Developmental regulation of ovarian-specific Mos expression.

To gain better insight into the physiologic role of the Mos proto-oncogene in mice we have been studying the cell type and developmental specificity of its expression. It was previously shown that in adult mice, Mos is transcribed predominantly in ovaries and in haploid spermatids of the testes. Using in situ hybridization techniques we now show that in the ovary, Mos is expressed in oocytes, but not in somatic cells. In these analyses Mos transcripts are not detected in primary resting oocytes, but accumulate soon after the oocyte enters the growth phase. High levels of Mos RNA are present throughout oocyte growth and maturation. Mos RNA is also abundant in ovulated eggs prior to fertilization. Following fertilization, however, there is a dramatic loss of Mos RNA, as evidenced by the failure to detect hybridization in late one-cell embryos. The narrow developmental window for Mos transcription defined by this study suggests a role for ovarian Mos in one or more of the processes of oocyte growth, meiotic maturation, ovulation, or fertilization.