Cellular differentiation of absorptive cells in the neonatal rat colon: an electron microscopic study.

Cellular differentiation of the absorptive cells in the neonatal-suckling rat colon was investigated using horseradish peroxidase as a macromolecular tracer with a scanning or transmission electron microscope. Numerous villi were lined on oblique folds in an orderly fashion and smaller villi were lined between the folds in the proximal colon in 0- to 12-day-old rats. Three different types of absorptive cells were observed in the epithelium covering their villi: type I cells, type II cells, and type III cells. The type I cells were most numerous, and similar to typical absorptive cells of the adult rat large intestine. The type II cells endocytosed macromolecules into an apical endocytic membrane system without giant lysosome from the large intestinal lumen. The type III cells were characteristic of the well-developed endocytic membrane system, including a large supranuclear giant lysosome specialized for endocytosis, storage, and intracellular digestion as an ileal absorptive cell of the suckling rat. At no time was macromolecular tracer detected in the Golgi apparatus of the absorptive cells or in the intercellular space of the epithelium. These results suggest that the villial absorptive cells of the proximal colon have differentiated to three types at specific neonatal-suckling times. The type II and type III cells were not observed in the epithelium after the disappearance of the villi.