Features Of The Gastrointestinal Tract Structure and Function of the Small Intestine

The small intestine is the principal site of food digestion, nutrient absorption as well as endocrine secretion. It is the longest component of the alimentary tract, measuring over 6 meters, and is divided into three anatomic regions: duodenum, jejunum and ileum. The duodenum begins at the pylorus of the stomach and is the proximal 20-25 cm of the small intestine. The jejunum spans about 2.5 meters in length. The ileum is approximately 3.5 meters long and an extension of the jejunum.

The absorptive surface area of the small intestine is greatly increased by tissue and cell specializations such as plicae circulares, villi and microvilli (Fig. 1). Plicae circulares are permanent transverse folds of the mucosa, forming semicircular or spiral elevations. They are abundant in the distal duodenum and beginning of the jejunum. Intestinal villi are finger-like outgrowths of mucosa protruding into the lumen of the small intestine. Microvilli are protrusions of the apical plasmalemma of the epithelial cells covering the intestinal villi, increasing the surface area of the small intestine 20 times. Therefore, these modifications immensely amplify the absorptive and interactive (with intestinal content, including the microbiota) surface area of the small intestine.

The mucosa comprises the lining epithelium, a lamina propria that houses glands and muscularis mucosa. There are at least 5 types of cells found in the intestinal mucosal

Villi Diagram

Figure 1 Schematic diagram of the mucosa, villi, and component cells of the small intestine.

Figure 1 Schematic diagram of the mucosa, villi, and component cells of the small intestine.

epithelium. They include enterocytes, goblet cells, Paneth cells, enteroendocrine cells and M cells (microfold cells). Both the enterocytes and the goblet cells line the villus and are the major cell types in the epithelium. The enterocytes are columnar in shape and have brush borders composed of microvilli which help to enhance the water ions and nutrient absorbing surface area. Goblet cells are unicellular mucin-secreting glands which produce mucinogen and mucin, a component of mucus. The number of goblet cells increases progressively down the gastrointestinal tract from the duodenum, to jejunum, ileum and colon, where they are most abundant. The Paneth cells' role is to maintain the innate immunity by secreting antimicrobial substances such as a-defensins (4,69). Enteroendocrine cells are present only in small numbers (~ 1%) and their functions include the production of panacrine and endocrine hormones (5). M cells are modified enterocytes overlying the enlarged lymphatic nodules in the lamina propria. Their function is to phagocytose and transport antigens present in the intestinal lumen to the underlying macrophages and lymphoid cells, which then migrate to other compartments of the lymphoid nodes, where immune responses to foreign antigens are initiated (5).

The lamina propria is rich in lymphoid cells, which will protect the intestinal lining from bacterial invasion. The loose connective tissue of lamina propria forms the main part of the villi, extending down to the muscularis mucosa. The epithelium may invaginate into the lamina propria between the villi to form glands, termed the crypts of Lieberkuhn. These tubular glands consist of enterocytes, goblet cells, regenerative cells, enteroendo-crine cells and Paneth cells. The rate at which the regenerative cells proliferate is high and they are capable of replacing other cell types in the intestinal epithelium. As mentioned above, the pyramidal-shaped Paneth cells secrete antibacterial agents, such as lysozyme and a-defensins or cryptdins, and internalized extracellular matter such as bacteria and immunoglobulin. Therefore, it is postulated that these cells help in regulation of the bacterial microenvironment in the small intestine.

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Responses

  • Maarit
    What structure protrudes from mucosa of small intestine?
    6 years ago
  • tero
    What features of the digestive tract extend surface area.?
    5 years ago

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