Human chorionic gonadotropin binding to human fetal testes as a function of gestational age

The characteristics of binding of hCG to testicular tissue obtained from human abortuses of 10-24 weeks gestational age were studied. Specific, saturable binding of [¹²⁶I]hCG was demonstrated using homogenates of human fetal testicular tissue. The equilibrium dissociation constant ranged from 0.4 × 10^-10 M to 5.5 ×10^-10 M, a finding that is indicative of a high affinity receptor. The capacity to bind hCG was low, but varied strikingly with gestational age. The binding capacity for hCG of tissues from abortuses of gestational age less than 15 weeks and greater than 22 weeks was consistently less than 10.0 pg × mg^-1 tissue (2.2 fmol × mg^-1 tissue). The binding capacity for hCG of tissues from abortuses of gestational age between 15-20 weeks ranged from 2.4-29.8 pg × mg^-1 tissue (0.5-6.5 fmol × mg^-1 tissue) with the majority of values being greater than 10 pg × mg ^-1 tissue (2.2 fmol × mg^-1 tissue). On the other hand, receptors for hCG in human fetal ovarian tissue were undetectable, irrespective of gestational age. It is concluded that specific high affinity binding sites for hCG are present in human fetal testes and that the binding capacity is maximum between gestational ages of 15-20 weeks. This increase in binding capacity parallels the surge in testosterone production known to occur during the same period of development. These results suggest that the increase in fetal plasma levels of testosterone during this time in gestation is the result of an increase in the sensitivity of the fetal testis to hCG caused by an increase in the number of hCG receptors, and that hCG most likely is responsible for stimulation of fetal testicular steroidogenesis in utero at this time of gestation.