OUR knowledge of the pathways of steroid biosynthesis in the ovary has been gained mainly by incubations of ovaries in vitro1,2. The tissues incubated have contained numerous cell types: granulosa cells, theca interna cells, stromal cells, interstitial cells, and sometimes luteal cells. Possibly such mixtures of two or more different cell types are able to secrete hormones that one cell type cannot secrete by itself3–9. Furthermore, during such incubations in vitro an exchange of precursors and products between different cell types may be facilitated because of breakdown of naturally occurring barriers, such as the basement membrane between the granulosa layer and the theca interna of the follicle.