Oocytes in the testis

Two-step oligoclonal development of male germ cells

During mouse development, primordial germ cells (PGCs) that give rise to the entire germ line are first identified within the proximal epiblast. However, long-term tracing of the fate of the cells has not been done wherein all cells in and around the germ-cell lineage are identified. Also, quantitative estimates of the number of founder PGCs using different models have come up with various numbers. Here, we use tetrachimeric mice to show that the progenitor numbers for the entire germ line in adult testis, and for the initiating embryonic PGCs, are both 4 cells. Although they proliferate to form polyclonal germ-cell populations in fetal and neonatal testes, germ cells that actually contribute to adult spermatogenesis originate from a small number of secondary founder cells that originate in the fetal period. The rest of the "deciduous" germ cells are lost, most likely by apoptosis, before the reproductive period. The second "actual" founder germ cells generally form small numbers of large monoclonal areas in testes by the reproductive period. Our results also demonstrate that there is no contribution of somatic cells to the male germ cell pool during development or in adulthood. These results suggest a model of 2-step oligoclonal development of male germ cells in mice, the second step distinguishing the heritable germ line from cells selected not to participate in forming the next generation.

Influence of sex chromosome constitution on the genomic imprinting of germ cells

Germ cells in XY male mice establish site-specific methylation on imprinted genes during spermatogenesis, whereas germ cells in XX females establish their imprints in growing oocytes. We showed previously that in vitro, sex-specific methylation patterns of pluripotent stem cell lines derived from germ cells were influenced more by the sex chromosome constitution of the cells themselves than by the gender of the embryo from which they had been derived. To see whether the same situation would prevail in vivo, we have now determined the methylation status of H19 expressed from the maternal allele, and the expression and methylation status of a paternally expressed gene Peg3, in germ cells from sex-reversed and control embryos. For these imprinted genes, we conclude that the female imprint is a response of the germ cells to undergoing oogenesis, rather than to their XX chromosome constitution. Similarly, both our XY and our sex-reversed XX male germ cells clearly showed a male rather than a female pattern of DNA methylation; here, however, the sex chromosome constitution had a significant effect, with XX male germ cells less methylated than the XY controls.

Genomic imprinting of XX spermatogonia and XX oocytes recovered from XX<-->XY chimeric testes

We produced XX<-->XY chimeras by using embryos whose X chromosomes were tagged with EGFP (X*), making the fluorescent green female (XX*) germ cells easily distinguishable from their nonfluorescent male (XY) counterparts. Taking advantage of tagging with EGFP, the XX* "prospermatogonia" were isolated from the testes, and the status of their genomic imprinting was examined. It was shown that these XX cells underwent a paternal imprinting, despite their chromosomal constitution. As previously indicated in sex-reversal XXsxr testes, we also found a few green XX* germ cells developed as "eggs" within the seminiferous tubules of XX*<-->XY chimeric testes. These cells were indistinguishable from XX* prospermatogonia at birth but resumed oogenesis in a testicular environment. The biological nature of the "testicular eggs" was examined by recovering the eggs from chimeric testes. The testicular eggs not only formed an egg-specific structure, the zona pellucida, but also were able to fuse with sperm. The collected testicular eggs were indicated to undergo maternal imprinting, despite the testicular environment. The genomic imprinting did not always follow the environmental conditions of where the germ cells resided; rather, it was defined by the sex that was chosen by the germ cells at early embryonic stage.

Germ cells enter meiosis in a rostro-caudal wave during development of the mouse ovary

Germ cells in the mouse embryo remain undifferentiated until about 13.5 days post-coitum (dpc), when male germ cells enter mitotic arrest and female germ cells enter meiosis. The molecular signals and transcriptional control mechanisms governing the differential fate of germ cells in males and females remain largely unknown. In order to gain insights into the behavior of germ cells around this period and into likely mechanisms controlling entry into meiosis, we have studied by wholemount in situ hybridization the expression pattern of two germ cell-specific markers, Oct4 and Sycp3, during mouse fetal gonad development. We observed a dynamic wave of expression of both genes in developing ovaries, with Oct4 expression being extinguished in a rostro-caudal wave and Sycp3 being upregulated in a corresponding wave, during the period 13.5-15.5 dpc. These results indicate that entry into meiosis proceeds in a rostro-caudal progression, in turn suggesting that somatically derived signals may contribute to the control of germ cell entry into meiosis in developing ovaries.

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.

Deficiency of Trp53 rescues the male fertility defects of Kit(W-v) mice but has no effect on the survival of melanocytes and mast cells

Mutations of the receptor tyrosine kinase, Kit, or its ligand, mast growth factor (Mgf), affect three unrelated cell populations: melanocytes, germ cells, and mast cells. Kit signaling is required initially to prevent cell death in these lineages both in vitro and in vivo. Mgf appears to play a role in the survival of some hematopoietic cells in vitro by modulating the activity of p53. Signaling by Mgf inhibits p53-induced apoptosis of erythroleukemia cell lines and suppresses p53-dependent radiation-induced apoptosis of bone marrow cells. We tested the hypothesis that cell survival in Kit mutant mice would be enhanced by p53 deficiency in vivo. Double-mutant mice, which have greatly reduced Kit receptor tyrosine kinase activity and also lack Trp53, were generated and the affected cell lineages examined. Mast cell, melanoblast, and melanocyte survival in the double Kit(W-v/W-v):Trp53(-/-) mutants was not increased compared to the single Kit(W-v/W-v):Trp53(+/+) mutants. However, double-mutant males showed an increase in sperm viability and could father litters, in contrast to their homozygous Kit mutant, wild-type p53 littermates. This germ cell rescue appears to be male specific, as female ovaries were similar in mice homozygous for the Kit mutant allele with or without p53. We conclude that defective Kit signaling in vivo results in apoptosis by a p53-independent pathway in melanocyte and mast cell lineages but that in male germ cells apoptosis in the absence of Kit is p53-dependent.

Ovotestes in B6-XXSxr sex-reversed mice

The sex-reversed mutation Sxr results in XX males. In the absence of any other mutations, testis differentiation in XXSxr fetuses is essentially normal and only one report of an XXSxr fetus with ovotestes is in the literature. We report that 84% (21/25) of 13 days postcoitum XXSxr fetuses on the B6 inbred genomic background have ovotestes. Ovotestes were found in fetuses from both Sxra and Sxrb variants. Examination of fetuses older than 13 dpc suggests that the presence of ovotestes is transient in most fetuses. However, one overt hermaphrodite was identified after birth. The development of ovotestes is associated with the inbred background and is exacerbated by the dominant spotting oncogene allele KitW-42J. We propose that spreading of X-inactivation into the Sxr region resulting in loss of Sry expression is more extensive in B6-Sxr strains.

Extracellular matrix abnormalities in testis and epididymis of XXSxr ("sex-reversed") mice

Sex-reversed (Sxr) is a duplication of the sex-determining region of the Y chromosome, which gets transposed to a paternal X chromosome. Chromosomally female (XX) zygotes that receive this XSxr chromosome develop as apparent males. Previous work on XXSxr mice (called pseudomales) showed extracellular matrix (ECM) ultrastructural abnormalities in the epididymis and testis. This study examined the biochemical nature of these abnormalities. More hydroxyproline (an indicator of collagen) was noted in the pseudomale testis and epididymis compared to normal male tissues. Western blot analysis showed increased collagen IV in the pseudomale testis and epididymis. In both the hydroxyproline and collagen IV studies, the epididymis was found to contain higher levels of these substances than the testis for both genotypes. There also appeared to be increased messenger RNA for tissue inhibitor of metalloproteinases (Timp), a regulator of collagen, in the pseudomale testis. Data from these studies seem to indicate that the XXSxr genotype influences ECM deposition and/or turnover and exerts a direct genetic influence on the development of the testis and epididymis. According to the existing paradigm of mammalian sexual development, the epididymis is expected to be normal in the presence of adequate androgenization and independent of chromosomal and genetic sex. The results presented here differ from what would be predicted by this paradigm.

Development of the mammalian gonad: the fate of the supporting cell lineage

Sex determination in mammals is mediated via the supporting cell lineage in the fetal gonad. In the very early stages of gonadal development, the fate of the supporting cell population is critically dependent on the expression of the male-determining gene on the Y chromosome. If this gene is absent or fails to be expressed, or is expressed too late or in too small a number of supporting cells, all supporting cells (XX or XY) differentiate as pre-follicle cells and development proceeds along the female pathway. Supporting cells in which the male-determining gene is expressed in a timely manner differentiate as pre-Sertoli cells; given sufficient such cells, testis cords form and development proceeds in a male direction. If XX supporting cells are also present, a few may be recruited into the pre-Sertoli population and participate in testis cord formation. The subsequent fate of pre-follicle cells depends critically on interaction with the germ cell population in the developing gonad: absence of germ cells may lead to partial masculinization of the gonad, and/or to disappearance of the supporting cell component.

Somatic and germ-cell sex in mammals

The phenotypic sex of an individual mammal is determined by the sex of its gonads, i.e. testes or ovaries. This in turn is determined by the presence or absence of a small region of the Y chromosome, located near the X-Y pairing region in man and on the short arm of the Y chromosome in the mouse. The testis-determining region of the Y appears to exert its primary effect by directing the supporting-cell lineage of the gonad to differentiate as Sertoli cells, acting at least in part cell-autonomously. The phenotypic sex of a germ cell, i.e. whether it undergoes spermatogenesis or oogenesis, is determined at least in the mouse by whether or not it enters meiotic prophase before birth. This depends not on its own sex chromosome constitution, but on its cellular environment. A germ cell in or near normal testis cords (made up mainly of Sertoli cells) is inhibited from entering meiosis until after birth; one that escapes this inhibition will develop into an oocyte even if it is in a male animal and is itself XY in chromosome constitution.

Ultrastructural abnormalities of epididymal tissues in XXSxr pseudomale (sex-reversed) mice

Sex reversed (Sxr), a duplication of the Y chromosomal testis-determining factor in mice, causes testis development in XXSxr animals. No effects of Sxr on nongonadal organs are expected. However, we have previously shown that the epididymis of XXSxr pseudomale (sex-reversed) mice lacks the Initial Segment. In the present study we examined the ultrastructure of the head of the epididymis of adult and 21-day old XXSxr pseudomale mice. Epithelial cells of both adult and juvenile XXSxr animals contain numerous vesicles, some within mitochondria. The basal lamina is thickened and infolded. The periepithelial layer is abnormally thick, with distorted smooth muscle cells and fibrocytes that also contain lysosomelike vesicles, often in mitochondria, and excessively wide intercellular spaces. Normal collagen fibrils are infrequent; they are in part replaced by wisps of nondiscrete material, possibly immature collagen. Sxr is known to affect spermatogenesis and Sertoli cells. The abnormal conjugation of sex-determining genes in XXSxr appears also to subvert mesenchymal-epithelial development in both epididymis and testis. We believe the most likely explanation of our data is that the XXSxr genotype is not testis specific but also influences the epididymis directly.

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.

Testis cell-specific proteins in testosterone-treated rat ovaries

Two-dimensional gel electrophoresis of rat gonads in various developmental stages (embryonic days 16, 17, 21; postnatal day 5) reveals an increasing number of polypeptide spots; 0.6%-2% of the polypeptides are gonad-specific and increase from embryonic day 16 onwards. Exposure of newborn rat ovaries to testosterone for 5 days results in the appearance of eight polypeptides. These polypeptides are absent in control ovaries but present in the testis from embryonic day 16 or 17 onwards. Three do not appear in the ovary at any developmental stage. These findings indicate that testosterone plays a physiologic role in normal testicular differentiation. After long-term testosterone treatment, ovaries are depleted of germ cells. This might explain the degeneration of oocytes in the abnormal environment of a testis (e.g., in XX true hermaphrodites).

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.

Functional capacity of sex-reversed (XX, Sxr/+) mouse germ cells as shown by progeny derived from XX, Sxr/+ oocytes of a female chimera

Mice of the genotype X/X, Sxr/+ (Sex-reversed) are sterile phenotypic males. Testis size is reduced and there is a failure of spermatogenesis during meiosis I. A female chimeric mouse has been produced whose germ line is entirely composed of X/X, Sxr/+ germ cells. The progeny of this chimera included several XX, Sxr/+ males. The finding that X/X, Sxr/+ germ cells can produce fully functional oocytes has implications for the function of the Sex-reversed gene, the control of germ cell differentiation, and the role of germ cells in primary sex determination.

The H-Y antigen and its functions: a review and a hypothesis

Having reviewed the status of H-Y as the sex-determining antigen concerned with the differentiation of the dominant gonad, we consider some of the problems deriving from the tests for this antigen, and from their application to the study of natural experiments. To reconcile the results of these studies with the alleged influence of H-Y on gonadal development, we propose and discuss a hypothesis on the genetic control of the synthesis of this antigen. This states that an autosomally-coded, positively cross-reacting precursor is rendered biologically active by a Y-chromosomal gene, and transformed (in a dose-dependent manner) into a biologically inactive, antigenically negative substance under the influence of an X-chromosomal gene.

Serological and cytological evidence for increased Y-chromosome related material in Sxr,XY (sex-reversed carrier, male) mice

In order to define the nature of the genetic lesion which gave rise to Sxr, the sex-limited autosomal dominant sex reversal condition in the mouse, cytological and serological studies were carried out comparing Sxr,XY (males carrying the sex-reversal gene) and normal XY male mice. Cytotoxic H-Y antisera were absorbed by splenocytes or sperm from both Sxr,XY and normal XY male mice. Our results indicate that for both types of tissue, Sxr,XY cells absorbed consistently greater amounts of cytotoxic activity than did normal XY cells. Whole-mount electron micrographs as well as light micrographs of silver-stained spermatocytes suggest that during meiotic prophase in Sxr,XY males, the paracentromeric region of the normal Y chromosome pairs with a supernumerary Y chromosomal fragment. This fragment can be identified as one of Y chromosomal origin by its thickened axial core. Taken together, our findings support the notion that the Sxr syndrome in the mouse can be due to a supernumerary Y chromosomal fragment containing male determining factors (including one or more H-Y structural or regulatory genes) rather than to a constitutive autosomal mutation.

An oocyte-specific antigen and its possible role in the organization of the ovarian follicle of the rat

An oocyte-specific antigen was detected by an antiserum produced in isogenic Lewis rats. The antigen was sensitive to trypsin treatment. Dissociation-reorganization experiments in vitro, using ovarian cells demonstrated that the antigen is required for the interaction of germ cells and somatic cells. A physiologic role is suggested for this differentiation antigen in follicular morphogenesis and ovarian function.

GPI expression in female germ cells of the mouse

The genetically determined oocyte-specific expression of glucose-phosphate isomerase activity in the mouse is first apparent at 6 to 7 days after birth, and occurs in XO as well as in XX oocytes. The regulator locus that controls oocyte-specific expression shows the same linkage relations as the structural gene, suggesting that both form part of a Gpi-1 gene complex.