Species differences in the ovarian distribution of 3beta-hydroxysteroid dehydrogenase/delta5-->4 isomerase (3beta-HSD) in two marsupials: the brushtail possum Trichosurus vulpecula and the grey, short-tailed opossum Monodelphis domestica.

The ovarian distribution of the steroidogenic enzyme 3beta-hydroxysteroid dehydrogenase/delta(5-->4) isomerase (3beta-HSD) was investigated by immunocytochemistry in two marsupial species throughout the reproductive cycle, using a rabbit polyclonal antibody raised against human placental 3beta-HSD. In the polyoestrous and polyovular South American opossum Monodelphis domestica, immunostaining was positive for 3beta-HSD in the adrenal cortex, the ovarian interstitial tissue, the corpus luteum and the granulosa cells of antral and atretic follicles. The theca interna was weakly positive for 3beta-HSD, but only in late preantral to early antral stages of follicular development. The adrenal medulla and smaller preantral follicles were completely negative for 3beta-HSD. In contrast, in the polyoestrous and monovular Australian brushtail possum Trichosurus vulpecula, immunostaining showed a strong positive reaction for 3beta-HSD in the theca, whereas the granulosa layer remained predominantly negative for 3beta-HSD except in the largest follicles. The atretic follicles were completely negative for 3beta-HSD. The ovaries of pregnant animals contained grossly enlarged, persistent, antral follicles, which reacted positively for 3beta-HSD. The function of these follicles in T. vulpecula and the 3beta-HSD-positive atretic follicles in M. domestica has not been determined. The differences between the two marsupials represent species variations. The situation in M. domestica does not represent a marsupial-eutherian dichotomy as previously conjectured.