Bcd

figure 8-3 Organization of the intestinal mucosa. (A) Diagrammatic representation of the intestinal villus at the macro level. (B) Representation of an intestinal columnar epithelial cell, which lines the intestinal mucosa. Microvilli (Mv) extend from the free surface and form the striated edge. The surfaces of the microvilli are coated with a filamentous mucopolysaccharide material termed the glycocalyx. The terminal web (TW) lies immediately under the microvilli. Abbreviations: G, Golgi; SER, smooth endoplasmic reticulum; Ly, lysosomes. (C) The junctional complex of an intestinal epithelial cell. The junction comprises three elements: the zonula occludens (ZO or tight junction), zonula adherens (ZA or intermediate junction), and macula adherens (MA or desmosome). Abbreviations: PM, plasma membrane; MA, macula adherens; Rt, filamentous rootlets; EC, glycocalyx or filamentous coat. All three elements appear to be involved in cell-to-cell attachment. (D) Goblet cells occur singly in the intestinal epithelium and large intestine. The mucous secretion of the goblet cells lubricates the lumen of the gastrointestinal tract. [Reproduced with permission from Lentz, T. L. (1971). "Cell Fine Structure" pp. 179, 181, 183. W. B. Saunders, Philadelphia, PA.]

figure 8-3 Organization of the intestinal mucosa. (A) Diagrammatic representation of the intestinal villus at the macro level. (B) Representation of an intestinal columnar epithelial cell, which lines the intestinal mucosa. Microvilli (Mv) extend from the free surface and form the striated edge. The surfaces of the microvilli are coated with a filamentous mucopolysaccharide material termed the glycocalyx. The terminal web (TW) lies immediately under the microvilli. Abbreviations: G, Golgi; SER, smooth endoplasmic reticulum; Ly, lysosomes. (C) The junctional complex of an intestinal epithelial cell. The junction comprises three elements: the zonula occludens (ZO or tight junction), zonula adherens (ZA or intermediate junction), and macula adherens (MA or desmosome). Abbreviations: PM, plasma membrane; MA, macula adherens; Rt, filamentous rootlets; EC, glycocalyx or filamentous coat. All three elements appear to be involved in cell-to-cell attachment. (D) Goblet cells occur singly in the intestinal epithelium and large intestine. The mucous secretion of the goblet cells lubricates the lumen of the gastrointestinal tract. [Reproduced with permission from Lentz, T. L. (1971). "Cell Fine Structure" pp. 179, 181, 183. W. B. Saunders, Philadelphia, PA.]

D. Hormone-Secreting Cells: Their Distribution in the Gastroenteropancreatic Complex

A variety of endocrine cells are dispersed throughout the gastric mucosa, small intestine, and colon. Two basic kinds of cells are identifiable by classical histological procedures of staining with silver and chrome: (a) enterochromaffin cells and (b) argophyl cells. The enterochromaffin cells are sparsely present in the gastric mucosa, but are fairly prevalent in the small and large intestines. Either both cell types are located between the bases of other gastrointestinal gland cells, or they extend from the basal lamina to the lumen of the intestine. In humans there are approximately twice as many argophyl cells as enterochromaffin cells in the intestine.

By using immunofluorescent techniques, it has been shown that in the gastroenteropancreatic complex there may be as many as 14 different types of hormone-secreting cells. It has been documented that these cells are capable of secreting gastrin, motilin, secretin, chole-cystokinin, somatostatin, GIP, VIP, enkephalins, insulin, and pancreatic polypeptide. In many cases, these cells have microvillar borders facing the lumen of the stomach or the intestine, so that they can respond to chemicals in the stomach or intestinal contents. Most of the cells appear to produce a single peptide hormone, but in some instances they also produce biogenic amines such as histamine or 5-hydroxytryptamine.

There are four general pathways by which peptide hormone-secreting cells of the gastrointestinal tract can make their chemical messengers available; these pathways include (a) paracrine delivery; (b) neurocrine delivery, (c) "open" endocrine cell delivery; and (d) "closed" endocrine cell delivery (see Figure 8-4). Examples of GI hormones that function via these pathways are as follows: paracrine fashion (PP, VIP), neurocrine (enkephalins, NPY, CGRP), open endocrine delivery (gastrin), and closed endocrine delivery (secretin, motilin, CCK).

A. Pearse and colleagues have proposed that all of the gastroenteropancreatic endocrine cells are derived from a common embryological ancestor, the APUD cell, which is originally derived from the neural crest. The acronym APUD stands for amine precursor uptake and decarboxylation, which reflects the common biochemical properties of all of these cells to produce and metabolize amines. Pearse noted that both endocrine cells and neurons that produce peptide hormones share similar ultrastructural, cytochemical, and biochemical properties.

E. Coordination of Gastroenteropancreatic Hormone Release

A number of gastrointestinal hormones can only be secreted by their cells of origin as a consequence of the active digestion and absorption of nutrients in the stomach and duodenum. Thus, the stomach has been recognized to be a major control center to coordinate digestion. The ingestion of nutrients is postulated to be the initiating signal for gastroenteropancreatic hormone release. Carbohydrate ingestion leads to the secretion of GIP and enteroglucagon, while fat ingestion stimulates CCK, GIP, neurotensin, and possibly motilin release. Because these hormonal responses occur within 15 min of ingestion, it seems probable that cellular absorption of the dietary component cannot have played a significant role in stimulating the prompt release of the GI hormones. Instead, it is now known that gastric hormones or releasing factors function as control signals for hormone release from jejunal and ileal cells, as well as from the pancreas. A gastrin-releasing protein has been isolated from the fundic portion of the stomach.

F. Brain-Gut Axis

There is now good neurophysiological and neuroendocrine evidence describing the brain-gut axis. A surprising number of gut hormones have also been found in the brain (e.g., CCK, gastrin, secretin, VIP, glucagon, PP, PYY, NPY). Further, some peptide hormones that clearly function in the gastrointestinal system were originally found in the brain (e.g., enkephalins, somatostatin, substance P, CGRP). It is also known that neurons of the central nervous system interact with neurons of the enteric nervous system via both afferent and efferent pathways to influence digestive processes. Finally, evidence is now emerging that GI hormones and peptides provide a feedback signal to the central nervous system to regulate satiety and thermoregulation.

Was this article helpful?

0 0
Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

Get My Free Ebook


Post a comment