Hormone storage in individual neurosecretory granules of the pituitary gland: A quantitative ultrastructural approach to hormone storage in the neural lobe.

A quantitative ultrastructural study of the neurosecretory granules in the neural lobe of the rat was performed in order to calculate the number of such granules in the neural lobe and thus, provided all hormone is intragranular, to derive the hormone content of individual granules. The results suggest that the gland contains 1 - 44 X 10(10) granules and that an individual granule (mean diameter 160 nm) contains about 84000 hormone molecules. The errors involved in the many measurements made are analysed, and the calculated value for the amount of space available for each hormone molecule within the granule is shown to agree well with independently reported data for the expected size of a molecule of protein-hormone complex, and for the size of subunits visualized in freeze-etched material. These results are compatible with exclusive intragranular hormone storage as a 'solid' core, and the physical form that this core might take is discussed. The data are used to express what is known of hormone turnover and release in terms of the numbers of granules involved. Comparison of different parts of the neural lobe suggests that, apart from the most anterior part of the gland, there is little regional variation in quantitative aspects of granule storage in the neural lobe.