Distribution of neurosecretory granules among the anatomical compartments of the neurosecretory processes of the pituitary gland: a quantitative ultrastructural approach to hormone storage in the neural lobe.

The distribution of neurosecretory granules in various anatomical compartments of neurosecretory axons of the neural lobe of the rat pituitary has been studied. Apart from the most anterior tip of the gland, where granules are largely restricted to undilated axons and a few 'swellings', the proportional compartmental storage of granules is essentially homogeneous for the rest of the gland: 13% of granules are found in undilated axons, 31% in axonal 'endings' (which contain microvesicles and abut the basement membrane) and 55% in axonal 'swellings' (which are devoid of significant numbers of microvesicles). These values indicate that the 'endings' contain a much greater proportion of the total number of granules stored in the neural lobe than would be predicted if the readily releasable pool of hormone were composed of all the granules in the 'endings'. Some further constraint on granule release either physiological or anatomical (e.g. the position of the granule in relation to the plasmalemma of the 'ending') must be operative.