Origin, differentiation and renewal of the four main epithelial cell types in the mouse small intestine IV. Paneth cells

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Abstract
The origin, differentiation and fate of Paneth cells in adult mouse small intestine was examined by electron microscopic radioautography at various times after a single injection of 3H‐thymidine.The Paneth cell population consists of 23% young Paneth cells with granules less than 2 μm in diameter, 63% mature Paneth cells with granules 2–3 μm in diameter, and 14% old Paneth cells with granules greater than 3 μm in diameter.Light and electron microscopic radioautography confirms that typical Paneth cells are not labeled 1 hour after 3H‐thymidine injection and, therefore, are not capable of proliferation. However, labeled cells with a few small Paneth cell granules are first observed 12 hours after an injection. Except for the presence of granules, these cells are indistinguishable from the highly proliferative crypt‐base columnar cells. It is, therefore, suspected that Paneth cells are derived from crypt‐base columnar cells.With time after injection, differentiation proceeds: stacks of rough endoplasmic reticulum cisternae appear, the granules become progressively more numerous and larger, and the nucleus acquires an irregular shape. Labeled mature Paneth cells are observed by five days after injection.Unlike the other three cell types, Paneth cells do not migrate up to the villus epithelium and thus are not lost through the extrusion zone. Instead they undergo degeneration in the crypt‐base and are eventually phagocytosed by an adjacent crypt‐base columnar cell.