The bat pituitary cleft: phasic production and release of secretory granules by microvillous cells.

The ultrastructure of the two types of cells, ciliated and microvillous, lining the wall of the bat pituitary cleft was examined at different periods during the annual life cycle. Ciliated lining cells remained unchanged in structure throughout the yearly activity cycle. In contrast, microvillous lining cells exhibited striking cyclic morphological changes. Throughout the active phase of the life cycle and during most of the hibernating period, the structure of microvillous cells is unchanged. However, toward the end of hibernation, the cells produce and store large numbers of round to ovoid dense secretory granules. Just prior to arousal, the microvillous cells are packed with secretory granules. At arousal, the cells undergo marked degranulation; the granules appear to be discharged into the lumen of the cleft. Degranulation is followed by a period of proliferation of rough endoplasmic reticulum, the cavity of which is often filled with large paracrystalline inclusions. Within 1 month, microvillous cells return to a resting state and are devoid of secretory granules.