Ectopic germ cells: natural model for the study of germ cell sexual differentiation

Sexual differentiation of germ cell deficient gonads in the medaka, Oryzias latipes

To determine whether germ cells perform any function in gonadal sexual differentiation, development of gonads in the medaka, Oryzias latipes, after exposure to busulfan was investigated. Busulfan suppressed proliferation of early germ cells, thus significantly reducing the number of germ cells and generating regions without germ cells in the developing gonads. Globular structures were observed in the parenchyma in these regions. The structure was male specific, developed at the same time as acinus (seminiferous tubule precursor), surrounded by the basal lamina, and contained characteristic desmosomes. These results strongly suggest that these globular structures are the precursors of seminiferous tubules devoid of germ cells. In the ovary, no follicles were observed but a well-developed ovarian cavity was evident. From these results we conclude that differentiation of gonadal parenchyma cells, except for follicular ones, is not germ cell dependent, though morphological differentiation of the somatic cells seems to follow the differentiation of germ cells.

Estrogen-induced gonadal sex reversal in the tammar wallaby

Estrogens have a feminizing effect on gonadal differentiation in fish, amphibians, reptiles, and birds. However, the role of estrogen during gonadal differentiation in mammals is less clear. We investigated the effect of estrogen on gonadal differentiation of male tammar wallabies. Male pouch young were treated orally with estradiol benzoate or oil from the day of birth, before seminiferous cords develop, to Day 25 postpartum and were killed at Day 50 postpartum. In all estrogen-treated neonates, a decrease in gonadal volume, volume of the seminiferous cords, thickness of the tunica albuginea, and number of germ cells was found. The stage of treatment affected the magnitude of the response. Two of three male young born prematurely after 25 days of gestation and treated subsequently with estradiol had ovary-like gonads, with well-developed cortical and medullary regions and primordial follicle formation. Furthermore, at Day 50 postpartum, many (21%) of the germ cells in these sex-reversed ovaries were in the leptotene and zygotene stages of meiosis, similar to female germ cells at the same stage of development. In the other males born on Day 26 of gestation or later, estradiol treatment from the day of birth caused development of dysgenetic testes, with abnormal Sertoli cells, atrophy of the seminiferous tubules and tunica albuginea, and absence of meiotic germ cells. In this marsupial, therefore, estradiol can induce either partial or complete transformation of the male gonads into an ovary with meiotic germ cells. These results confirm that estrogen can inhibit early testicular development, and that testis determination occurs during a narrow window of time.

Sex determination: lessons from families and embryos

Genetic studies in familial cases of sex reversal and in human embryos have contributed to the understanding of human sex determination and its disorders. For some heritable disorders of sex reversal, the gonadal phenotype was frequently overlooked until sex reversal was discovered fortuitously by chromosome analysis, often resulting in preventable complications. Within families, the phenotypes are variable and, in some instances, these can be explained by known genetic mechanisms. When a novel molecular marker is shared by family members affected with sex reversal, the level of confidence is higher that this marker may play a role in the development of the phenotype. The identification of pedigrees with sufficient power to generate significant linkage of disorder (lod) scores from genomewide screens can now lead to the identification of novel sex-determining genes. Studies of the gonads of 46,XY human embryos have shown that SOX9 expression follows a pattern similar to that of SRY and, in both instances, stands in contrast to the expression observed in the mouse. Differences between human and mouse embryonic gonads have also been observed for the temporal expression of DAX1, suggesting that the mechanisms of action of SRY, SOX9, and DAX1 may vary between these and other species.

Secretion of zona pellucida glycoprotein mZP2 by growing oocytes from mZP3(+/+) and mZP3(-/-) mice

The mouse egg extracellular coat, or zona pellucida (ZP), is composed of three glycoproteins, called mZP1-3, which are synthesized and secreted concomitantly by growing oocytes. Disruption of the mZP3 gene by targeted mutagenesis yields mice that are homozygous nulls (mZP3(-/-)). Growing oocytes from mZP3(-/-) mice do not synthesize mZP3 mRNA or protein and, as a result, do not assemble a ZP. Here, we examined secretion of mZP2 by growing oocytes and eggs from mZP3(-/-) mice, as well as incorporation of mZP2 into the ZP of oocytes from mZP3(+/+) mice. Laser scanning confocal microscopy (LSCM) of antibody-labeled samples showed that, indeed, mZP2 was synthesized and secreted by oocytes isolated from mZP3(-/-) mice and cultured in vitro. Nascent mZP2 was found in the culture medium, associated with the surface of the plasma membrane of growing oocytes, and in the oocyte cytoplasm. By contrast, mZP2 was barely detectable at any of these sites when ovulated eggs from mZP3(-/-) mice were examined. Examination of oocytes from wild-type (mZP3(+/+)) mice showed that, while a portion of nascent mZP2 was assembled into the ZP (approximately 40%), here too a significant fraction was secreted into the culture medium (approximately 60%). Similar results also were obtained when intact pre-antral follicles were isolated from mZP3(+/+) mice and cultured in vitro. Several of these observations are consistent with previous results obtained with oocytes from heterozygous null mice (mZP3(+/-)). Furthermore, the results suggest that ZP assembly from nascent glycoproteins may be a stochastic process that requires the presence of both mZP2 and mZP3 and occurs completely outside the growing oocyte.

Germ cell nuclear antigen (GCNA1) expression does not require a gonadal environment or steroidogenic factor 1: examination of GCNA1 in ectopic germ cells and in Ftz-F1 null mice

The germ cell lineage is first recognized as a population of mitotically proliferating primordial germ cells that migrate toward the gonadal ridge. Shortly after arriving at the gonadal ridge, the germ cells begin to initiate a commitment to gamete production in the developing gonad. The mechanisms controlling this transition are poorly understood. We recently reported that a mouse germ cell nuclear antigen 1 (GCNA1) is initially detected in both male and female germ cells as they reach the gonad at 11.5 days postcoitum (dpc). GCNA1 is continually expressed in germ cells through all stages of gametogenesis until the diplotene/dictyate stage of meiosis I. Since GCNA1 expression commences soon after primordial germ cells arrive at the gonadal ridge, we wanted to determine whether the gonadal environment was essential for induction of GCNA1 expression. By examining GCNA1 expression in germ cells that migrate ectopically into the adrenal gland, we determined that both the gonadal and adrenal gland environments allow GCNA1 expression. We also examined GCNA1 expression Ftz-F1 null mice, which were born lacking gonads and adrenal glands. During embryonic development in the Ftz-F1 null mice, the gonad and most germ cells undergo apoptotic degeneration at about 12.5 dpc. While most of the germ cells undergo apoptosis without expressing GCNA1, a few surviving germs cells, especially outside the involuting gonad clearly express GCNA1. Thus, although the Ftz-F1 gene is essential for gonadal and adrenal development, induction of GCNA1 expression in germ cells does not require Ftz-F1 gene products. The finding that germ cell GCNA1 expression is not restricted to the gonadal environment and is not dependent on the Ftz-F1 gene products suggests that GCNA1 expression may be initiated in the germ cell lineage by autonomous means.

Aetiology of intersexuality in female (XX) pigs, with novel molecular interpretations

This review considers the problem of ovotestis formation in animals of 38,XX chromosome complement. After a clinical description, attention focuses on the condition of the gonads and genital tract. A complete spectrum of gonadal types has been found, ranging from a single ovotestis almost invariably on the right-hand side to both gonads appearing as testicular-like structures, sometimes with a distinct tunica albuginea. The ovotestis or testis-like structure may have descended to an inguinal or scrotal location. Although interstitial cells of Leydig and seminiferous tubules were always abundant in testicular tissue, germ cells were never present. The lumen of the seminiferous tubules was packed with pale-staining, Sertoli-like cells. A bicornuate uterus was characteristic but suppression of the proximal portion of the Müllerian duct always adjoined an ovotestis; a corresponding development of the Wolffian duct featured as a convoluted epididymis. Inhibition of the Fallopian tube was attributed to a local influence of AMH from the Sertoli cells, as was the failure of small Graafian follicles within an ovotestis to respond to injected gonadotrophins. As to the aetiology of an ovotestis, defective colonisation of the genital ridges by primordial germ cells is considered, as is evidence for incorporation of adrenal cells into the embryonic gonad. Molecular probing has failed to reveal the classical sex-determining gene, Sry, and other Y-related DNA sequences such as Zfy and DYZI in almost all the intersex animals examined. Currently favoured as an explanation for ovotestis formation is a mutation in the inhibin gene within granulosa cells of Graafian follicles. Such a mutation would prompt secretion of the closely comparable glycoprotein molecule AMH in these genetic females, with a resultant progressive virilisation of gonadal tissue. The proposed mutation may be carried as an autosomal recessive gene by certain boars. Varying amounts of AMH secretion or differing timescales for the transition from inhibin to AMH could in part explain differing degrees of ovotestis formation. Despite this proposition, interactions between genes that prescribe functional testicular tissue, enhanced rates of gonadal development, and left-right asymmetries between the paired gonads now require systematic study.

Targeted disruption of the mZP3 gene results in production of eggs lacking a zona pellucida and infertility in female mice

Mammalian eggs are surrounded by a thick extracellular coat, the zona pellucida, that plays important roles during early development. The mouse egg zona pellucida is constructed of three glycoproteins, called mZP1, mZP2, and mZP3. The gene encoding mZP3 is expressed only by growing oocytes during a 2- to 3-week period of oogenesis. Here, the mZP3 gene was disrupted by targeted mutagenesis using homologous recombination in mouse embryonic stem cells. Viable female mice homozygous for the mutated mZP3 allele (mZP3-/-) were obtained. These mice are indistinguishable in appearance from wild-type (mZP3+/+) and heterozygous (mZP3+/-) littermates. However, although ovaries of juvenile and adult mZP3-/- females possess growing and fully grown oocytes, the oocytes completely lack a zona pellucida. Consistent with this observation, eggs recovered from oviducts of superovulated, adult mZP3-/- females also lack a zona pellucida. Thus far, mZP3-/- females mated with wild-type males have failed to become pregnant.

A parallel between development and evolution: germ cell recruitment by the gonads

In gonad-bearing animals gametogenesis can be divided into three main phases. During embryonic development the primordial germ cells move towards the gonadal primordia. A long, intra-gonadal phase follows during which the germ cells grow and differentiate. Mature germ cells are finally released from the gonads and brought to the exterior. Thus, germ cells are successively motile, non-motile and motile again. This complex life history is given here a simple evolutionary interpretation. The basic assumption is that primitive Metazoa already had germ cells, but no gonads to harbour them. Higher animals acquired gonads, which sequestered the germ cells, thus creating the temporary confinement experienced by germ cells in most present-day Metazoa. This evolutionary scheme may explain why several steps of germ cell differentiation are totally or partially independent of the gonads. These steps presumably existed in primitive, gonad-free Metazoa, and conserved their autonomy in higher animals.

X-chromosome activity of the mouse primordial germ cells revealed by the expression of an X-linked lacZ transgene

We have determined the timing of the inactivation and reactivation of the X chromosome in the mouse primordial germ cells (PGCs) by monitoring the expression of an X-linked HMG-lacZ reporter gene. PGCs were identified by their distinct alkaline phosphatase activity and they were first localised in the primitive streak and allantoic bud of the 7.5-day gastrulating embryo. Although inactivation of the transgene was found in some PGCs at these sites, at least 85% of the population were still expressing the lacZ gene. This suggests that, although X-inactivation has commenced during gastrulation, the majority of PGCs still possess two active X chromosomes. Transgene activity remained unchanged during the relocation of PGCs to the hindgut endoderm, but decreased abruptly when PGCs left the hindgut and migrated through the mesentery. X-inactivation was completed during the migration of PGCs, but not simultaneously for the whole population. The first wave of PGCs entering the genital ridge at 9.5 days did not immediately re-activate the silent transgene until about 24 hours later. Re-activation of the transgene took place in over 80% of PGCs entering the genital ridge at 10.5-13.5 days p.c., preceding the entry into meiosis. About 90% of the meiotic germ cells in the 14.5-15.5 day fetal ovary expressed the transgene. Similar profiles of transgene activity were observed in PGCs of embryos that have inherited the lacZ transgene from different parents, showing unequivocally that X-inactivation in the germ cell lineage is not related to parental legacy.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental arrest of fertilized eggs from the B6.YDOM sex-reversed female mouse

When the Y chromosome of a Mus musculus domesticus mouse strain is placed onto the C57BL/6J (B6) inbred background, the XY progeny develop ovaries or ovotestes but never normal testes during fetal life. While some of the hermaphroditic males become fertile, none of the XY females produces litters. Here, we examined the fertility and development of oocytes derived from the XY female mouse. With or without preceding injection of gonadotropins, female mice were mated with normal B6 males, and their embryos were recovered at various developmental stages. In vitro fertilization was performed with the eggs recovered from the oviduct after treatment with gonadotropins. Development of embryos was examined by both light and electron microscopy. The results indicate that the oocytes released from the B6.YDOM ovary were efficiently fertilized and often initiated the first cell cleavage, but all embryos died during early preimplantation periods. Even when oocytes were fertilized in vitro, minimizing their exposure to the XY oviduct/uterus environment, most embryos died at the 1- or 2-cell stage. A few exceptional embryos reached the 4- or 8-cell stage, but abnormalities were evident in both nuclear and cytoplasmic structures of all embryos. After cleavage, neighbouring blastomeres were only loosely associated, and microvilli were abundant at the intercellular interfaces. We postulate that oocytes of the B6.YDOM female mouse become defective during XY ovarian differentiation, and, hence, fail to proceed through normal embryonic development.

Development of primordial germ cells in the mouse

Primordial germ cells in the mouse are known to be derived from the epiblast. They can be identified histochemically, by their high alkaline phosphatase activity. At 8 d post coitum they have been observed within the embryonic part of the egg cylinder, at the posterior end of the primitive streak. Earlier, at 7.25 days post coitum, we have observed them embedded in the extra-embryonic mesoderm, as a tight clump. Germ cell counts over the 7-8 d period of gastrulation have been made. They are consistent with either of two models: (1) derivation of the germ cell lineage from a very small stem cell pool, followed by a constant rate of proliferation, and (2) derivation from a larger initial stem cell pool, followed by a period when germ cells are differentiating but not dividing. From their initial extra-embryonic location, germ cells spread into the mesoderm of the primitive streak, and the endoderm of the yolk sac and hind gut. Active locomotion is probably required for their passage up the dorsal mesentery and into the genital ridges. Mutant alleles at two loci, W (White-spotting) and Sl (Steel), drastically reduce the number of germ cells reaching the ridges. Since those that succeed in reaching the ridges suffer little if any delay, the defect is unlikely to be due to reduced powers of locomotion, but rather to a failure of proliferation or survival. W acts cell-autonomously: its gene product is the c-kit polypeptide, a transmembrane tyrosine kinase receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Oocyte numbers are reduced in developing mouse ovaries cultured in testis-conditioned medium

Reduced numbers of oocytes were present in fetal ovaries of 13 d post coitum (dpc) mice, cultured for 4 d in medium conditioned (-CM) for 2 d by 13 dpc testes, compared with ovaries maintained in standard unconditioned medium. This effect was abolished by heat-inactivation of the testis-CM. Electrophoretic analysis of conditioned media revealed differences between testis-CM and ovary-CM. Ovarian differentiation was otherwise unaffected by the testis-CM and gonadal volume was not significantly reduced. Organisation of ovigerous cords proceeded, even though the full complement of oocytes was absent, and connective tissue septa developed normally between the small cords. The reduction in oocyte numbers occurred without any inhibition of müllerian duct development. Since others, using transgenic mice, have shown, that higher concentrations of antimüllerian hormone are required to decrease oocyte numbers than are necessary for duct regression, our results suggest that an additional factor is involved in producing this modified effect.

Sertoli cells and testicular differentiation in the rat fetus

The fetal testis is not merely a precursor of the adult organ: it is indeed an endocrine gland whose function is the masculinization of the fetus. It differs physiologically and morphologically from the adult testis. In this paper, the first stages of testicular differentiation in the rat are described, with special emphasis on the ultrastructural aspects. At the stage of 13.5 days after fertilization, the first Sertoli cells differentiate; they are characterized by a voluminous and little electron dense cytoplasm, a well-developed RER formed by vesicles and short cisternae filled with a flocculent material. Progressively, they polarize and adhere to one another by adherens-like junctions and cytoplasmic interdigitations to form the differentiating seminiferous cords. In the basal part of the Sertoli cells, a mat of microfilaments differentiates under the plasmalemma, while cytoplasmic blebs protruding in the extracellular space tend to disappear. A continuous basal lamina delineating the seminiferous cords begins to appear on day 14.5 and becomes widespread on day 15.5. These observations, when compared with other data from the literature, emphasize the fact that the differentiation of the Sertoli cells is the first morphological event during testicular differentiation. A possible role of the Sertoli cells in the subsequent organogenesis of the testis is suggested.

Germinal cell ectopism in the strepsirhine prosimian Galago crassicaudatus crassicaudatus

The presence of germinal cells outside of the embryonal and fetal gonads of the strepsirhine prosimian Galago crassicaudatus crassicaudatus is described. Forty-three embryos and fetuses from day 26 or 27 of gestational age to near term were studied: more than 90% possessed germinal cells in ectopic sites situated either far from (extragonadal ectopism) or close to the gonads (perigonadal ectopism). The first sites were the walls of the aorta and mesenteric artery, the stroma between the aorta and the cardinal vein, and the retroperitoneal neuroganglia. The second were the mesenchyme dorsal to the gonads and around the vestigia of the mesonephric glomeruli and tubules, and the rete ovarii and testis. The ectopic cells were generally present in conscpicuous numbers, in some animals being more numerous than in the gonads. Those situated far from the gonads underwent degeneration and decreased significantly in numbers during post-embryonal stages of development, while the others remained numerous and functionally active up to near term. While the differentiation of the extragonadal germinal cells after day 60 of gestational age could not be studied due to technical difficulties, the XX and XY cells in perigonadal sites appeared to follow patterns of differentiation identical to those of their entopic counterparts.

Germ cell deficiency causes testis cord differentiation in reconstituted mouse fetal ovaries

Sex-reversal in fetal ovaries was studied by using a dissociation-reconstitution technique. Gonads of 12.5 gestation-day male and female mouse fetuses were dissociated into single cells. To eliminate germ cells, the dissociated cells were cultured for 14 h, and then somatic cells attached to culture dishes were harvested and aggregated by gyratory culture for 24 h. The aggregates were then transplanted into ovarian bursa in ovary-ectomized nude mice. The recovered explants were examined histologically. Male somatic cells developed into testes containing Sertoli cells, Leidig cells, and tunica albuginea. Female somatic cells formed testis cords and differentiated into Sertoli cells, but they did not differentiate into other testis components or ovarian tissues. However, aggregates consisting of both female and male somatic cells differentiated into well-developed testes containing Leidig cells and tunica albuginea as well as Sertoli cells. Enzyme marker analysis showed significant contributions of female cells in these organized testes. In contrast, aggregates containing both female germ cells and somatic cells developed into ovaries and did not differentiate into any testicular tissues. The results indicate that female somatic cells in fetal gonads at 12.5 gestation day have the potency to form testis cords and differentiate into Sertoli cells. The subsequent steps in testis development require the contributions of male cells. The present study also suggests that testicular differentiation is independent of germ cells but ovarian development involves the interaction between germ cells and somatic cells.

Control mechanisms of testicular differentiation

In this paper the importance of unknown factors responsible for the initial differentiation of a gonadal primordium is stressed. The hypothesis that in the absence of testis determining genes (TDG) the indifferent gonad is programmed to become an ovary is considered further. The TDG(s) are expressed only among cells already marked as gonadal cells, and they seem mainly to change the chronological sequence and intensity of expression of processes common to both sexes. The chronology of the normal events necessary for testicular differentiation and the fact that some of these events can be dissociated from one another under experimental conditions in vitro, suggest that many genes are involved in testicular differentiation and that the so-called testis-determining genes are probably regulatory genes.

The development of the sexually indifferent gonad in the prosimian, Galago crassicaudatus crassicaudatus

The morphogenesis of the sexually indifferent gonad of the primate Galago crassicaudatus crassicaudatus was studied by high-resolution light microscopy and electron microscopy in 15 embryos aged 26 to 33 days. Onset of gonadal development follows the morphogenesis of the mesonephros by a conspicuous interval and is identified as the time when the first primordial germinal cells arrive in the region ventral to the central third of the mesonephros; this is followed by intense proliferation of the coelomic mesothelial cells lining the area. They become organized into short piles that deepen in the underlying mesenchyme, enclosing the germinal cells in the process. Rapidly, the piles become confluent forming a compact mass, the gonadal blastema, which is soon cleaved into gonadal cords by stroma and vascular lacunae. The mesonephros becomes involved in the morphogenesis of the gonad only in late stages of development when anatomic continuities become established between the capsules of its regressing glomeruli and the elongating gonadal rete cords. These observations show that in the Galago the somatic cells of the gonadal blastema, i.e., the precursors of the definitive testicular and ovarian sustentacular cells, derive from the coelomic mesothelium in contrast to other mammals, e.g., ruminants and rodents, where they are of mesonephric derivation. This important point is discussed in light of the differences that exist among species with regard to the structural complexity, functionality, and stages of differentiation/involution of their mesonephroi on the one hand, and the time of gonadal development on the other.

Morphological development of the mouse gonad in tda-1 XY sex reversal

tda-1 XY sex reversal occurs when the Y chromosome of at least some populations of wild Mus musculus domesticus is placed on the C57BL/6J genomic background. Gross anatomical observations have previously revealed morphological similarities among fetal ovotestes of tda-1 and Tas-inherited XY sex reversals and BALB/cWt mosaic hermaphrodites. We studied the histology of tda-1 XY sex-reversed gonads, ranging in age from day 14 of gestation to adult. The obtained data revealed additional similarities with ovotestes of BALB/cWt mosaic hermaphrodites as well as with ovotestes of hermaphrodites found in XXSxr and XX/XY chimeras. It is proposed that ovotestes occurring in these various hermaphroditic conditions may be formed through a common pathway.

Quantitative histology of human fetal testes in chromosomal disease

Morphometric studies on male gonads were performed in 35 midterm fetuses aborted after prenatal diagnosis of a chromosome anomaly and in 11 chromosomally normal controls. A significant reduction of the number and volume percentage of premeiotic germ cells was observed in the chromosomally abnormal cases. Germ cell depletion was correlated with the severity of the chromosomal disease. It was least expressed in the XYY condition. In trisomy 13 and 18, depletion lead to values of less than a half or even a fourth the values of controls. Complex anomalies with XXY or XYY in addition to an autosomal disorder showed a moderate effect on germ cell reduction. No morphological differences were observed in germ cells or in Sertoli cells in a comparative electron microscopic study. Paucity of fetal germ cells can result from impaired colonization of the gonadal ridge, from low mitotic activity, or from increased premeiotic cell loss. All three factors seem to contribute to the above findings.

Differentiation of mouse ectopic germinal cells in intra- and perigonadal locations

Four hundred and thirteen ectopic germinal cells in the testicular and extratesticular stroma and in the rete testis of mouse fetuses from day 13 of uterine development to term were studied together with 161 ectopic germinal cells in the rete ovarii and periovarian stroma of female fetuses at days 17 and 18 of intrauterine life. The morphology and the differentiation of these ectopic germinal cells were compared to those of germinal cells within seminiferous and ovigerous cords. While the ectopic germinal cells in the testis and in the rete testis followed patterns of differentiation identical with those in the seminiferous cords throughout the period included in the study, those in the extratesticular stroma behaved like entopic germinal cells only through day 17, since at days 18 and 19 many of them entered meiotic prophase just like XX germinal cells in the ovigerous cords. No differences were noted between ectopic and entopic ovarian germinal cells. The results of this study show that the factors responsible for the male differentiation of XY germinal cells are not limited to the seminiferous cords but operate throughout the testicular territory, and confirm that outside the testis, XY germinal cells differentiate as female; our study also corroborates the thesis that the differentiation of XX germinal cells is an autonomous and ubiquitous process.

Foetal meiosis in the testis of the rodent Octodon degus

Different stages of meiotic prophase have been studied in foetal testes of the rodent, Octodon degus, using the light and electron microscope. Special attention was focused on the ultrastructural morphology of these meiotic cells in comparison to pre-spermatogonia of foetal testes and meiotic spermatocytes of the adult male testis. Meiosis occurs in only a few cells located among fibroblasts of the tunica albuginea or in the region of the gonadal blastema. The foetal meiotic process resembles adult meiosis in its ultrastructural characteristics; typical pachytene synaptonemal complexes and leptotene or diplotene axial elements appear associated to the chromatin. This process occurs at the same foetal age that meiosis commences in the ovary, thus reinforcing the idea that both meiosis-inducing and meiosis-preventing substances are secreted in both sexes. The intra-or extracordonal localization of the germ cells would be an important factor in determining the cells' response to these substances.

Studies on mouse germ cells inside and outside the gonad

Soon after entering the genital ridge, mouse primordial germ cells take either a male or a female pathway of development. Which direction they follow, and how successful their subsequent gametogenesis turns out to be, depends on various factors, including their own genotype and the phenotype of the gonad they inhabit. Experimental situations that have helped to throw light on this problem include XX in equilibrium with XY chimeras, XO females, and mice carrying the sex-reversed gene (Sxr). Further information can be obtained by studying germ cells that fail to enter the genital ridge, as well as culturing isolated germ cells.

Germ cell differentiation in mouse adrenal glands

The differentiation of germ cells in the adrenal glands of 26 male and female Swiss albino mice was studied in sequential stages of development, from day 12 1/2 of intrauterine life to postnatal day 21; the study was performed by means of high-resolution light microscopy and electron microscopy. In 12 1/2- and 13-day-old embryos, the ectopic cells had morphologic characteristics typical of primordial germ cells, whereas in 14- and 15-day-old fetuses they were identifiable as oogonia. In male and female fetuses from day 17 to term, all ectopic germinal elements entered meiotic prophase, reached diplotene, and differentiated into oocytes in perfect adherence to mouse ovarian timetables. In the postnatal animals, females as well as males, all oocytes progressed through the postmeiotic phase of growth just as they normally do in ovarian follicles, and, in the 2- and 3-week-old animals, they displayed features identical to those exhibited by oocytes in large antral follicles, including a zona pellucida. Germinal elements were no longer seen in the adrenals of animals older than 3 weeks. Our study shows that mammalian germ cells are capable of developing even outside the gonads, and that in ectopic sites they all differentiate as oocytes irrespective of their genetic sex.