Ontogeny of the sexually dimorphic nucleus of the preoptic area

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Abstract
An area of increased cell density in the medial preoptic area of the adult rat brain which is markedly larger in volume in the male has been named the Sexually Dimorphic Nucleus of the Preoptic Area (SDN‐POA). It has been further shown that the presence of the testes or exogenous androgen during the first week of postnatal life significantly increases the volume of the SDN‐POA in the brain of the adult. The present study was conducted to describe the time course of the prenatal and postnatal development of the SDN‐POA in the intact male and female rat. Sprague‐Dawley females were housed with males on proestrus. The presence of sperm in the vaginal smear on estrus was used to define day 1 of gestation. Male and female prenatal and postnatal pups were sacrificed and perfused with 10% neutral formalin on days 20, 21, 22, 23, 24, 25, 26, 27, 28, 30, and 32 postfertilization. Following histological sectioning at 60 m̈m and staining with thionin, three investigators independently drew the boundaries of the SDN‐POA on successive sections, using a microprojector at a magnification of 43.5. A fourth investigator averaged the three drawings; from this average, the nuclear volume was determined with a calibrated planimeter. All drawings and measurements were performed without knowledge of age or sex since the brains were coded according to a random number generator. The volume of the SDN‐POA was found to be significantly larger in males than in females on days 23, and 25 through 32. Moreover, the volume of the SDN‐POA increased significantly with age in the male, but there was no change in SDN‐POA volume in the female. In order to test the specificity of the sexual dimorphism of the SDN‐POA, five different linear measurements were taken of brain size. There was a significant increase in these parameters in both sexes with increasing age; however, there was no significant sex difference found. Thus, the sex difference in volume of the SDN‐POA cannot be accounted for by sex differences or age‐related changes in brain size per se. These data suggest that the development of the SDN‐POA (as measured by volume) is itself sexually dimorphic. There are dramatic increases in the male, but not in the female, during a time‐period which is known to be critical for sexual differentiation of the brain.