As in the small intestine, all of the mucosal epithelial cells of the large intestine arise from stem cells located at the bottom of the intestinal gland. The lower third of the gland constitutes the intestinal stem cell niche, where newly generated cells undergo two to three more divisions as they begin their migration up to the luminal surface, where they are shed about 5 days later. The intermediate cell types found in the lower third of the intestinal gland are identical to those seen in the small intestine.
The turnover times of the epithelial cells of the large intestine are similar to those of the small intestine (i.e., about 6 days for absorptive cells and goblet cells and up to 4 weeks for enteroendocrine cells). Senile epithelial cells that reach the mucosal surface undergo apoptosis and are shed into the lumen at the midpoint between two adjacent intestinal glands.
Although the lamina propria of the large intestine contains the same basic components as the rest of the digestive tract, it demonstrates some additional structural features and greater development of some others. These include the following:  Collagen table, which represents a thick layer of collagen and proteoglycans that lies between the basal lamina of the epithelium and that of the fenestrated absorptive venous capillaries. This layer is as much as 5  m thick in the normal human colon and can be up to three times that thickness in human hyperplastic colonic polyps. The collagen table participates in regulation of water and electrolyte transport from the intercellular compartment of the epithelium to the vascular compartment.
 Pericryptal fbroblast sheath, which constitutes a well-developed fibroblast population of regularly replicating cells. They divide immediately beneath the base of the intestinal gland, adjacent to the stem cells found in the epithelium (in both the large and small intestines). The fibroblasts then differentiate and migrate upward in parallel and synchrony with the epithelial cells. Although the ultimate fate of the pericryptal fibroblast is unknown, most
FIGURE 17.29 • Electron micrograph of dividing goblet cells.
This electron micrograph demonstrates that certain cells of the intestine continue to divide even after they have differentiated. Here, two goblet cells (GC) are shown in division. Typically, the dividing cells move away from the basal lamina toward the lumen. One of the goblet cells shows mucinogen granules (M) in its apical cytoplasm. The chromosomes (C) of the dividing cells are not surrounded by a nuclear membrane. Compare with the nuclei (N) of the nondividing intestinal epithelial cells. The lumen of the gland (L) is on the right. CT, connective tissue; E, eosinophil. 5,000.
of these cells, after they reach the level of the luminal surface, take on the morphologic and histochemical characteristics of macrophages. Some evidence suggests that the macrophages of the core of the lamina propria in the large intestine may arise as a terminal differentiation of the pericryptal fibroblasts:  GALT, which is continuous with that of the terminal ileum. In the large intestine, GALT is more extensively developed; large lymphatic nodules distort the regular spacing of the intestinal glands and extend into the submucosa. The extensive development of the immune system in the colon probably reflects the large number and variety of microorganisms and noxious end products of metabolism normally present in the lumen.