To date, there is no conclusive evidence that ERs are present in preimplantation embryos. There are reports that estrogen is made by the rabbit blastocyst (61), and estrogens have been used to induce implantation in mice (62), but whether estrogens act through ERs in the embryo or in the maternal uterus is not known. ERs may be present in early embryos, but if so, levels are below the methods of detection used thus far. Perhaps with more sensitive immunodetection methods, it may be possible to detect ERs in embryos if they are present. Using PCR, messenger RNA for ER has been detected as early as the oocyte stage in mouse embryos (Q. Hou and J. Gorski, unpublished results). This was confirmed recently by Wu et al. (83a). Figure 7 shows a model for the pattern of ER expression in the developing mouse fetus based on the various reports discussed in this review. ERs are present in the 10-day mouse fetus, possibly in the developing ambisexual reproductive tract. Analysis of seven individual 10-day-old fetuses taken from the same litter showed similar levels of an immunostained protein the size of the ER in each fetus (57). The pattern of expression of ER between implantation and the development of the reproductive tract may be the same in male and female mice. Estrogen, acting through ERs, may be one factor (of many) that determines which cells are destined to be part of the indifferent reproductive tract. We were not able to isolate fetal mouse reproductive tracts at an indifferent stage (day 10) due to their very small size. One way to study ER in the indifferent reproductive tract would be to examine these tissues in a larger animal, such as the bovine, using similar immunodetection methods. The distribution of ER in the fetal mouse reproductive tract on fetal days 13 (before sexual differentiation) and 15 (initiation of sexual differentiation) is similar in males and females (71, 72). Thus, estrogen does not appear to be responsible for the initiation of sexual differentiation. Early experiments by Jost (41) showed that removal of the gonad from male or female rabbit fetuses resulted in the female phenotype, which lent weight to the hypothesis that ovarian hormones are not critical in the development of the female phenotype, whereas testicular hormones are essential for the development of the male phenotype.(ABSTRACT TRUNCATED AT 400 WORDS)
An immunocytochemical assay for estrogen receptor (ER) was used to study the distribution of receptor in fetal and immature female mouse reproductive tracts. Immunoblots confirmed that a single band, the size of the ER, immunostained in extracts from day 15 and 17 fetal reproductive tracts. Staining was observed over nuclei of epithelial cells of the Mullerian duct and over nuclei of cells of the developing connective tissue (mesenchymal cells) of the reproductive tract on fetal day 15. By day 17 when a primitive uterus could be distinguished, ERs were detected in nuclei of mesenchymal cells, but in only a small portion of epithelial cells. A different pattern of immunocytochemical staining was observed in uteri from animals killed on the day of birth; cells of the connective tissue contained ER, but the epithelial cells did not. By 4 or 6 days after birth, more nuclei in the connective tissue stained for ER with a greater intensity compared to nuclear staining in epithelial cells. ERs were detectable in nuclei of both uterine epithelial cells and connective tissue cells on days 10 and 19 after birth.
Bovine placental lactogen (bPL) has been isolated from bovine trophoblast and characterized as a 32 K mol wt protein which exists in three different forms which differ in their isoelectric point values and their amino acid compositions. Two of the three forms have been shown to have both bovine GH (bGH)- and bovine PRL (bPRL)-like activities equal on a molar basis to bGH and BPRL in radioreceptor assays. It has been postulated that, in sheep, PL is delivered to the maternal circulation by the migration of fetal binucleate cells from the trophoblast across the fetal-maternal boundary into the uterine epithelium. To determine whether an analogous situation exists in the cow, antibodies to bPL were used to localize bPL in bovine placentomes and to measure its concentration in fetal and maternal sera. For cytology, bPL was localized on sections of placentomes from midgestation and term bovine placentas using an indirect immunoperoxidase technique. Stained binucleate cells were demonstrated throughout the trophoblast, often in close association with the microvillous boundary which separates the trophoblast from the maternal epithelium. In cross-sections of fetal villi, binucleate cells with cytoplasmic processes extending into and through the uterine epithelium were immunostained as well as cells within the plane of the uterine epithelium in close approximation or apposition to the maternal basement membrane. RIA demonstrated bPL to be present in maternal sera in concentrations of 1-2 ng/ml and in fetal sera at 5-12 ng/ml. These data are consistent with the hypothesis that binucleate cell migration accomplishes the delivery of bPL to the maternal circulation.
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