The fetal development of the luteinizing hormone-releasing hormone (LHRH) neuronal systems of the guinea pig brain.

We have studied the distribution of LHRH-like immunoreactive material in fetal guinea pig brains beginning at day 25 of gestation. Cells and processes were first detected throughout the peripheral, intracranial and central course of the nervus terminalis at 28 (but not 25) days of gestation. The localization of LHRH in this structure preceded its appearance in the hypothalamus and coincided with the initial detection of immunoreactive LH in the pituitary gland. The possible role of the LHRH neuronal network within the nervus terminals in the development of reproductive function is discussed. Comparisons between the brains of littermates of both sexes were made at each age (days 28 through 60 of gestation) to determine possible differences between the sexes in the development of the LHRH neurosecretory systems. No sexually dimorphic features were evident in these systems throughout the prenatal period except at days 40 and 45. At these ages, differences in the number of LHRH neurons in the arcuate nucleus were found between the sexes in some but not all of the brains examined. These differences in LHRH concentrations may reflect the onset of testicular activity as indicated by an increase in serum testosterone levels. Increased serum testosterone concentrations were observed in the male fetuses beginning at 45 days of gestation. However, cell counts made within this nucleus from days 40 through 60 of gestation indicated no comparable sexual dimorphism in the total neuronal population which appeared to be relatively stable throughout this period of brain growth. The number of immunoreactive LHRH neurons visible throughout the brain increased from days 30 through 45 and fewer LHRH cells were seen on days 50 and 60 of gestation, particularly in the arcuate nucleus. The apparent decrease in visible LHRH neurons was concomitant with an increase in number and more extensive distribution of immunoreactive processes throughout the hypothalamus and in certain extrahypothalamic areas of the brain.