Stereological analysis of nutritive phagocytes and gametogenic cells during the annual reproductive cycle of the green sea urchin, Strongylocentrotus droebachiensis

Abstract. The germinal epithelium of sea urchin gonads contains two interdependent populations of cells: somatic cells called nutritive phagocytes (NPs) and germ cells—ovarian and testicular gonial cells and their derivatives, gametocytes and gametes. Annually, NPs vary their structure and function to produce a changing microenvironment for germ cells during gametogenesis and after spawning. Here, we describe seasonal changes in the NPs as they interact with germ cells during successive gametogenic stages in both sexes of the green sea urchin, Strongylocentrotus droebachiensis. Monthly samples were collected from the Gulf of Maine at a depth of 10 m and analyzed with light microscopy and stereology. Shorter day length and falling seawater temperatures were correlated with nutrient mobilization from NPs, initiation of gonial cell mitosis, and subsequent gametogenesis. During gametogenesis, NPs in both sexes were depleted of nutrients and eventually phagocytize residual ova or spermatozoa. Observations from this study are important for understanding both the cellular aspects of the reproductive biology of sea urchins and those environmental factors that affect the onset and progress of gametogenesis.     

Reproductive cycle and gonadal development of the Atlantic sea urchin Arbacia punctulata in the Gulf of Mexico: changes in nutritive phagocytes in relation to gametogenesis

ABSTRACT

The annual reproductive cycle comprises steady gametogenic activities that synchronize gonadal maturation and spawning rhythms, which are important for aquatic organisms including marine echinoderms (Echinodermata, Echinoidea). In this study, we report the annual reproductive cycle, gonadal development, and changes in nutritive phagocytes (NPs, which accumulate nutrients in germ cells) in relation to gametogenesis of the Atlantic sea urchin (Arbacia punctulata, an edible echinoid) in the Gulf of Mexico. Monthly changes in gonadal development and maturation were observed morphologically and histologically. We calculated gonadosomatic index (GSI) and compared the stages of gonadal development in order to determine the NPs index, and characteristics of germ cells (eggs and sperm) during the annual reproductive cycle. According to GSI and histological analyses, gametogenic activities were classified into four stages of both sexes: mature (June–August), spent (September–November), recovery (December–March), and growing (April–May). The GSI values in both sexes were high during summer months. In males, testicular lobules were densely packed with sperm from June to August. In females, however, mature eggs first appeared in some ovaries in May, numerically increased from June to July, and decreased in August. During gametogenesis, on the other hand, NPs in both testes and ovaries were depleted from June to August. Collectively, our results suggest that the Atlantic sea urchin spawns during summer months in the Gulf of Mexico. This is the first report, to the best of our knowledge, on gonadal development and changes in NPs during the annual reproductive cycle of any Arbacia species in the Gulf of Mexico.     

Ultrastructure of the gonadal primordia of the viviparous lizard (Lacerta vivipara Jacquin) during the period of colonization by the germ cells]

This ultrastructural study was carried our during colonization of the gonadal primordia by the germ cells which reach the gonads after interstitial migration. During the period of colonization, the germinal epithelia have no basal membrane. The epithelial cells are linked together by desmosomal junctions; they contain many free ribosomes, some lipid droplets, few granular reticula. The Golgi apparatus and the agrangular reticulum are well developed and situated at the distal pole of the cells. The outline of the germinal epithelia is regular in front of the coelomic cavity. At first, the outline of the basal surface is very irregular because the epithelial cells put out many cytoplasmic processes. Then, cytoplasmic processes become more sparse and the outline of the basal surface more regular. The germinal epithelia do not show swellings linked with a merocrine type of excretory process as in the chick (Cuminge and Dubois, 1971). However, this does not rule out a chemotactic type attraction of the germ cells. The first germ cell which arrive in the gonadal areas are incorporated into the epithelia. Later on, the germ cells are immobilized by the mesenchymal cells of the gonadal primordia which prevent them from reaching the epithelia. These germ cells stay in the medullary area of the young gonad which contain a greater number of germ cells than the epithelia.     

The sea urchin major yolk protein is synthesized mainly in the gut inner epithelium and the gonadal nutritive phagocytes before and during gametogenesis

Major yolk protein (MYP), the predominant component of yolk granules in sea urchin eggs, is also contained in the coelomic fluid and nutritive phagocytes of the gonad in both sexes. MYP is stored in ovarian and testicular nutritive phagocytes prior to gametogenesis and is used during gametogenesis as material for synthesizing proteins and other components necessary for eggs and sperm. To reveal the expression profile and the main production site of MYP, we analyzed MYP mRNA expression in immature and maturing Pseudocentrotus depressus. Real‐time reverse‐transcribed polymerase chain reaction analysis showed that MYP mRNA was expressed predominantly in the digestive tract (stomach, intestine and rectum) and the gonad of both sexes. The total amounts of MYP mRNA in the whole digestive tract and in the whole gonad were at similar levels in both immature and maturing sea urchins. MYP mRNA was also detected in white morula cells and vibratile cells separated from the coelomic fluid by density gradient centrifugation, but the expression levels in these cells were very low compared with those in the digestive tract and the gonad. Using in situ hybridization analysis, MYP mRNA was detected in the inner epithelium of the digestive tract and in nutritive phagocytes of the ovary and testis, but was not detected in the germ cells. We conclude that the adult sea urchin has two predominant production sites for MYP regardless of sex and reproductive stage: the inner epithelium of the digestive tract and the nutritive phagocytes of the gonad. Mol. Reprod. Dev. 77: 59–68, 2010. © 2009 Wiley‐Liss, Inc.     

Proliferating cell nuclear antigen in gonad and associated storage tissue of the Pacific oyster Crassostrea gigas: seasonal immunodetection and expression in laser microdissected tissues

To understand the processes involved in tissue remodeling associated with the seasonal reproductive cycle of the oyster Crassostrea gigas, we used immunodetection and expression measurements of proliferating cell nuclear antigen (PCNA). The expression of the PCNA gene was measured by real-time polymerase chain reaction in the whole gonadal area compared with laser microdissected gonad and storage tissue. Results underlined the advantage of the laser microdissection approach to detect expression, mainly for early stages of spermatogenesis. In the storage tissue, PCNA expression was reduced in the gonadal tubules, but immunolabeled hemocytes and vesicular cells were detected when the storage tissue was being restored. In the gonadal tubules, the PCNA gene was more highly expressed in males than in females. As soon as spermatogenesis was initiated, PCNA expression showed a high and constant level. In females, the expression level increased gradually until the ripe stage. The immunological approach established the involvement of peritubular cells in gonadal tubule expansion during early gametogenesis. In both sexes, gonial mitosis was immunodetected throughout the reproductive cycle. In males, the occurrence of two types of spermatogonia was ascertained by differential immunolabeling, and intragonadal somatic cell proliferation was noted. As expected, immunolabeling was never observed from stage II spermatocytes to spermatozoa. In females, positively stained cells were detected from oogonia to growing oocytes with various labeled intracellular locations.     

Morphological development of the gonads in zebrafish

Gonadogenesis of zebrafish Danio rerio was investigated by means of light microscopy to test the suitability of gonad histology as an endpoint in hazard assessment of endocrine‐active compounds. At age 2 weeks post‐fertilization (pf), primordial germ cells were found in a dorsocaudal position in the body cavity. At 4 weeks pf, the majority of the fish (86%) possessed paired gonads with meiotic germ cells; these gonads represented presumptive ovaries. At week 5 pf, 87% of the fish examined had ovaries with perinucleolar oocytes. Further development of the gonads in female zebrafish up to week 11 pf was characterized by an increase in gonad size as well as in the number and size of perinucleolar oocytes. Starting with week 5, some fish showed alterations of gonad morphology, including a decrease in the number and size of the oocytes, an enhanced basophilia and irregular shape of the oocytes, and finally their degeneration into residual bodies. With the decline in oocyte number, stromal cells became more numerous and they infiltrated the gonadal matrix. In several 7 week‐old zebrafish with altered gonadal morphology, enhanced numbers of gonial cells arranged in cyst‐like groups appeared. These gonads were interpreted as presumptive testes. In one fish out of 32 individuals examined, spermatocytes were detected, in addition to the gonial cells. During the subsequent weeks, the percentage of fish showing early testes with spermatogonia, spermatocytes and spermatids increased and reached 40% at 11 weeks pf. The sequence of gonadal alterations taking place in some of the individuals from week 5 pf onwards was interpreted to reflect the transition of protogynic ovaries into testes. The developmental pattern described identifies zebrafish to be a juvenile hermaphrodite. The results of this study are of relevance for the use of gonadal histopathology as endpoint in endocrine disruption testing, particularly in order to avoid false diagnoses of ‘intersex gonads’ in zebrafish.     

Gonadal restructuring and correlative steroid hormone profiles during natural sex change in protogynous honeycomb grouper (Epinephelus merra)

The honeycomb grouper shows protogynous hermaphroditism. The endocrine mechanisms involved in gonadal restructuring throughout protogynous sex change are largely unknown. In the present study, we investigated changes in the gonadal structures and levels of serum sex steroid hormones during female to male sex change in the honeycomb grouper. On the basis of histological changes, entire process of sex change was assigned into four developmental phases: female, early transition (ET), late transition (LT), and male phase. At the female phase, the oocytes of several developmental stages were observed including gonial germ cells in the periphery of ovigerous lamellae. At the beginning of ET phase, perinucleolar and previtellogenic oocytes began degenerating, followed by proliferation of spermatogonia toward the center of lamella. The LT phase was characterized by further degeneration of oocytes and rapid proliferation of spermatogenic germ cells throughout the gonad. At the male phase, no ovarian cells were observed and testis had germ cells undergoing active spermatogenesis. Serum levels of estradiol-17beta (E2) were high in females in the breeding season, but low in the non-breeding female, transitional and male phase, and those of 11-ketotestosterone (11-KT) and testosterone (T) were low in females and gradually increased in the transitional and male phase. The present results suggest that low serum E2 levels and degeneration of oocytes accompanied by concomitant increase in the 11-KT levels and proliferation of spermatogenic germ cells are probably the events mediating protogynous sex change in the honeycomb grouper.     

Sex differentiation and aspects of gametogenesis in shortnose sturgeon Acipenser brevirostrum Lesueur

Shortnose sturgeon Acipenser brevirostrum gonad samples were collected from industry-reared fish and wild broodstock at various developmental stages to elucidate patterns of gonadal differentiation and maturation. Genital ridges, containing germ cells, were present in 26 day-old fish and distinct gonads were present by day 54. Sturgeon gonads are known to consist of two tissue types (adipose and gametogenic) and both were present at 72 day. Anatomical differentiation of gonads occurred by 6 months and was advanced by 15 months. Ovaries had distinct lamellae while testes remained non-lamellate. Gonial proliferation had occurred by 15 months, but the cells were not identifiable as spermatogonia or oogonia. Small white 'pinhead' oocytes were macroscopically visible in ovaries as early as 36 months. At 43 months ovaries were clearly organized, with some areas containing only immature oocytes and other containing oocytes apparently developing as cohorts. Individual fish showed considerable variation: the level of development remained unchanged at 84 months in some females, while others showed clear progression towards sexual maturation at 48 months. Sperm cells were present in males as early as 52 months. Advanced development of ovarian follicles was observed only in biopsies of re-conditioned broodstock of wild origin. In the year before spawning, the most advanced oocytes became pigmented, the chorion thickened, the nucleus (germinal vesicle) migrated towards the micropyle complex at the animal pole, and ovulation occurred in May under appropriate environmental conditions.     

Hormonal regulation of gametogenesis in insect

The review discusses the role of juvenile hormone (JH), ecdysone and brain in the regulation of oogenesis and spermatogenesis in insects. The early period of gametogenesis (gonial mitoses, the meiotic prophase) in both sexes is controlled mainly by ecdysone and neurosecretory cells of the brain. In periods of cytoplasmic growth of oocytes and vitellogenesis the main role in the regulation belongs to JH. The modern views on hormonal regulation of vitellogenin synthesis and follicular epithelium differentiation are under consideration with a special reference of the role of ecdysteroids in Diptera and Lepidoptera oogenesis.     

Ultrastructure of the somatic portion of the gonads in asteroids,with emphasis on flagellated-collar cells and nutrient transport

Somatic portions of gonads in two phanerozonian sea-stars, Ctenodiscus crispatus and Hippasteria phrygiana, were similar in all aspects of gross structure and histology seen previously in both forcipulate and spinulosan asteroids. For the first time, detailed ultrastructural observations have been made of cells and tissues that reveal several features believed to be of universal occurrence in the gonads of asteroids. These include flagellated-collar cells in the visceral peritoneum and other coelomically derived epithelia, muscular-flagellated-collar cells in the visceral peritoneum and genital coelomic (perihaemal) sinus, the digestion of collagen fibers by cells in the connective tissue layer, and the intimate relationship of the genital haemal sinus and the entire germinal epithelium. Structural and functional compartmentalization are discussed in relation to major activities of the gonad throughout the annual reproductive cycle. The distinctive ultrastructure and current generation of flagellated-collar cells found in the visceral peritoneum are analyzed relative to their role in nutrient transport to gonadal tissues. The single flagellum of each flagellated-collar cell beats in coordination with those on neighboring cells to produce extremely rapid, oriented currents of coelomic fluid. The form of beating in an individual flagellum is planar, and the resulting synchronized activity of many adjacent flagella is non-metachronal; both of these characteristic aspects of current production have, thus far, been encountered together only in the Echinodermata. Flagellated-collar cells are efficient in generating currents which mix contents of the coelomic fluid, and they can presumably supply themselves with nutrients. It is concluded that nutrients so obtained are generally not passed through the wall of the gonad to the germinal epithelium and, as a result, have little to do with nutrition of somatic and germinal cells of the germinal epithelium. Alternatively, well-developed genital portions of the haemal system of the sea-star are advanced as the major channels supplying nutrients to germinal epithelia during gametogenesis.     

Sexual phenotype of avian chimeric gonads with germinal and stromal cells of opposite genetic sexes

The respective roles of germinal and stromal cells in determining the sexual phenotype of the gonad were analyzed in chimeric gonads obtained by surgical recombination between young avian blastodiscs in ovo. Equivalent territories were exchanged between two blastodisc, in order that the germinal crescent and the gonad territory had a different origin (fig. 3). Embryos used for these experiments carried a sex linked pigment mutation, that made it possible to diagnose the genetic sexes of germ cells and stroma at the time when the gonad was retrieved for examination. On the basis of species, three types of combination were performed: chick germ cells in chick or quail stroma, quail germ cells in chick stroma. In each chimera, the genetic sexes of the two gonadal cell populations could be identical or opposite. However it appeared that the germ cell population was not always homogeneous. In some grafting schemes, ectopic germ cells, located outside the germinal crescent, contributed to the colonization of the experimental gonad. These germ cells were from the same territory as the stroma element of the gonad, i.e., they were of the same species and the same genetic sex. Whatever the case, in 87 chimeras that were studied, the sex phenotype of the gonads always corresponded to the genetic sex of the stroma. Thus the genetic sex of germ cells has no role in the sexual differentiation of the gonadal rudiments.     

Localization of a fibronectin-like molecule in the ovotestis of the snail Helix aspersa

Summary An antifibronectin antibody has been prepared which recognises a fibronectin-like substance isolated from Helix aspersa hemolymph. By use of the indirect immunofluorescence technique, the distribution of fibronectin in embryos and in the ovotestis at selected development stages from hatching to the adult has been investigated. In embryos, the basement membranes and the epithelia were immunoreactive, whereas the mesenchyme and the gonadal rudiment were not. After hatching, the fibronectin was present in ovotestis. It was localized on the epithelia of gonadal acini and at the periphery of the periacinar vesicular tissue. This adhesive molecule is present on the nurse cells, which sustain the group of male cells, while it is absent on differentiating male cells in the lumen of acini. The membrane of oocyte did not exhibit fluorescence. By contrast the surface of follicular cells were clearly labelled. Inside the vitellogenic oocytes, granular fluorescence was also observed.The participation of fibronectin in gonadal organogenesis is discussed in relation to cellular adhesion and movement. Its role in the metabolic exchanges between the germinal cells and the surrounding tissues is suggested.     

中华鳖vasa基因克隆及在卵母细胞中的表达分析

研究利用中华鳖为研究模型进行爬行类生殖细胞发育分化成熟等生物学研究,克隆了中华鳖vasa基因的cDNA序列,全长3865 bp,包括5'端非编码区90 bp,3'端非编码区1699 bp,开放阅读框长2076 bp,共编码691个氨基酸。中华鳖Vasa氨基酸序列包含DEAD-box家族蛋白8个保守保守功能域,在N末端有4个RGG重复序列和2个GG富集区,与小鼠Vasa蛋白的同源性较高（72%）。荧光定量PCR的结果表明,中华鳖vasa mRNA主要精巢和卵巢中表达,其他体组织中均难检测到表达。卵巢冰冻切片原位杂交结果显示:中华鳖vasa mRNA在生殖细胞中特异表达;在卵子发生过程中的不同发育期卵母细胞中呈现动态的变化。即vasa mRNA在初级卵母细胞及生长期卵母细胞中表达最强,且均匀分布在细胞质中,随着卵母细胞的逐渐增大,信号逐渐减弱,直至在成熟的卵母细胞中几乎检测不到表达信号,说明vasa可能在中华鳖早期卵母细胞发育中起重要作用。同时,vasa基因可作为中华鳖生殖细胞分子标记物,根据其mRNA的表达水平来鉴别不同发育时期的卵母细胞。研究结果为进一步开展中华鳖胚胎生殖细胞发育及配子生成,特别是研究中华鳖,乃至爬行类原始生殖细胞（Primordial Germ Cells,PGCs）的起源、迁移、分化等研究奠定了基础。     

Conserved form and function of the germinal epithelium through 500 million years of vertebrate evolution

The germinal epithelium, i.e., the site of germ cell production in males and females, has maintained a constant form and function throughout 500 million years of vertebrate evolution. The distinguishing characteristic of germinal epithelia among all vertebrates, males, and females, is the presence of germ cells among somatic epithelial cells. The somatic epithelial cells, Sertoli cells in males or follicle (granulosa) cells in females, encompass and isolate germ cells. Morphology of all vertebrate germinal epithelia conforms to the standard definition of an epithelium: epithelial cells are interconnected, border a body surface or lumen, are avascular and are supported by a basement membrane. Variation in morphology of gonads, which develop from the germinal epithelium, is correlated with the evolution of reproductive modes. In hagfishes, lampreys, and elasmobranchs, the germinal epithelia of males produce spermatocysts. A major rearrangement of testis morphology diagnoses osteichthyans: the spermatocysts are arranged in tubules or lobules. In protogynous (female to male) sex reversal in teleost fishes, female germinal epithelial cells (prefollicle cells) and oogonia transform into the first male somatic cells (Sertoli cells) and spermatogonia in the developing testis lobules. This common origin of cell types from the germinal epithelium in fishes with protogynous sex reversal supports the homology of Sertoli cells and follicle cells. Spermatogenesis in amphibians develops within spermatocysts in testis lobules. In amniotes vertebrates, the testis is composed of seminiferous tubules wherein spermatogenesis occurs radially. Emerging research indicates that some mammals do not have lifetime determinate fecundity. The fact emerged that germinal epithelia occur in the gonads of all vertebrates examined herein of both sexes and has the same form and function across all vertebrate taxa. Continued study of the form and function of the germinal epithelium in vertebrates will increasingly clarify our understanding of vertebrate reproduction. J. Morphol. 277:1014–1044, 2016. © 2016 Wiley Periodicals, Inc.     

Histological and ultrastructural investigation of early gonad development and sex differentiation in Adriatic sturgeon (Acipenser naccarii,Acipenseriformes, Chondrostei)

Gonad development and sex differentiation from embryos to 594‐day‐old individuals were investigated in farmed Acipenser naccarii using light and transmission electron microscopy. The migrating primordial germ cells first appear along the dorsal wall of the body cavity in embryos 1.5 days before hatching. The gonadal ridge, containing a few primary primordial germ cells (PGC‐1) surrounded by enveloping cells, appears in 16‐day‐old larvae. At 60 days, the undifferentiated gonad is lamellar and PGC‐1 multiply, producing PGC‐2. In 105‐day‐old juveniles, a distinct germinal area with advanced PGC‐2 appears on the lateral side near the mesogonium and the first blood vessels are visible. At 180 days, putative ovaries with a notched gonadal epithelium and putative testes with a smooth one appear, together with adipose tissue on the distal side. In 210‐day‐old juveniles, active proliferation of germ cells begins in the putative ovaries, whereas putative testes still contain only a few germ cells. The onset of meiosis and reorganization of stromal tissue occurs in ovaries of 292‐day‐old individuals. Ovaries with developed lamellae enclosing early oocyte clusters and follicles with perinucleolar oocytes occur at 594 days. Meiotic stages are never found, even in anastomozing tubular testes of 594‐day‐old individuals. Steroid producing cells are detected in the undifferentiated gonad and in the differentiated ones of both sexes. Anatomical differentiation of the gonad precedes cytological differentiation and female differentiation largely precedes that of the male. Gonad development and differentiation are also associated with structural changes of connective tissue, viz. collagen‐rich areas are massive in developing testes and reduced in ovaries. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Gonadal development and fertility of mice treated prenatally with cadmium during the early organogenesis stages

Gonadal development was studied in mouse embryos that were exposed to cadmium during the early organogenesis stages. At 13.5 days, both the male and the female embryos had small genital ridges. Fewer primordial germ cells were found in the male embryos. In both sexes, many primordial germ cells were left outside the genital ridges, presumably as a result of retarded cell migration. In 16.5-day embryos, the size of the testes and ovaries and the number of differentiating germ cells were reduced. Many germ cells degenerated during the differentiation to spermatogonia and meiotic oocytes. The perturbed gonadal development was less likely to be caused primarily by a defective hypothalamopituitary axis but was more a part of the general cadmium-induced damage. The fertility of the male offspring was impaired by the prenatal cadmium insult, but the females were apparently fertile. The epididymal spermatozoa of the cadmium-affected offspring showed a lower fertilizing capacity in vitro. The impaired fertility of the cadmium-affected mice was the result of poor gonadal growth, paucity of germ cells, and defective maturation of the gametes.     

Localization and quantification of gonad serotonin during gametogenesis of the surf clam, Spisula solidissima

In the surf clam, Spisula solidissima, serotonin was reported to induce spawning when injected into the gonads. At nanomolar concentrations, it facilitates the fertilizability of oocyte by sperm, at micromolar concentration, it triggers the meiotic maturation of prophase 1-arrested oocytes, thus mimicking the effect of sperm. To further understand the role of serotonin in the gametogenic and spawning processes, we used both immunohistochemistry and high-pressure liquid chromatography linked with electrochemical detection to detect serotonin in the gonads of the surf clam. We found serotonin-containing varicose fibers covering the surface of the germinal epithelium in both sexes. The area occupied by the serotonergic innervation field encircling gonad acini varied according to the gonadal stages (active phase, ripe phase, partially spawned phase, spent phase). We also found large variations in the serotonin concentration between specimens during the gametogenic cycle. The serotonin concentration was correlated with gonad growth: it decreased in the ripe phase in comparison with the previous phase, the active phase. We attribute the decrease to the increase of total gonad mass in this stage. In contrast, as spawning begins, the total gonad mass declines while the gonad serotonin concentration increases to a level similar to that found in active phase. The finding that prior to spawning, serotonin is present in the gonads within fibers exhibiting distinct varicosities suggests that it is implicated in spawning.     

A germline-specific gap junction protein required for survival of differentiating early germ cells

Germ cells require intimate associations and signals from the surrounding somatic cells throughout gametogenesis. The zero population growth (zpg) locus of Drosophila encodes a germline-specific gap junction protein, Innexin 4, that is required for survival of differentiating early germ cells during gametogenesis in both sexes. Animals with a null mutation in zpg are viable but sterile and have tiny gonads. Adult zpg-null gonads contain small numbers of early germ cells, resembling stem cells or early spermatogonia or oogonia, but lack later stages of germ cell differentiation. In the male, Zpg protein localizes to the surface of spermatogonia, primarily on the sides adjacent to the somatic cyst cells. In the female, Zpg protein localizes to germ cell surfaces, both those adjacent to surrounding somatic cells and those adjacent to other germ cells. We propose that Zpg-containing gap junctional hemichannels in the germ cell plasma membrane may connect with hemichannels made of other innexin isoforms on adjacent somatic cells. Gap junctional intercellular communication via these channels may mediate passage of crucial small molecules or signals between germline and somatic support cells required for survival and differentiation of early germ cells in both sexes.     

Strongylocentrotus drobachiensis oocytes maintain a microtubule organizing center throughout oogenesis: implications for the establishment of egg polarity in sea urchins

Although it has been known for over a century that sea urchin eggs are polarized cells, very little is known about the mechanism responsible for establishing and maintaining polarity. Our previous studies of microtubule organization during sea urchin oogenesis described a cortical microtubule-organizing center (MTOC) present during germinal vesicle (GV) migration in large oocytes. This MTOC was localized within the future animal pole of the mature egg. In this study we have used electron microscopy and immunocytochemistry to characterize the structure of this MTOC and have established that this organelle appears prior to GV migration. We show that the cortical MTOC contains all the components of a centrosome, including a pair of centrioles. Although a centrosome proper was not found in small oocytes, the centriole pair in these cells was always found in association with a striated rootlet, a structural remnant of the flagellar apparatus present in precursor germinal cells (PGCs). The centrioles/striated rootlet complex was asymmetrically localized to the side of the oocyte closest to the gonadal wall. These data are consistent with the previously proposed hypothesis that in echinoderms the polarity of the PGCs in the germinal epithelium influences the final polarity of the mature egg.