Histology of the digestive tract
The digestive tract includes four major layers (Fig. 17.4). Listed from the lumen outward, they are (1) the mucosa, (2) the submucosa, (3) the muscularis externa, and (4) the serosa.
These layers vary somewhat according to region of the digestive tract, so the following description applies to the small intestine.Mucosa
The mucosa layer is a mucous membrane lining the inside of the digestive tract. It consists of three sublayers including a layer of epithelial tissue in direct contact with the contents of the digestive tract, the lamina propria, and the muscularis mucosae.
Within the oral cavity, pharynx, esophagus, and anal canal, the epithelial tissue is stratified squamous epithelium that performs a protective function. The remainder of the tract is mostly simple columnar epithelium and mucus-producing goblet cells. Shortly after birth, the simple columnar epithelial cells develop tight junctions, thus forming a barrier between the lumen contents and the body. Prior to the formation of these tight junctions, an animal is able to absorb antibodies found in the colostrum until the epithelial cells undergo closure, or until the development of tight junctions. Closure generally occurs within a couple days after birth. Also found scattered among the epithelial cells are endocrine cells, collectively called enterendocrine cells, which secrete hormones coordinating digestive functions.
The epithelial cells have a life span of approximately 2-3 days in the esophagus and up to 6 days in the large intestine. These cells are always being sloughed off, and they are continually replaced.
The lamina propria, consisting of areolar connective tissue, binds the epithelial cells to the muscularis mucosae. This layer also contains blood vessels, sensory neurons, lymphatic vessels, smooth muscle cells, and lymphatic nodules that are part of the mucosa-associated lymphatic system (MALT). The MALT is present along the digestive tract and contains cells of the immune system.
The appendix and tonsils are part of the MALT.The muscularis mucosa is a thin layer of smooth muscle fibers. This layer helps create folds in the stomach and small intestine, thus increasing their surface area.
Submucosa
The submucosa consists of dense irregular connective tissue and contains large blood vessels, lymphatic vessels, and—in some regions—exocrine glands secreting buffers and enzymes into the lumen.
Muscularis externa
Within the oral cavity, pharynx, and parts of the esophagus, depending on the species, this layer contains skeletal muscle that controls swallowing. The external anal sphincter also generally contains skeletal muscle permitting voluntary control of defecation. Along the remainder of the digestive tract, the muscularis externa generally consists of two layers of smooth muscle: an inner layer of circular smooth muscle fibers and an outer layer of longitudinal smooth muscle fibers. These layers control peristalsis and segmental contractions.
Serosa
Most portions of the digestive tract lie within the peritoneal cavity. The outermost portions, or superficial layer, of the digestive tract is lined with the adventitia. As the digestive tract passes through the diaphragm and enters the peritoneal cavity, this layer is composed of the visceral portion of the peritoneum, also called the serosa. There is no serosa around the oral cavity, pharynx, esophagus, or rectum. Instead, there is a layer of collagen fibers attaching the digestive tract to surrounding structures.