Structural Considerations

Most digestion and, for practical purposes, all absorption of nutrients take place in the small intestine. The macroscopic surface area of the human small intestine is approximately 2.5 m2, but structural modifications increase the overall surface area manyfold so that the exposure of nutrients to absorptive cells and membrane-bound digestive enzymes is greatly enhanced. The surface of the small intestine is thrown up into longitudinal folds, called folds of Kerckring. Finger-like villi project from these folds 0.5-1.5 mm into the lumen (Fig. 1). The villi are longest in the duodenum and shortest in the distal ileum. The surface of the villus is a layer of columnar epithelial cells, called enterocytes, interspersed with mucus-secreting goblet cells. The apical surface of an enterocyte is covered by hundreds of tiny processes called microvilli (Fig. 1). Owing to its appearance, the microvillous surface is called the brush border and greatly increases the exposure of luminal contents to the enterocyte membrane. Collectively, these structural modifications result in a total small intestinal absorptive area of approximately 400 m2. Tube-like crypts project down into the surface at the base of each villus. There are three crypts per villus, and these are lined with cells that are continuous with those of the villi. Crypts are approximately 0.3-0.5 mm deep, with the deepest found in the duodenum and the shallowest in the terminal ileum.

The various cell types along the crypt-villus unit have different functions (Fig. 1). The crypt cells themselves are the proliferative cells of the intestine. Crypts are monoclonal, meaning that there is only one stem cell per crypt. After division, one daughter cell remains anchored as the stem cell and the other divides several times while migrating up the crypt. Crypt cells are also capable of secreting fluid and electrolytes. As the crypt cells divide, the daughter cells migrate out of the crypts and up on to a villus. Some daughter cells differentiate into mucus-secreting goblet cells. The function of intestinal mucus is not totally clear, but it no doubt plays a role in physical, chemical, and perhaps immu-nologic protection. Most daughter cells differentiate into enterocytes, which are the principal digestive and absorptive cells of the small bowel. As new cells migrate onto a villus, old cells are extruded from the tip. During the course of its migration up the villus, an enterocyte

FIGURE 1 Schematic diagram of intestinal villi and crypt. Cell types and functions of individual cells are noted.

matures, the brush border becomes highly developed, the activities of its membrane-bound enzymes increase, and specific transport processes and membrane carriers differentiate. Thus, cells near the villus tip are much more capable of digestion and absorption than those at the base. Additional villous cells differentiate into hormone-producing enteroendocrine cells. These cells, for example, are responsible for the synthesis and release of secretin and cholecystokinin. A few cells migrate to the bottom of the crypts and become Paneth cells, whose functions are not entirely understood. Paneth cells contain lysozyme, a bacteriolytic enzyme, as well as immunoglobulin. This coupled with findings of degenerating bacteria and protozoa within their lysosomal elements suggest that Paneth cells may regulate the microbiological flora in the gut.

The entire epithelium of the small intestine is replaced every 3-6 days, making it one of the fastest growing tissues of the body. The rates of cell division and turnover are influenced by luminal contents, including nutrients and growth factors, and by gastrointestinal hormones. In general, increased cell loss results in higher rates of cell proliferation. The mechanisms balancing cell loss and renewal are poorly understood, but result in rapid repair of damaged mucosa. The high mitotic rate of the gastrointestinal mucosa makes it susceptible to x-radiation and cancer chemotherapy with resulting diarrhea and other problems.

Get Rid of Gallstones Naturally

Get Rid of Gallstones Naturally

One of the main home remedies that you need to follow to prevent gallstones is a healthy lifestyle. You need to maintain a healthy body weight to prevent gallstones. The following are the best home remedies that will help you to treat and prevent gallstones.

Get My Free Ebook


Post a comment