If examined closely, the lumenal surface of the small intestine appears similar to velvet due to its being covered by millions of small projections called villi which extend about 1 mm into the lumen. Villi are only the most obvious feature of the mucosa which houses a dynamic, self-renewing population of epithelial cells that includes secretory cells, endocrine cells and the mature absorptive epithelial cells which take up nutrients from the lumen and transport them into blood, fulfilling the basic function of the digestive system. Understanding how the small intestine functions requires looking at the structure of the mucosa in more detail.
The mucosa of small intestinal mucosa is arranged into two fundamental structures:
The system described above is really quite elegant. Stem cells in the crypts divide to form daughter cells. One daughter cell from each stem cell division is retained as a stem cell. The other becomes committed to differentiate along one of four pathways to become an enterocyte, enteroendocrine cell, goblet cell or Paneth cell. Cells in the enterocyte lineage divide several more times as they migrate up the crypts, and as they migrate onto the villi, differentiate further into the mature absorptive cells that express all the transport proteins and enzymes characteristic of those cells. To put it another way, enterocytes are born at the bottom of the crypts, pass through childhood migrating up the walls of the crypts, then settle down briefly to enjoy an absorptive adulthood on the villi.
Inside the Villus
Virtually all nutrients, including all amino acids and sugars, enter the body across the epithelium covering small intestinal villi. As shown in the diagram above, each villus contains a capillary bed and a blunt-ended lymphatic vessel referred to as the "central lacteal."
After crossing the epithelium, most of these molecules diffuse into a capillary network inside the villus, and hence into systemic blood. Some molecules, fats in particular, are transported not into capillaries, but rather into the lymphatic vessel, which drains from the intestine and rapidly flows into blood via the thoracic duct.
Source: Republished with permission by Richard Bowen - Hypertexts for Biomedical Sciences