Implication of absence of HCG-like gonadotrophin in the blastocyst for control of corpus luteum function in pregnant rabbit

Abstract

IN the primate, the corpus luteum formed after ovulation has a relatively short life span. If the ovum becomes fertilised, however, the presence of the embryo in the uterus leads to ‘rescue’ of the corpus luteum and a prolongation of its functional life¹. Circumstantial evidence suggests that this is achieved through the elaboration of a luteotrophic substance (chorionic gonadotrophin) by the trophoblastic cells of the implanting embryo^1,2. Based on differences in the plasma progesterone levels of pregnant and pseudopregnant animals, a similar situation has been proposed for the rabbit³. In support of this, the blastocyst fluid from 6-d pregnant rabbits has been reported to contain a substance similar to human chorionic gonadotrophin (HCG), by a radioreceptor assay⁴, and a substance similar to luteinising hormone (LH), by a heterologous radioimmunoassay⁵. Our study was initiated to identify definitively the source of the HCG-like substance in the rabbit blastocyst. Using a highly sensitive in vitro bioassay method we have examined the concentration of HCG-like material in the serum throughout pregnancy and in the blastocyst from days 4 to 7 of pregnancy. We report here that the rabbit blastocyst does not contain or secrete any gonadotrophin which has significant HCG-like activity.