Steroid secretion by different cell types of the horse conceptus

Horse conceptuses were recovered non-surgically at Day 12-Day 15 and were dissociated with collagenase. Separation of the cells on a 31.8% Percoll gradient gave two bands of cells and indirect evidence suggests that the low density cells (LDC) are endoderm and the higher density cells (HDC) are trophectoderm. Each band was incubated for 24 h in Minimum Essential Medium and concentrations of oestradiol and progesterone in the medium were measured by radioimmunoassay (RIA). The LDC secreted predominately progesterone (log oestradiol/progesterone = -0.994 +/- 0.141; N = 15) whereas the HDC secreted mainly oestradiol (log oestradiol/progesterone = 0.522 +/- 0.135). When incubated for a further 24 h in fresh medium the oestradiol production decreased by 85% and 84% whereas the progesterone production decreased by 57% and 50% for LDC and HDC respectively. When cells from each band were incubated for 24 h with [3H]pregnenolone, separation of the radiolabelled metabolites by reversed-phase high-pressure liquid chromatography revealed a different steroid profile for each cell type. Therefore, in the intact embryo, an interaction between cells may be important for oestrogen synthesis and thus the maintenance of pregnancy.