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Immunolocalization of steroidogenic enzymes P450scc, 3βHSD, P450c17 and P450arom in the corpus luteum of the Hokkaido brown bear (Ursus arctos yesoensis) in relation to delayed implantation

The objective was the immunocytochemical localization of steroidogenic enzymes in the corpus luteum of Hokkaido brown bears during the period of delayed implantation. Cholesterol side-chain cleavage cytochrome P450 (P450scc), 3β-hydroxysteroid dehydrogenase (3βHSD), 17-hydroxylase cytochrome P450 (P450c17) and aromatase cytochrome P450 (P450arom) were localized as biosynthetic sites of pregnenolone, progesterone, androgens, and oestrogens, respectively. Ovaries containing corpora lutea were obtained from three mature bears during the expected delayed implantation period and ovarian sections were immunostained by the avidin–biotin–peroxidase complex method using polyclonal antibodies generated against steroidogenic enzymes of mammalian origin. P450scc and 3βHSD were localized in all luteal cells, whereas P450c17 (0.4–5.1% of 1000 cells) and P450arom (7.1–11.2% of 1000 cells) were localized in only a few luteal cells. These data suggest that luteal cells contain steroidogenic enzymes required for progesterone synthesis but also have a minimum capability for synthesizing androgen and oestrogen during the delayed implantation period in Hokkaido brown bears.