ONTOGENY OF GONADAL LUTEINIZING-HORMONE AND FOLLICLE-STIMULATING-HORMONE RECEPTORS IN THE FETAL PIG AND RELATED CHANGES IN GONADOTROPIN ANDTESTOSTERONE SECRETION

Fetuses of Large White and Meishan sows were collected at 28, 35, 49, 56, 75, 90, 103, and 113 days of gestation. LH, FSH, and testosterone concentrations were measured either in amniotic fluid (Days 28-56) or in arterial umbilical blood (Days 75-113). Gonads were analyzed for th eir content of LH and FSH receptors and RNA transcripts. Most of these parameters were similar in the two breeds, except that the mean testo sterone concentration was higher (p < 0.05) in the plasma of 75-113-da y Meishan fetuses (309 and 136 pg/ml in males and females, respectivel y) than in Large White fetuses (152 and 109 pg/ml). Higher testosteron e concentrations were detected in males than in females, either in amn iotic fluid (114 vs. 81 pg/ml, p < 0.05) or in plasma (230 vs. 122 pg/ ml, p < 0.01). In contrast, higher gonadotropin concentrations were fo und in the plasma of females than in that of males (2 vs. 1.6 ng/ml I. H and 4.2 vs. 1.4 ng/ml FSH, p < 0.05). In the testis, 2-3 pmol/g gona d of LH and FSH receptors were detected as early as Day 28. This value increased to 10-13 pmol/g between 35 and 56 days before declining to 3-5 pmol/g after Day 90. In ovaries, LH receptors were detected in the earlier period (Days 28-56) at 1-3 pmol/g before diminishing to 0.1-0 -4 pmol/g. FSH receptors were higher at Day 28 (1 pmol/g) than at any subsequent stage (0.1-0.4 pmol/g). The gonadal content of RNA transcri pts was significantly higher in testes than in ovaries. The changes in LH and FSH receptors in males may be related to changes in the relati ve testicular content of Leydig and Sertoli cells. It is likely that t he low expression of gonadotropin receptors in the ovary is related to the absence of growing follicles during fetal life, along with a lack of ovarian steroid negative feedback on pituitary gonadotropin secret ion. Therefore, in the testis the expression of gonadotropin receptors after sexual differentiation appeared as constitutive, while in the o vary some restriction of gonadotropin receptor expression would be exe rted after 75 days post-coitum (p.c.)