4. Major layers of the GI tract. The innermost layer is the mucosal layer, which is made up of epithelial cells, the lamina propia, and the muscularis mucosae. Beyond the mucosa is the submucosal layer consisting of submucosal glands and blood vessels. The muscularis layer is made up of a layer of inner circular muscle and a layer of outer longitudinal muscle. The serosa makes up the outermost layer and contains arteries, veins, and lymph vessels. Illustration by Cari Lewis. 

Contexts in source publication

Context 1

... results of the mediation analyses are visible in Tables 10.3 and 10.4. Here, the direct and indirect effects of PIR and education on colorectal cancer incidence through diet, physical activity, BMI, and allostatic load are demonstrated for each population. Overall, education had a significant negative association with allostatic load (β = -0.1666, P < 0.05) and colorectal cancer incidence (β = -0.2974, P < 0.05). Education attainment also exhibited a significant negative association with CRC via associations with physical activity, diet, and BMI (β = -0.001, P < 0.05), as well as diet, BMI, and allostatic load (β = -0.001, P < 0.05) (Figure 10.2). Direct and indirect effects of education attainment on colorectal cancer incidence across all populations; "*" denotes a P value < 0.05, "**" denotes a P value < 0.01, "***" denotes a P value < ...

Context 2

... are many types of epithelial cells, which are typically distinguished into two main groups depending on their function and structure: covering epithelia and glandular epithelia. The covering epithelial cells are those that cover external surfaces or line body cavities, and are classified based on morphology, seen in Figure 2.3. Glandular epithelial cells line the large intestine and specialize in mucus secretion (McGurk 2013;Young, et al. 2013). Glandular epithelial tissue is particularly relevant in colorectal cancer because adenocarcinomas (gland- forming epithelial tumors) arise from these cells and are the most common type of cancer in the colon (McGurk 2013). ...

Context 3

... process of digestion begins with ingestion, when food materials enter the mouth (see Figure 2.6 above for a diagram of the digestive system). Next, mechanical processing by the teeth and tongue takes place as the materials are broken down into smaller fragments, which make them easier to travel through the digestive tract (Moini 2012;Moore, et al. 2013). In addition to mechanical processing, enzymes, such as amylase from the salivary glands, also begin to break down particles, even while food is still in the mouth. After food leaves the mouth, enzymatic breakdown of materials continues in the stomach and intestines. Food begins to be broken down from large food pieces into smaller particles, so that they can be absorbed by the epithelial walls of the mucosa; these large molecules include polysaccharides, triglycerides, and proteins, which are too large to be absorbed whole (Moini 2012;Moore, et al. 2013). Next, water, acids, buffers, enzymes, and salts are released by the epithelium and glandular organs of the alimentary canal, which eventually triggers the movement of organic molecules, vitamins, electrolytes, and water across the epithelium into the interstitial fluid of the submucosa (Moore, et al. 2013). Waste products of any secreted bodily fluids mix with undigested leftover residues and become fecal material, which is eliminated from the body via defecation (Moini ...

Context 4

... GI tract (alimentary canal) of the digestive system is a muscular tube that extends 8 meters from the lips to the anus and includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum and anus (Moore, et al. 2013). The walls of the alimentary canal 26 itself consist of four layers that specialize in various functions, seen in Figure 2.4. The ...

Context 5

... stem cells are located at the base of the intestinal crypts. The epithelial cells that originate from these stem cells divide asymmetrically, which results in another identical stem cell and a progenitor (Figure 2.7). The stem cell proceeds to differentiate into an adult intestinal cell, while the progenitor retains its original ability to divide (Hall, et al. 1994;Potten, et al. 2002). In general, stem cell stability and function is maintained by the microenvironment in which it is located, wherein paracrine secretions of cytokines and growth factors influence stem cell differentiation (Brittan and Wright 2004). Once stem cells are differentiated into epithelial cells, they migrate to the top of the intestinal crypt where they become functional enterocytes (i.e., intestinal absorption cells). Overall, the turnover rate of intestinal cells in the epithelial lining is very rapid, with all cell populations being renewed every five days (Garagnani, et al. 2013). This is particularly relevant because with such a high rate of mitosis, there is more opportunity for mutation in cell proliferation or apoptosis genes compared to other cells in the body that divide much more slowly (e.g., osteocytes). ...

Context 6

... small intestine extends from the stomach to the large intestine and is made up of the duodenum, jejunum, and ileum. The small intestine receives both pancreatic and liver secretions to complete the digestion of nutrients from food material; the small intestine is where 90 percent of nutrient absorption occurs (Moini 2012). Intestinal villi make up the inner walls of the small intestine, seen in Figure 2.5. The villi are most numerous in the proximal jejunum and the duodenum (Moini 2012;Moore, et al. 2013). In general, they increase the surface area of ...

Context 7

... the primitive gut tube is made up of four general sections (as shown in Figure 2.1): 1) the pharyngeal gut, 2) the foregut, 3) the midgut, and 4) the hindgut. The locations of these sections and what they form are described as follows (Carlson ...

Context 8

... colon is formed from both the mid-and hindgut during development. The midgut forms the cecum, ascending colon and the proximal two-thirds of the transverse colon, while the hindgut forms the distal one-third of the transverse colon, the descending colon, the sigmoid colon, and the upper section of the anal canal (Figure 2.2). By 5 weeks, the midgut of the developing embryo is suspended to the dorsal abdominal wall by mesentery (Carlson 2013;Sadler 2011). Development of this part of the gut tube is characterized by rapid ...