Endocytosis by absorptive cells in the middle segment of the suckling rat small intestine.

In neonatal suckling rats, the absorptive cells of the duodenum, jejunum and ileum endocytose maternal milk macromolecules, including antibodies from the lumen. To determine whether the absorptive cells from the middle segment of small intestine have an apical endocytic membrane system that is the same as that of the duodenum, jejunum or ileum, we applied horseradish peroxidase by intraluminal injection, and examined the absorptive cells using light and transmission electron microscopy. In the middle segment of the small intestine, the absorptive cells had an apical endocytic membrane system including apical coated pits, apical invaginations, coated vesicles, vesicles, tubules, early endosomes, late endosomes and a large homogeneous electron-dense lysosome at the supranuclear region. This would suggest that the endocytic membrane system in the absorptive cells from the middle segment of the small intestine is specialized for quick and active intracellular digestion. The apical endocytic membrane system of the absorptive cells varied according to the segment of the small intestine, and the absorptive cells made their transition gradually from jejunal type to middle type, and then from middle type to ileal type at the suckling stage. The jejunal and ileal type of absorptive cells increased with growth, in contrast to the middle type.