Protein distribution and gene expression of collagen type IV in the neonatal rat ovary during follicle formation.

Protein distribution and mRNA expression of basement membrane collagen (type IV) during follicle formation were studied using serial sections from 24, 48 and 72 hrs. old rat ovaries. Collagen type IV, a protein found only in the basal lamina of the basement membrane, was localized under light microscope using a polyclonal antibody. During the first 24 hrs. postpartum, immunostaining was found as thin septa separating the clusters from the stroma. By 72 hrs. postpartum, immunostaining was found around each newly formed primordial follicle. The cell types involved in collagen type IV synthesis were determined by in situ hybridization using a biotinylated riboprobe. Before the follicles had been formed, the stromal cells showed intense staining while the epithelial presumptive granulosa cells showed a pale staining. However, after a follicle had been formed, some of the granulosa cells enclosed within the follicular basement membrane showed strong staining for the message. The presumptive granulosa cells are presumed to be the progenitors of granulosa cells. If so, these observations suggest that the expression of the message coding for collagen type IV by the granulosa cells may be a marker for commitment of the undifferentiated cell to the granulosa cell lineage.