The effect of hypothalamic lesions on the length of gestation in fetal sheep.

Twenty-nine sheep fetuses were subject to stereotaxic surgery at 106 to 110 days of gestation. Electrolytic lesions were placed bilaterally in the anterior hypothalamus. Sham-operated controls (n = 4) were delivered at 146.3 +/- 4.3 days. Of the fetuses with lesions, two were excluded because histologic confirmation of the lesion was not possible. Those fetuses with bilateral lesions of the suprachiasmatic nuclei (n = 4) were delivered at 148 +/- 10 days. In 10 animals with bilateral lesions of the paraventricular nuclei, delivery was significantly (p less than 0.0001) prolonged to at least 165.6 +/- 5.1 days. In nine animals with lesions not involving the endocrine hypothalamus, delivery was at 148.1 +/- 4.3 days. All animals that were delivered after 157 days (n = 9) had lesions including the paraventricular nuclei bilaterally (p less than 0.01). The adrenal glands of fetuses with prolonged gestation were normal in weight and light microscopic appearance. These observations demonstrate that fetal neural pathways involving the paraventricular nuclei are essential for parturition in the sheep. However, fetal adrenal growth can continue without such influences.