CHANGES IN THE APPEARANCE OF HYPOTHALAMO-NEUROHYPOPHYSIAL NEUROSECRETORY GRANULES ASSOCIATED WITH THEIR MATURATION

Abstract

SUMMARY The appearance of neurosecretory granules (NSG) in different parts of the hypothalamo-neurohypophysial system (paraventricular and supraoptic neurosecretory neurones and their processes within the hypothalamus and in the neural lobe) of the rat was studied after fixation at different pH's (5·0, 6·0, 7·0 or 8·0) using a triple-aldehyde mixture followed by osmium tetroxide. Differences between the areas studied were most marked at pH 8·0. At this pH a significantly greater proportion of granules retained an electron-dense core within the perikarya and adjacent processes than within the neural lobe, where only 5% of all granules remained dense-cored. At pH 8·0 comparison of NSG in paraventricular and supraoptic areas showed that a significantly higher proportion of granules remained dense-cored in the paraventricular region. This difference was particularly marked in the neurosecretory processes. The difference in response to fixation at pH 8·0 between hypothalamus and neural lobe may reflect changes in the nature of the granule resulting from 'maturation'. If so, maturation starts within the perikaryon and proceeds as the granules pass down the axons to the neural lobe, where very few remain immature. The differences between paraventricular and supraoptic regions might be explained as a result of faster maturation of granules containing vasopressin than those containing oxytocin.