Spermatogenesis in XO,Sxr mice: role of the Y chromosome

The goal of this investigation was to evaluate the role of the Y chromosome in spermatogenesis by a quantitative and qualitative analysis of spermatogenesis as it occurs in the absence of a significant portion of the Y chromosome, i.e., in XO,Sxr male mice. Although these mice have the testis-determining portion of the Y chromosome on their single X chromosome, they lack most of the Y chromosome. Since it was found that all sperm-specific structures were assembled in a normal spatial and temporal pattern in spermatids of XO,Sxr mice, the genes controlling these structures cannot be located on the Y chromosome outside of the Sxr region, and are more likely to be on autosomes or on the X chromosome. In spite of the assembly of the correct sperm-specific structures, spermatogenesis was not quantitatively normal in XO,Sxr mice and significantly reduced numbers of spermatids were found in the seminiferous tubules of these mice. Furthermore, two size classes of spermatids were found in the testes of XO,Sxr mice, normal and twice-normal size. These findings are suggestive of abnormalities of meiosis in XO,Sxr spermatocytes, which lack one of the two sex chromosomes, and may not implicate function of specific genes on the Y chromosome. Morphological abnormalities of spermatids, which were not unique to XO,Sxr mice, were observed and these may be due to either a defective testicular environment because of reduced numbers of germ cells or to the lack of critical Y chromosome-encoded products. Since pachytene spermatocytes of XO,Sxr mice exhibited a sex vesicle, it can be concluded that the assembly of this structure does not depend on the presence of either a complete Y chromosome or the pairing partner for the X chromosome.     

X chromosome expression during oogenesis in the mouse.

The activities of glucose-6-phosphate dehydrogenase (G6PD) and lactate dehydrogenase (LDH) have been assayed in mouse oocytes at several stages of follicle development isolated from XX and XO female mice. Throughout the entire growth period the activity of G6PD was proportional to the number of X chromosomes present in the oocyte, whereas no difference in LDH activity was detected between XX and XO oocytes. It is concluded, therefore, that both X chromosomes are functional throughout oogenesis.     

Nonrandom segregation of the mouse univalent X chromosome: evidence of spindle-mediated meiotic drive

A fundamental principle of Mendelian inheritance is random segregation of alleles to progeny; however, examples of distorted transmission either of specific alleles or of whole chromosomes have been described in a variety of species. In humans and mice, a distortion in chromosome transmission is often associated with a chromosome abnormality. One such example is the fertile XO female mouse. A transmission distortion effect that results in an excess of XX over XO daughters among the progeny of XO females has been recognized for nearly four decades. Utilizing contemporary methodology that combines immunofluorescence, FISH, and three-dimensional confocal microscopy, we have readdressed the meiotic segregation behavior of the single X chromosome in oocytes from XO females produced on two different inbred backgrounds. Our studies demonstrate that segregation of the univalent X chromosome at the first meiotic division is nonrandom, with preferential retention of the X chromosome in the oocyte in approximately 60% of cells. We propose that this deviation from Mendelian expectations is facilitated by a spindle-mediated mechanism. This mechanism, which appears to be a general feature of the female meiotic process, has implications for the frequency of nondisjunction in our species.     

Bcl-2 and Bax regulation of apoptosis in germ cells during prenatal oogenesis in the mouse embryo.

Apoptosis is the main cause of primordial germ cell and oocyte degeneration in the developing fetal ovary. In this study we examined by immunohistochemistry and immunoblotting the expression of the anti- and pro-apoptotic proteins Bcl-2 and Bax in primordial germ cells and fetal oocytes during pre natal oogenesis in the mouse embryo. While Bcl-2 and Bax were not detectable in primordial germ cells in vivo, both proteins were upregulated when they undergo apoptosis in culture. Treatment with the stem cell factor (SCF), a growth factor known to partially reduce primordial germ cell apoptosis, resulted in decreased Bax expression. Bcl-2 was barely detectable in oocytes entering into meiosis and its expression did not change during the stage of meiotic prophase I examined. On the contrary, high levels of Bax was expressed in degenerating oocytes while low levels of the protein was present in many apparently healthy oocytes between 15.5 days post coitum (d.p.c.) and birth, when Bax was downregulated. Oocytes isolated from 15.5 days post coitum (d.p.c.) ovaries that progress through prophase I and undergo a wave of apoptosis at the stage of pachytene/diplotene in vitro, showed a pattern of Bax expression similar to the in vivo condition. Although the addition of SCF to the culture medium reduced significantly apoptosis in oocytes at the pachytene/diplotene stages, it was not possible to directly correlate this effect with the downregulation of Bax in the surviving oocytes. These findings indicate that whereas a balance between Bcl-2 and Bax might regulate apoptosis of proliferating primordial germ cells under a partial control by SCF, Bax-mediated apoptosis in meiotic oocytes may be due to intrinsic meiotic checkpoints which act to monitor aberrant DNA recombination rather than to a growth factor-dependent process. Elimination of supernumerary oocytes might be a subsequent apoptotic phenomenon controlled by the availability of growth factors such as SCF within the ovary.     

Teratogen susceptibility of XO mothers and XO embryos in mice

We examined whether the chromosomal imbalance inherent in an XO constitution in mice is more susceptible to teratogenic influence of biotin deficiency using a newly established mouse colony with pure X monosomy. We hypothesized that XO mothers or XO embryos might be more susceptible to certain teratogens. Contrary to our expectation, the incidence of external malformations induced by biotin deficiency did not differ either between XX dams and XO dams or between XX fetuses and XO fetuses.     

Evidence that postnatal growth retardation in XO mice is due to haploinsufficiency for a non-PAR X gene

XO Turner women, irrespective of the parental source of the X chromosome, are of short stature, and this is now thought to be largely a consequence of haploinsufficiency for the pseudoautosomal region (PAR) gene SHOX. X(p)O mice (with a paternal X) are developmentally retarded in fetal life, are underweight at birth, and show reduced weight gain in the first few weeks after birth. X(m)O mice, on the other hand, are more developmentally advanced than their XX siblings in fetal life; their postnatal growth has not hitherto been assessed. Here we show that X(m)O mice are not underweight at birth, but they nevertheless show reduced weight gain postnatally. The fact that postnatal growth is affected in X(p)O and X(m)O mice, means that this must be due to X dosage deficiency. In order to see if haploinsufficiency for a PAR gene was responsible for this growth deficit (cf SHOX deficiency in Turner women), X(m)Y*(X) females, in which the Y*(X) chromosome provides a second copy of the PAR, were compared with XX females. These X(m)Y*(X) females were also growth-retarded relative to their XX sibs, suggesting that it may be haploinsufficiency for a non-dosage-compensated X gene or genes outside the PAR that is responsible for the postnatal growth deficit in XO mice. The X genes known to escape X inactivation in the mouse have closely similar Y homologues. X(m)YSRY-negative females were therefore compared with XX females to see if the presence of the SRY-negative Y chromosome corrected the growth deficit; this proved to be the case. The postnatal growth deficit of XO mice is therefore probably due to haploinsufficiency for a non-dosage-compensated X gene that has a Y homologue that provides an equivalent function in the somatic tissues of males.     

Oocyte development in XO foetuses of man and mouse: the possible role of heterologous X-chromosome pairing in germ cell survival

The pairing characteristics of the X axis in XO human and mouse oocytes were studied by the spreading technique throughout meiotic prophase. In three human XO foetuses, germ cell development was seen to be largely blocked at the preleptotene stage. In XO mice on the other hand, oocytes surviving through pachytene increasingly show the X axis making a non-homologous association with itself or with an autosome. Such associations take the form hairpins or rings when self pairing occurs or triradial structures when involvement is with an autosome. Pairing initiation in the autosomes involved is disturbed by the X axis suggesting that the heterologous pairing seen is taking place at the earliest stage of synaptonemal complex formation, namely zygotene. It is suggested, that in the XO mouse, and perhaps also in rare fertile XO humans, survival, of a population of oocytes into the adult is ensured by the ability of the X axis to pair non-homologously at meiotic prophase, thus satisfying pairing requirements.     

High susceptibility for diploidy in ovulated oocytes from XO mice

Summary Adult female mice of the sensitive NMRI/Han strain ovulate diploid oocytes after gonadotropin treatment. Other mouse strains are non-sensitive with respect to the ovulation of such diploid oocytes. In this study we combined the impaired ovarian situation in the XO karyotype with the trait diploidy, which is determined genetically, by mating Ta/O (Ta=Tabby) females of C3Hx101 background to males of the NMRI/Han strain. The adult female F₁ hybrids were stimulated to ovulation by gonadotropins and identified by their karyotype (XX or XO). The cytogenetic analysis of ovulated oocytes revealed a low level of diploidy in the XX littermates (1.0%), but a very high level in females with the XO karyotype (24.6%). All of the XO females ovulated at least one diploid oocyte. We suggest that it is the XO status which drastically impairs meiosis I in our gonadotropin-sensitive F₁ females due to (1) alterations of the developmental program within the oocyte, (2) a disturbed communication between oocyte and follicle, (3) a preferential maturation and ovulation of follicles at risk, or (4) an exceptional recruitment of many such follicles, by, e.g., a premature responsiveness to gonadotropins in our XO females. An interdependence of several such mechanisms is possible.     

Germ cell differentiation and synaptonemal complex formation are disrupted in CPEB knockout mice.

CPEB is a sequence-specific RNA binding protein that regulates translation during vertebrate oocyte maturation. Adult female CPEB knockout mice contained vestigial ovaries that were devoid of oocytes; ovaries from mid-gestation embryos contained oocytes that were arrested at the pachytene stage. Male CPEB null mice also contained germ cells arrested at pachytene. The germ cells from the knockout mice harbored fragmented chromatin, suggesting a possible defect in homologous chromosome adhesion or synapsis. Two CPE-containing synaptonemal complex protein mRNAs, which interact with CPEB in vitro and in vivo, contained shortened poly(A) tails and mostly failed to sediment with polysomes in the null mice. Synaptonemal complexes were not detected in these animals. CPEB therefore controls germ cell differentiation by regulating the formation of the synaptonemal complex.     

Studying Early Lethality of 45,XO (Turner's Syndrome) Embryos Using Human Embryonic Stem Cells

Turner''s syndrome (caused by monosomy of chromosome X) is one of the most common chromosomal abnormalities in females. Although 3% of all pregnancies start with XO embryos, 99% of these pregnancies terminate spontaneously during the first trimester. The common genetic explanation for the early lethality of monosomy X embryos, as well as the phenotype of surviving individuals is haploinsufficiency of pseudoautosomal genes on the X chromosome. Another possible mechanism is null expression of imprinted genes on the X chromosome due to the loss of the expressed allele. In contrast to humans, XO mice are viable, and fertile. Thus, neither cells from patients nor mouse models can be used in order to study the cause of early lethality in XO embryos. Human embryonic stem cells (HESCs) can differentiate in culture into cells from the three embryonic germ layers as well as into extraembryonic cells. These cells have been shown to have great value in modeling human developmental genetic disorders. In order to study the reasons for the early lethality of 45,XO embryos we have isolated HESCs that have spontaneously lost one of their sex chromosomes. To examine the possibility that imprinted genes on the X chromosome play a role in the phenotype of XO embryos, we have identified genes that were no longer expressed in the mutant cells. None of these genes showed a monoallelic expression in XX cells, implying that imprinting is not playing a major role in the phenotype of XO embryos. To suggest an explanation for the embryonic lethality caused by monosomy X, we have differentiated the XO HESCs in vitro an in vivo. DNA microarray analysis of the differentiated cells enabled us to compare the expression of tissue specific genes in XO and XX cells. The tissue that showed the most significant differences between the clones was the placenta. Many placental genes are expressed at much higher levels in XX cells in compare to XO cells. Thus, we suggest that abnormal placental differentiation as a result of haploinsufficiency of X-linked pseudoautosomal genes causes the early lethality in XO human embryos.     

Complete chromomere map of mid/late pachytene human oocytes.

A complete chromomere map of the mid/late human pachytene oocyte has been developed from ovaries of 35 fetuses at 18-22 weeks gestation. Bivalents, which were all specifically identifiable, were more extended than comparable human spermatocyte bivalents. The total number of chromomeres found was 639, exceeding both the number of human pachytene spermatocytes and the number of mitotic bands seen in metaphase somatic chromosomes. Each oocyte bivalent contained more chromomeres than the number of corresponding prometaphase chromosome bands. Similarities and differences were noted in regional comparisons of chromomere distribution between oocytes and spermatocytes.     

棕色田鼠XO雌体育性研究

通过对棕色田鼠外形特征,卵巢切片,怀胎和生产雌鼠染色体鉴定等方面研究,证实了该鼠ＸＯ雌鼠可孕,并具有生殖能力。染色体鉴定表明,ＸＸ雌体中的两条Ｘ性染以体,一条为Ｍ类型另一条为ＳＭ类型;ＸＯ雌体中的Ｘ性染色体为Ｍ类型。所以ＸＯ雌性的生育能力可能与Ｘ染色体有关,其上可能存在雌性育性基因。     

XYY spermatogenesis in XO/XY/XYY mosaic mice

The relative frequencies of XYY and XY cells in XO/XY/XYY mosaic mice were compared between somatic cells (bone marrow) and spermatogonia, and between spermatogonia and pachytene or MI spermatocytes. The results indicated there was no selection either for or against XYY spermatogonia. There was, however, a strong selection against XYY spermatocytes during pachytene, with their almost total elimination by the first meiotic metaphase. At pachytene, most XYY cells had trivalent or X univalent/YY bivalent configurations. These findings are contrasted with previous studies of XYY spermatogenesis in mice and are discussed with respect to a model that invokes sex-chromosome univalence as the cause of XYY spermatogenic failure.     

Effects of sex chromosome dosage on placental size in mice

Mice of the XO genotype with a paternally derived X chromosome (XpO) have placental hyperplasia in late pregnancy, although in early pregnancy the ectoplacental cone, a placental precursor, is smaller in XpO mice than in their XX sibs. This early size deficiency of the ectoplacental cone is apparently a consequence of Xp imprinting, because XmO embryos (with a maternally derived X chromosome) are unaffected. In the present study we sought to establish whether XpO placental hyperplasia in late pregnancy is also a consequence of Xp imprinting. Placental weight data were first collected from litters that included XpO or XmO fetuses and XX controls. Comparison of XO placentae with XX placentae showed that XpO and XmO placentae are hyperplastic. This finding suggested that the hyperplasia might be an X dosage effect, and this hypothesis was supported by the finding that XY male fetuses from the same crosses also had larger placentae than their XX sibs. Further analysis of a range of sex-chromosome variant genotypes, including XmYSry-negative females and XXSry transgenic males, showed that mouse fetuses with one X chromosome consistently had larger placentae than littermates with two X chromosomes, independent of their gonadal/androgen status.     

Gonadal development and birth weight in X*X and X*Y females of the wood lemming, Myopus schisticolor

A quantitative histological analysis of ovaries from 8- to 10-day-old wood lemmings revealed significant differences between females with X*Y and X*X sex chromosome constitutions. The ovarian volume of X*Y females was on average 57% of X*X, and the number of oocytes was less than half in X*Y compared to X*X. However, the frequency of growing oocytes in relation to the total number was 6.5% for X*Y compared to 3.0% for X*X. Oogenesis in X*Y wood lemmings resembles in many respects that of mice heterozygous for certain translocations and with tertiary trisomy (Ts31H), and those with X0 monosomy. The fertility in X*Y wood lemmings is not reduced. On the contrary, X*Y females have a higher reproductive fitness than X*X and XX. This is discussed in relation to the present findings. The body weight at birth was 8% higher in X*Y than in X*X.