Formation of the sexually dimorphic nucleus of the preoptic area: neuronal growth, migration and changes in cell number.

The sexually dimorphic nucleus of the preoptic area (SDN-POA) appears to be a morphological marker of the process of sexual differentiation of the rat brain. A portion of the presumptive SDN-POA neurons can be specifically identified utilizing autoradiography following the administration of [3H]thymidine on day 18 of gestation. In the present study we have utilized this fact in order to describe the general pattern of formation of the SDN-POA during the perinatal period. Following the administration of [3H]thymidine, fetal pups were sacrificed and perfused with neutral formalin either two hours after the injection or on day 20 or 22 postfertilization. Neonatal pups were either sacrificed and perfused or assigned to one of the treatment groups. Male pups were either gonadectomized or sham gonadectomized; females were all sham gonadectomized. These pups were then sacrificed and perfused on either day 26, 28 or 32 postfertilization. All brains were processed for autoradiography. The size, number and location of labeled cells within the medial preoptic area (MPOA) was determined. In general it appears that the labeled cells grow in size during the early postnatal period. These cells also migrate from the more ventral aspects of the MPOA to aggregate and form the SDN-POA. Furthermore, there is a significant decrease in the number of labeled cells by day 32 postfertilization (day 10 of postnatal life) which appears to contribute to the specific labeling of the SDN-POA of the adult animal. However, results obtained in this study from quantitative analyses do not indicate that sex or the postnatal steroid hormone environment influence the processes of growth, migration and decrease in MPOA cell number of those presumptive SDN-POA cells born specifically on day 18 of gestation and analyzed in this study.