The small intestine consists of the serosa, the muscularis, the submucosa and the mucosa (Figure 6.1). The serosa is an extension of the peritoneum, and consists of a single layer of flattened mesothelial cells overlying some loose connective tissue. The muscularis has an outer longitudinal layer and an inner circular layer of muscle. The submucosa consists largely of dense connective tissue sparsely infiltrated by lymphocytes, fibroblasts, macrophages, eosinophils, mast and plasma cells. The submucosa contains an extensive lymphatic network.
The intestinal mucosa itself can be divided into three layers:
a) the muscularis mucosa, which is the deepest layer consisting of a sheet of muscle 3 to 10 cells thick that separates the mucosa from the submucosa.
b) the lamina propria, the middle layer, is mainly connective tissue and forms the core of the numerous villi and surrounds the crypts. The lamina propria usually contains many types of cells, e.g. plasma cells, lymphocytes, mast cells, macrophages, smooth muscle cells and non-cellular elements such as collagen and elastin fibres. The lamina propria provides structural support, and there is increasing evidence that it has an important role in preventing the entry of microorganisms and foreign substances.
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