Possible relations between the secretory cycle of the neurosecretory cells in the rat paraventricular nucleus and the estrous cycle.

The ultrastructure of neurosecretory cells in the paraventricular nucleus of the female rat in various stages of the estrous cycle was studied by electron microscopy. On account of the secretory cycle, the neurosecretory neurons were classified into four types, and the incidence of each type was counted throughout the estrous cycle. In proestrous afternoon, neurons at the phase of protein synthesis (Type I neuron) increased in number. In estrus and metestrus, neurons at the phase of granule production (Type II neuron) and granule storage (Type III neuron) increased in number. In diestrus and in early morning of the proestrus, neurons at the phase of granule transport (Type IV neuron) and Herring bodies increased in number and in size. The secretory cycle in the paraventricular nucleus of the female rat was intimately related to the estrous cycle. We also observed the neurons in the supraoptic nucleus, but there were no changes accompanying the estrous cycle.