Diagram of general organization of the alimentary canal. This composite diagram shows the wall structure of the alimentary canal in four representative organs: esophagus, stomach, small intestine, and large intestine. Note that villi, a characteristic feature of the small intestine, are not present in other parts of the alimentary canal. Mucosal glands are present throughout the length of the alimentary canal but sparingly in the esophagus and oral cavity. Submucosal glands are present in the esophagus and duodenum. The extramural glands (liver and pancreas) empty into the duodenum. Diffuse lymphatic tissues and nodules are found in the lamina propria throughout the entire length of the alimentary canal (shown here only in the large intestine). Nerves, blood vessels, and lymphatic vessels reach the alimentary canal via the mesenteries or via the adjacent connective tissue (as in the retroperitoneal organs).
cretory products provide mucus for protective lubrication, as well as buffering of the tract lining and substances that assist in digestion, including enzymes, hydrochloric acid, peptide hormones, and water (see Fig. 16.1). The mucosal epithelium also secretes antibodies that it receives from the underlying connective tissue.
The glands of the alimentary tract (see Fig. 16.1) develop from invaginations of the luminal epithelium and include
• Mucosal glands that extend into the lamina propria.
• Submucosal glands that either deliver their secretions directly to the lumen of mucosal glands or via ducts that pass through the mucosa to the luminal surface.
• Extramural glands that lie outside the digestive tract and deliver their secretions via ducts that pass through the wall of the intestine to enter the lumen. The liver and the pancreas are extramural digestive glands (see Chapter 17) that greatly increase the secretory capacity of the digestive system. They deliver their secretions into the duodenum, the first part of the small intestine
The lamina propria contains glands, vessels that transport absorbed substances, and components of the immune system
As noted, the mucosal glands extend into the lamina propria throughout the length of the alimentary canal. In ad-
edition, in several parts of the alimentary canal (e.g., the esophagus and anal canal), the lamina propria contains aggregations of mucus-secreting glands. In general they lubricate the epithelial surface to protect the mucosa from mechanical and chemical injury. These glands are described below in relation to specific regions of the digestive tube.
In segments of the digestive tract in which absorption occurs, principally the small and large intestines, the absorbed products of digestion diffuse into the blood and lymphatic vessels of the lamina propria for distribution. Typically, the blood capillaries are of the fenestrated type and collect most of the absorbed metabolites. In the small intestine, lymphatic capillaries are numerous and receive some absorbed lipids and proteins.
The lymphatic tissues in the lamina propria function as an integrated immunologic barrier that protects against pathogens and other antigenic substances that could potentially enter through the mucosa from the lumen of the alimentary canal. The lymphatic tissue is represented by
• Diffuse lymphatic tissue consisting of numerous lymphocytes and plasma cells, located in the lamina propria, and lymphocytes transiently residing in the intercellular spaces of the epithelium
• Lymphatic nodules with well-developed germinal centers
• Eosinophils, macrophages, and sometimes neutrophils
The diffuse lymphatic tissue and the lymphatic nodules are referred to as gut-associated lymphatic tissue (GALT). In the distal small intestine, the ileum, extensive aggregates of nodules, called Peyer's patches, occupy much of the lamina propria and submucosa. They tend to be located on the side of the tube opposite the attachment of the mesentery. Aggregated lymphatic nodules are also present in the appendix.
The muscularis mucosae forms the boundary between mucosa and submucosa
The muscularis mucosae, the deepest portion of the mucosa, consists of smooth muscle cells arranged in an inner circular and outer longitudinal layer. Contraction of this muscle produces movement of the mucosa, forming ridges and valleys that facilitate absorption and secretion. This localized movement of the mucosa is independent of the peristaltic movement of the entire wall of the digestive tract.
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