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Integrins are a family of cell surface receptors that mediate cell-cell and cell-matrix interactions in a variety of different cellular systems. Here we show that unfertilized mouse oocytes express β1 class integrins both at mRNA and protein levels. Using the reverse transcription polymerase chain reaction and oligonucleotide primers based on the DNA sequence of mouse integrins, the RNA transcripts for the β1, α5 and α6 subunits were detected in unfertilized oocytes. The expression of the mRNAs is paralleled by the expression of the corresponding proteins, in fact, theα 5/ β1 and the α6/ β1 complexes can be immunoprecipitated with specific antibodies from 125I-surface-labeled oocytes. Using subunit-specific antibodies we also demonstrate the presence of the α3/ β1 at the oocyte surface but α1, α2, α4 or αV subunits were not detectable. Since the mouse 3 DNA sequence is not available, we have not tested for the corresponding transcript. Integrin subunits α6 and β1 were differently distributed on the oocyte surface, as visualized by immunofluorescence staining and by immunoelectron microscopy. α6 antigen was mainly confined to the microvillous area of the oocyte surface, while β1 was more homogeneously distributed over the whole oolemma. These data demonstrate for the first time the expression of three β1 integrin complexes in unfertilized mouse oocytes. Such proteins may have a role in sperm-egg interaction or during very early steps of embryogenesis.

The adhesiveness of female and male mouse primordial germ cells (PGCs) to somatic cell monolayers of various origin has been studied in the definite conditions of an in vitro system. PGCs were isolated from the gonads of embryos of various post coital ages according to the method of De Felici & McLaren (1982), and seeded on the cell monolayers. PGCs from 12·5 to 15·5 days post coitum (dpc) embryos specifically adhered to Sertoli and follicular cells obtained from adult gonads. The percentage of female PGCs which adhered to follicular cell monolayers was significantly higher than that of male PGCs. No significant adhesion was seen between PGCs and somatic cell monolayers obtained from various embryonic and adult tissues. The results obtained indicate that the simple in vitro assay described in the present paper might help to characterize the cellular interactions between somatic and germ cells during gonadal development.

During migration and for about 2 days after their arrival in the gonadal ridges, primordial germ cells (the embryonic precursors of gametes of the adult animal) proliferate actively. Certain growth factors, such as stem cell factor and leukemia inhibitory factor, seem to be essential for survival, proliferation and possibly differentiation of mouse primordial germ cell in vivo and/or in vitro. Similarly, increase in intracellular cAMP is followed by a marked enhancement of primordial germ cell proliferation, at least in culture. In the present study, we show that pituitary adenylate cyclase-activating polypeptides (PACAP-27 and PACAP-38), two neuropeptides of the secretin-glucagon-vasoactive intestinal polypeptide-GH-releasing hormone family, stimulate in vitro proliferation of mouse primordial germ cells, bind to primordial germ cells and gonadal somatic cells (possibly to type I PACAP receptor) and activate adenylate cyclase in the same cells. Moreover, PACAP-like immunoreactivity was found in gonadal ridges, mostly on germ cell surface. In conclusion, evidence is provided that PGC proliferation can be stimulated by certain bioactive polypeptides, thus suggesting a novel regulatory role for such compounds in early gonad development.

Granulosa cells (2 × 106per ml) obtained from pregnant mare’s serum gonadotropin (PMSG)-primed mice induce within 24 h the expansion in vitro of cocultured mouse cumuli oophori. Experiments with conditioned media showed that the expansion-promoting action of granulosa cells is due to diffusible factor(s) released into the culture medium. Studies with prostaglandin synthetase inhibitors and direct measurements of prostaglandin E2(PGE2) released by granulosa cells in the culture medium have also been performed. The results strongly suggest that the cumulus oophorus expansion-promoting action of granulosa cells is mediated by PGE2, and support the hypothesis (Downs and Longo, 1983) that granulosa cells might play a similar role in the mechanism of cumulus expansion in vivo. The suggestion is advanced that coculture with granulosa cells might be of help to allow physiologic expansion in culture of immature cumuli obtained from preovulatory follicles in in vitro fertilization programs.