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Immunolocalization of 3β-hydroxysteroid dehydrogenase, cytochrome P450 17-hydroxylase/17,20-lyase and cytochrome P450 aromatase in the equine corpus luteum of dioestrus and early pregnancy

The onset of equine chorionic gonadotrophin (eCG) secretion in pregnant mares is associated with an increase in luteal androgen and oestrogen production. The luteal cell type(s) responsible for the increased production of androgens and oestrogens has not been identified in the equine corpus luteum. In this study, we examined the pattern of expression of 3β-hydroxysteroid dehydrogenase (3β-HSD), cytochrome P450 17-hydroxylase/17,20-lyase (P450₁₇) and cytochrome P450 aromatase (P450_arom) by immunohistochemistry in equine luteal tissue collected during dioestrus (days 7–10; n = 4) and early pregnancy, before (days 29–35; n = 4) and after (days 39–45; n = 4) the onset of endogenous eCG secretion. All luteal cells expressed 3β-HSD, P450₁₇ and P450_arom. The distribution of 3β-HSD, P450₁₇ and P450_arom did not differ with stage of the reproductive cycle. The intensity of immunohistochemical staining for 3β-HSD did not appear to differ with reproductive stage. In contrast, the intensity of immunostaining for P450₁₇ increased after the onset of eCG secretion. The intensity of immunostaining for P450_arom increased during pregnancy before the onset of eCG secretion and diminished after the onset of eCG secretion to the intensity seen in dioestrous corpora lutea. This finding suggests that androgen and oestrogen production is not compartmentalized within the equine corpus luteum. Both large and small luteal cells express the steroidogenic enzymes necessary for oestrogen production, and the intensity of immunostaining for P450₁₇ and P450_arom appears to be stage-specific.