The small intestine

Small Intestine
The small intestine is a hollow organ of small diameter that is typically 6 to 7 m long. It is the major site for the absorption of nutrients. Important features of the small intestine are villi and microvilli, which increase surface area for absorption. macroscopically, the lining of the small intestine shows a series of permanent circular or semilunar folds (plicae circulares), consisting of mucosa and submucosa , which are best developed in the jejunum. Densely covering the entire mucosa of the small intestine are short (0.5- to 1.5-mm) mucosal outgrowths called villi that project into the lumen . These finger- or leaf like projections are covered by a simple columnar epithelium of absorptive cells called enterocytes, with many interspersed goblet cells.

Each villus has a core of loose connective tissue that extends from the lamina propria and contains fibroblasts, smooth muscle fibers, lymphocytes and plasma cells, fenestrated capillaries, and a central lymphatic called a lacteal. Intestinal glands called glands (crypts) of Lieberkühn are located in the lamina propria of the small intestine. Villi project into the lumen of the intestine; the glands of Lieberkuhn open into the mucosa at the base of the villi.

The epithelium of the intestinal glands includes differentiating cells and pluripotent stem cells for all the cell types of the small intestine. These include the following:

a. Enterocytes, the absorptive cells, are tall columnar cells, each with an oval nucleus located basally. The apical end of each enterocyte displays a prominent ordered region called the striated (or brush) border. Ultrastructurally the striated border is seen to be a layer of densely packed microvilli covered by glycocalyx through which nutrients are taken into the cells, each microvillus is a cylindrical protrusion of the apical cytoplasm approximately 1 ?m tall and 0.1 ?m in diameter containing actin filaments and enclosed by the cell membrane. Each enterocyte has an average of 3000 microvilli and each 1 mm2 of mucosal surface contains about 200 million of these structures. Microvilli, villi, and the plicae circulares all greatly increase the mucosal surface area in contact with nutrients in the lumen, which is an important feature in an organ specialized for nutrient absorption.

b. Goblet cells are interspersed among the absorptive enterocytes.They secrete glycoprotein mucins that are then hydrated to form mucus, whose main function is to protect and lubricate the lining of the intestine.