Relationship between ovarian delta 5-3 beta-hydroxysteroid dehydrogenase activity and implantation in non-lactating and lactating mongolian gerbils (Meriones unguiculatus).

The distribution and intensity of delta 5-3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) activity was examined histochemically in ovaries from Mongolian gerbils during pregnancy and undergoing lactational delay. In all of the ovaries examined the corpora lutea showed the strongest level of activity, but like the interstitium and granulosa, there was no relationship between the enzyme pattern and embryonic development. Thecal 3 beta-HSD activity was only observed immediately before, and after implantation on day 8 p.c. in pregnancies without lactation, and was totally absent throughout lactational delay, appearing for the first time after day 15 in association with implantation. The absence of this key enzyme in the theca interna throughout the pre-implantation period means that the biosynthesis of androstenedione and progesterone cannot occur, thereby precluding the production of oestrogen, an essential steroid for implantation in this species.