Δ⁵-3²-Hydroxysteroid Dehydrogenase Activity in Rat Embryos on Days 1 Through 7 of Pregnancy

The combined results of the presentand a previous study (2) showed that, there was noA5-3/3-hydroxysteroid dehydrogenase (HSD) activityin unfertilized rat eggs and in Day-1, -2, and -3embryos. HSD activity was first observed on Day 4(08.00 hr), it peaked on Day 5, sharply declined onDay 6, and was again absent on Day 7. If it isassumed that quantitative changes in HSD activityreflect changes in synthesis of steroid hormones, itcan be postulated that steroid hormones producedby the embryo play a key role in the phenomena ofmorula to blastocyst transformation and implantationof the blastocyst. (Endocrinology95:321, 1974)