Quantitative electron microscopic observations on Paneth cells of germfree and ex-germfree Wistar rats.

Ultrastructural changes of Paneth cells of germfree (GF) rats which had been inoculated with bacteria-containing feces from conventionally-reared (SPF) rats were quantitatively examined. 12 and 24 h after inoculation, the Paneth cells showed a striking decrease in the number of secretory granules and the occurrence of large vacuoles. Phagosomes containing bacteria were not seen. After 4 days, the secretory granules reaccumulated and smooth-surfaced apical vesicles increased in number. It is discussed that the large vacuoles may be related to membrane-retrieval events following the massive extrusion of secretory granules whereas the apical vesicles appear to serve this function when exocytosis is not pronounced. In addition to the large secretory granules ca. 10% of Paneth cell profiles contained a few dense-cored vesicles measuring about 150 nm in diameter which resemble peptidergic neurosecretory granules.