Immunolocalization of steroidogenic enzymes in the corpus luteum and placenta of the Japanese black bear, Ursus thibetanus japonicus, during pregnancy

The Japanese black bear, Ursus thibetanus japonicus, is a seasonal breeder and shows delayed implantation for several months during pregnancy. The obj ective of this study was to clarify the steroidogenic capability of the cor pus luteum and placenta during pregnancy, including both delayed implantati on and fetal development, by immunolocalization of steroidogenic enzymes in these organs of the Japanese black bear. Ovaries and placentae from 15 wil d Japanese black bears, which had been killed legally by hunters and were t hought to be pregnant, were used in an immunocytochemical study to localize the cholesterol side chain cleavage cytochrome P450 (P450scc), 3 beta -hyd roxysteroid dehydrogenase (3 beta HSD), 17 alpha -hydroxylase cytochrome P4 50 (P450c17) and aromatase cytochrome P450 (P450arom) by the avidin-biotin- peroxidase complex method using polyclonal antisera raised in mammals again st P450scc, 3 beta HSD, P450c17 and P450arom. P450scc and SPHSD were locali zed in all luteal cells throughout pregnancy. P450c17 was present in a few luteal cells, especially in the outer area of the corpus luteum throughout pregnancy, but the number of positively immunostained cells decreased durin g the post-implantation period. Cells positively immunostained for P450c17 were significantly smaller than negatively immunostained cells (P < 0.01). P450arom was present sporadically in a few luteal cells throughout pregnanc y, but the number of positively immunostained cells decreased during the po st-implantation period. The size of cells positively immunostained for P450 arom was not significantly different from that of negatively immunostained cells. The whole placenta was negatively immunostained for P450scc, 3 beta HSD and P450c17, but P450arom was present in the syncytiotrophoblasts and e ndothelial cells of maternal blood vessels. These results indicate that, in the Japanese black bear, corpora lutea are a source of progesterone which may play an important role in the maintenance of delayed implantation and f etal development during pregnancy. Corpora lutea have a minimum capability to synthesize androgen in small luteal cells and oestrogen in normal-sized luteal cells during pregnancy, and placentae have the ability to synthesize oestrogen during late pregnancy.