Digestive tract wall

The digestive tract wall has the same basic structure from the esophagus through and including the colon. The four major layers within the wall are  

Mucosa. The innermost layer of the wall is the mucosa, which consists of a mucous membrane, the lamina propria, and the muscularis mucosa. The mucous membrane provides important protective and absorptive functions for the digestive tract. The nature of the epithelial cells lining the tract varies from one region to the next. Rapidly dividing stem cells continually produce new cells to replace worn out epithelial cells. The average life span of these epithelial cells is only a few days. The lamina propria is a thin middle layer of connective tissue. This region contains the capillaries and small lymphatic vessels that take up the digested nutrient molecules. The muscularis mucosa is a thin layer of smooth muscle. Contraction of this muscle may alter the effective surface area for absorption in the lumen. 

Submucosa. The submucosa is a thick middle layer of connective tissue. This tissue provides the digestive tract wall with its distensibility and elasticity as nutrient materials move through the system. 

Muscularis externa. The outer layer of the wall is the muscularis externa. In most regions of the tract, it consists of two layers of muscle an inner circular layer and an outer longitudinal layer. Contraction of the circular layer narrows the lumen of the tube. Contraction of the longitudinal layer causes the tube to shorten. 

Muscular activity, or gastrointestinal motility, is enhanced by stretching the muscle, as occurs with the presence of food materials and distension of 

---

Describe the anatomical and functional characteristics of each of the four layers of the digestive tract wall mucosa, submucosa, muscu-laris externa, and serosa... 

Adhesion to the digestive tract wall to prevent colonisation by pathogenic microorganisms Detrimental bacteria, such as E. coli, need to become attached to the gut waU to exert their harmful effects. Attachment is achieved by means of hairlike structures, called fimbriae, on the bacterial surface. The fimbriae are made up of proteins, called lectins, which recognise and selectively combine with specific oligosaccharide receptor sites on the gut wall. Lactobacilli successfully compete for these attachment sites, as shown in Fig. 24.1. 

Historically, dietary fiber referred to iasoluble plant cell wall material, primarily polysaccharides, not digested by the endogenous enzymes of the human digestive tract. This definition has been extended to iaclude other nondigestible polysaccharides, from plants and other sources, that are iacorporated iato processed foods. Cellulose [9004-34-6] (qv) is fibrous however, lignin [9005-53-2] (qv) and many other polysaccharides ia food do not have fiberlike stmctures (see also Carbohydrates). 

The function of the digestive system is to make ingested food available to the cells of the body. Most ingested food is in the form of very large molecules that must be broken down by mechanical and biochemical processes into their smaller components (see Table 18.1). These smaller units are then absorbed across the wall of the digestive tract and distributed throughout the body. Not all ingested materials may be completely... 

The enormous interest in augmenting the world s supply of protein from microbial sources (135) has focussed much attention on this source of protein as a nutrient for animals as well as humans. However, there is some evidence that if intact yeast cells are included in a diet, the tough polysaccharide-containing cell wall may constitute at least a partial barrier in the effective utilization of the protein of the cytoplasm. One reason for this is the absence of enzymes in the digestive tract of humans and most warm-blooded animals capable of hydrolyzing the microfibrillar / -glucan component of the cell wall (135,136). Other factors may include the inability of the proteolytic enzymes of the digestive track to make effective contact with the cells protein. 

Le Douarin, N.M. and Tiellett (1973) Migration of neural crest cells to the wall of the digestive tract in avian embryo. J. Embryol. Exp. Morphol. 30 31-48. 

---