XX Embryos Research Articles

Identical action of Y chromosome and sxr factor in early testicular development in the mouse.

The Sxr (sex reversed) factor in the mouse described by Cattanach et al. (1971) causes sex reversal of genetic female mice, thus producing XX males. Its primary effect is induction of testicular development in XX embryos. In the present study, the early steps of testicular development in sex reversed XX and in XY mouse embryos are examined. The embryos are bred from male carriers of the Sxr mutation. After determination of the sex chromosomes from chromosomal spreads, the appearance of testis cords is evaluated in sex reversed XX embryos and their XY littermates at 12 1/2 and 13 days of pregnancy. In sex reversed XX embryos testis cords appear at the same stage of development as in XY embryos. From day 13 to 17 of pregnancy, the volume of the testis anlage is determined by planimetry in sex reversed XX and in normal XY embryos from separate litters. No significant difference is found in the growth rate of XX and XY testes. Measurements of the diameter of the testis cords also indicate the same developmental pattern in both groups. Thus, during induction of a testis the Sxr factor perfectly imitates the Y chromosome in all parameters investigated.

X-chromosome activity in preimplantation mouse embryos from XX and XO mothers

Embryos from XO female mice begin development with half the activity levels of an enzyme (HPRT) coded for by a gene on the X chromosome, compared with embryos from XX females. Groups of unfertilized eggs and individual embryos at the 8-cell, morula and blastocyst stages were assayed for HPRT activity. An autosomally coded enzyme (APRT) was assayed simultaneously in the same reaction mix as a control. There is a substantial increase in HPRT activity by the 8-cell stage. However, the mean activity of HPRT in embryos of XO mothers remains half that in embryos of XX mothers. This suggests a significant maternally inherited component of HPRT activity in 8-cell embryos. By the 9- to 16-cell morula stage the HPRT activities in the two groups of embryos become similar due, presumably, to a transition to embryo-coded activity; HPRT activities in individual morulae from XX mothers show a bimodal distribution consistent with the hypothesis that both X-chromosomes are active in XX embryos at this stage.

Cell interactions in the sex organs of sex reversed mice heterozygous for testicular feminization.

The X-linked mutation for testicular feminization (Tfm) described by Lyon and Hawkes (1970) causes androgen insensitivity of the testosterone target cells. The molecular basis of the phenotype is a defect of the androgen receptor protein (Attardi and Ohno, 1974; Gehring and Tomkins, 1974). Due to random X inactivation, XX mice heterozygous for Tfm are mosaics with respect to androgen sensitivity. They can be converted to males by the autosomal dominant mutation for “sex reversal” (Sxr) described by Cattanach, Pollard, and Hawkes (1971). By imitating the Y-chromosome, Sxr induces formation of testes in XX embryos. The testosterone produced by the embryonic testes in turn induces formation of male sex organs. In sex-reversed embryos heterozygous for Tfm, testosterone encounters a mixed population of androgen-insensitive and androgen-sensitive cells. In the androgen-insensitive cells the X chromosome bearing Tfm is active. The Tfm cells express the defective receptor protein and therefore are constitutively bound to form female organs. In the androgen-sensitive cells the wild-type X is active. The wild-type cells therefore express an intact androgen receptor protein. They respond to testosterone and are bound to form male sex organs.

Cytogenetic analysis of oocytes and early preimplantation embryos from XO mice

The meiotic segregation pattern of the X chromosome in XO mice was determined by counting second meiotic metaphase chromosomes. A total of 584 oocytes was collected from superovulated but unmated XO and XX mice. Second meiotic metaphase (MII) counts indicated that the X chromosome in XO mice did not segregate randomly but remained preferentially in the oocyte. Increased maternal age had no effect upon this segregation pattern. Selection against XO and YO preimplantation embryos was evaluated by observing their relative frequencies during first and second cleavage divisions. A combined total of 1650 preimplantation eggs was collected from superovulated and mated XO and XX female mice. Eggs were obtained during metaphase of first (CI) or second cleavage (CII) divisions by injecting mated females with colchicine at 28 or 49 hr, respectively, after human chorionic gonadotrophin (HCG) administration. Prior to analysis, chromosomes were banded by trypsin-Giemsa, which facilitated specific chromosome identification, including the X and Y. The proportion of XX, XY, XO, and YO embryos observed at CI from XO females was not significantly different from expected values based on MII counts. At CII, a significant difference was observed from the expected proportion of embryo classes resulting from a reduction in XO and YO embryos. The results provide cytogenetic evidence emphasizing that, in order for development to continue, at least one functional X chromosome is necessary as early as the two-cell stage. Triploid one- and two-cell embryos were observed more frequently in embryos from XO than XX females. The sex ratio was not significantly different from 1:1 during either cleavage stage regardless of the type of female from which the embryos were derived.

Number of germ cells in known male and known female genotypes of vertebrate embryos (Oryzias latipes).

The number of primordial germ cells in embryos of known genotypic sex was the same in XY males and XX females until the gonad became recognizable as testis or ovary. It has been claimed that the heterogametic sex chromosome causes the gonad to differentiate as a testis in mammals and as an ovary in birds as a result of earlier and more mitoses. This claim was not supported in the present study where a sex difference in numbers of germ cells was first noted during differentiation of oocytes in the XX embryos.