glycogen-like granules are present in some. Mitochondria form a juxtanuclear mass within which groups of several mitochondria surround a dense granule. “Nuage” granules also are found unassociated with mitochondria. Oocytes are present in stages of meiotic prophase up to diplotene. Synaptinemal complexes are seen in several (pachytene) cells. The cytoplasm of oocytes differs from that of oogonia in that mitochondria do not form groups, and nuage and glycogen are absent, whereas small round vesicles and large irregular vesicles are common. The ultrastructural similarities in germ cells of a reptile as compared to those of other vertebrates strengthens the notion that germ-line cells possess (or lack) qualities related to the undifferentiated state of these cells.     

Different modes of programmed cell death during oogenesis of the silkmoth Bombyx mori

It is increasingly recognized that programmed cell death includes not only apoptosis and autophagy, but also other types of nonapoptotic cell death, such as paraptosis, which are all characterized by distinct morphological features. Our findings indicate that all three types of programmed cell death occur in the ovarian nurse cell cluster during late vitellogenesis (formation of the egg yolk) of Bombyx mori (Lepidoptera), whereas middle vitellogenesis is exclusively characterized by the presence of a nonapoptotic type of cell death, known as paraptosis. During middle vitellogenesis, nurse cells exhibit clearly cytoplasmic vacuolization, as revealed by ultrastructural examination performed through conventional light and transmission electron microscopy, while no signs of apoptotic or autophagic features are detectable. Moreover, nurse cells of developmental stages 7, 8 and 9 contain autophagic compartments, as well as apoptotic characteristics, such as condensed chromatin, fragmented DNA and activated caspases, as revealed by in vitro assays. We propose that paraptosis precedes both apoptosis and autophagy during vitellogenesis, since its initial activation is detectable during middle vitellogenesis, whereas no apoptotic nor autophagic features are observed. In contrast, at the late stages of Bombyx mori oogenesis, paraptosis, autophagy and apoptosis operate synergistically, resulting in a more efficient elimination of the degenerated nurse cells.

Addendum to: Mpakou VE, Nezis IP, Stravopodis DJ, Margaritis LH, Papassideri IS. Programmed cell death of the ovarian nurse cells during oogenesis of the silkmoth Bombyx mori. Dev Growth Differ 2006; 48:419–28.     

Comparative Spiralian Oogenesis--Structural Aspects: An Overview

Considerable variety exists in ovarian structure and cellularinteraction in spiralians. During their development the eggsof Dwpatra cuprea, are associated with nurse cells; there areno follicle cells in this species. The nurse cells have prominentnuclei and connect to oocytes via cytoplasmic bridges throughwhich ribosomes, mitochondria and other inclusions pass. Morecommonly, follicle cells surround a portion of, or entire, oocytesin many species of spiralians. Usually, as in Ilyanassa, theyhave a well developed compliment of organelles. The structureand distribution of organelles within follicle cells impliesthat they are functionally active, but precisely in what mannerduring oogenesis is poorly understood. Other cell types, suchas Leydig and interstitial cells also seem to play a role inoogenesis. Within the oocyte, a host of components includingyolk, lipid, mitochondria, ribosomes, membranous cisternae,cortical granules, etc. are accumulated. Autosynthetic yolkformation is prevalent among spiralians. Surface differentiationincludes microvillar development. This may be uniform in someeggs or restricted to certain regions (e.g., the animal hemisphere)in other oocytes. Oocyte-follicle cell interactions change duringoogenesis. The topographical association of the oocyte withother ovarian cells influences subsequent animal-vegetal polarityand other ooplasmic differences. Examples of ooplasmic localizationsare discussed. Conventional EM has revealed no unusual corticalstructure in many oocytes although occasionally microtubulesand microfilaments are present.     

Oogenesis and the ovarian cycle in Salamandra salamandra infraimmaculata Mertens (Amphibia; Urodela; Salamandridae) in fringe areas of the taxon's distribution

Reproductive cycle and oogenesis were studied in specimens of Salamandra salamandra infraimmaculata Mertens that inhabit fringe areas of the taxon's distribution in the Mediterranean region. Both ovarian mass and length are correlated significantly with body mass and length. Ovarian length is also correlated with the number of oocytes. During the oogenetic cycle six stages in oocyte development were recognized. Three occur during previtellogenesis: stage 1, in which oogonia divide and form cell nests; stage 2 in which oogonia differentiate into oocytes; and stage 3, in which the oocyte cytoplasm increases in volume. In the vitellogenic phase two additional stages, 4 and 5, were recognized: stage 4, in which lipid accumulates in vacuoles in the periphery followed by the appearance of yolk platelets near the cytoplasmic margin; and stage 5, in which oocyte volume increases rapidly due to increased number of yolk platelets until it reaches its maximal size. During postvitellogenesis one stage was recognized: stage 6, in which the beginning of maturation is characterized by movement of the nucleus toward the animal pole. Oogenesis continues year-round. The first four stages were seen in all ovaries examined. The ovarian cycle is independent of season and reproductive stage apart from the number of mature, postvitellogenic oocytes that increases following gestation toward the beginning of spring (March-April). J. Morphol 231:149–160, 1997. © 1997 Wiley-Liss, Inc.