Estrogen formation from cholesterol in rabbit preimplantation blastocysts and corpora lutea in vitro

Rabbit blastocysts and corpora lutea (CL) obtained six days after coitus were incubated with [4-14C]cholesterol. In the CL, the conversion of [4-14C]cholesterol to radioactive progesterone was about twice that to radioactive estrone. On the other hand, in the preimplantation blastocyst, estrone formation from the substrate was far higher than progesterone formation. Both in the blastocyst and CL, [4-14C]cholesterol was least incorporated into estradiol among the steroids identified. It is suggested that steroids biosynthesized in rabbit preimplantation blastocysts and CL are quite different both in quality and in quantity at the time of implantation.