Gastrointestinal tract





The bowelles ben cominly called the guttes. TREVISA Barth. De P.R. v.xlii (1492) 1398 .


Development of the gastrointestinal tract


The epithelium and glands of the gastrointestinal tract are endodermal in origin except for ectodermal contributions to the primitive mouth and to the proctodeum.


The primitive gut


The three divisions of the primitive gut are based on the three branches of the dorsal aorta: (1) the foregut on the celiac artery, (2) the midgut on the superior mesenteric artery , and (3) the hindgut on the inferior mesenteric artery ( Fig. 6-1 ).




FIGURE 6-1.


The foregut extends from the mouth to the site of entrance of the common bile duct into the duodenum. At first, the pharyngeal portion is dominant, but the caudal portion elongates and at the beginning of the fifth week the stomach is formed as a dilation just proximal to the opening into the yolk stalk . The stomach appears to descend because of differential growth of the cephalad structures. A mesogastrium develops, continuous with that of the small intestine. The left side of the tubular stomach develops more rapidly than the right, so that the stomach appears to rotate, placing the greater curvature to the left. Rugae appear and are followed by gastric pits and glands.


Midgut and hindgut


The midgut is based on the left side of the superior mesenteric artery , where numerous branches extend to supply the proximal part of the umbilical loop as far as the yolk stalk ( Fig. 6-2 ). The midgut becomes the jejunum and ileum , the cecum and appendix, the ascending colon , and the majority of the transverse colon . It is supplied by the ileocolic and the superior mesenteric arteries , which together form a large loop centered on the terminal ileum that extends from the point of the entry of the common bile duct just proximal to the future splenic flexure of the transverse colon.




FIGURE 6-2.


During the fourth week, the previously open communication with the yolk sac becomes the narrow yolk stalk . The persistence of the proximal part of this stalk creates Meckel diverticulum (ileal diverticulum), which may be found in adults on the antimesenteric border of the ileum about 40 cm from the ileocecal valve.


As the midgut lengthens, the midgut loop expands into the base of the umbilical cord, the umbilical celom . At 6 weeks, a diverticulum appears at the distal end of the loop; this sac will form the rudimentary cecum and appendix. By the tenth week, the midgut lies once again within the abdominal cavity.


The hindgut , based on the inferior mesenteric artery , starts medial to the distal third of the transverse colon and terminates at the cloacal membrane . From it will develop the distal part of the transverse colon , the descending and sigmoid colon , the rectum , and the upper part of the anal canal to the level of the anal valves at the juncture of the cloaca with the proctodeum and also part of the bladder and urethra.




Greater and lesser omenta


Formation of the greater and lesser omenta


As part of the transverse septum, the dorsal mesogastrium joins the spleen and stomach to the dorsal body wall (lienorenal ligament and gastrosplenic ligament), whereas the ventral mesogastrium joins the stomach and liver to the anterior wall ( lesser omentum and falciform ligament ) ( Fig. 6-3 A). At about 16 weeks, the greater omentum develops by caudal extension of the free margin of a fold of the primitive mesogastrium.




FIGURE 6-3.


From the dorsal mesogastrium, a posterior fold arises consisting of a layer of mesenchyme between peritoneal covers; its vasculature comes from the left gastroepiploic artery. An anterior fold develops from the ventral mesogastrium, supplied by the right gastroepiploic artery. The two folds form the greater omentum, with its two-layered blood supply ( Fig. 6-3 B).


Maturation of the omenta and fusion of the peritoneal surfaces


The double-thickness anterior layer of the greater omentum extends caudad from the greater curvature of the stomach , passes anterior to the transverse colon as anterior leaves , and returns to the pancreas as posterior leaves , thereby forming the lesser sac or omental bursa behind the stomach ( Fig. 6-4 A).




FIGURE 6-4.


The anterior and posterior leaves of the greater omentum , each composed of two layers of peritoneum, fuse distally. The dorsal surface of the greater omentum becomes attached to the underlying transverse mesocolon and anterior surface of the transverse colon . The final result of the rotation is to place a triple layer of peritoneum over the kidney and adjacent structures that consists of two layers of colonic peritoneum and one layer of primary dorsal peritoneum (called primary because it subsequently is covered by the secondary peritoneum of the colonic mesentery) ( Fig. 6-4 B). Over the right kidney, the fused mesoduodenum is interposed between the colonic layer and the primary peritoneal surface (see Fig. 6-6 ).




Intestinal rotation


Rotation of the intestine


In the sixth week, the primitive intestine forms a simple arc, the midgut loop , with the yolk stalk at the apex ( Fig. 6-5 A). Rotation of the gut about the axis of the yolk stalk begins at this time in a counterclockwise direction. The effect is to transpose the mesentery, placing the left side to face posteriorly and the right side, anteriorly.






FIGURE 6-5.


The coils of small intestine returning to the abdomen force the descending colon against the primary peritoneum that covers the left posterior body wall, where the left surface of the colonic mesentery fuses with the original dorsal peritoneum. In this way, the descending colon loses its mesentery. The rotation places the ileocolic artery above and to the right of the superior mesenteric artery and leaves the colon inverted.


On the right, the future ascending colon lies at first at an oblique angle over the duodenum with the ileum below and medial to it. The ileocolic artery now lies above and lateral to the superior mesenteric artery ( Fig. 6-5 B).


As the cecum descends, the adjacent bowel is formed into the ascending colon and the transverse colon ( Fig. 6-5 C). The left side of the mesentery of the ascending portion is fixed to the right primary dorsal peritoneum in the same way as it is on the left, with fusion occurring between the left mesenteric surface and the primary peritoneum. This portion of the large bowel thus loses its mesentery. The colon adheres to the duodenum as it passes anteriorly, but in its transverse portion, it maintains its mesentery, which is attached to the pancreas. As noted in Fig. 6-5 A, the mesentery to the descending colon disappears and the bowel becomes fixed to the body wall.




Development of the cecum and rectum


Cecum


The cecum is at first short and cone-shaped, but as it develops, it elongates, principally in the upper part, leaving the appendix in a more dependent portion. Two saccules usually develop on either side of the anterior tenia, the right one growing faster than the left. The result is formation of a new apex from the extension of the right saccule, moving the former apex with the appendix toward the left. Alternatively, the fetal conical cecum (or some variation) may persist. In any case, the tenia of the longitudinal muscle coat terminate at the base of the appendix. The distal part of the diverticulum forming the cecum does not expand as fast as the proximal part but remains as the vermiform appendix.


During the seventh month, lymph nodules form in the wall; these will increase in number until puberty.


Rectum and anal canal


This terminal part of the gut is formed from the portion of the hindgut caudal to the connection of the allantoic duct. Their development is closely associated with that of the bladder (see Fig. 13-8 ).


An imperforate anus may present as a low defect involving the anus or as a high anorectal defect. Low defects include anal stenosis, membranous atresia, and anal agenesis (with or without a fistula); anorectal defects include anorectal agenesis (with or without a fistula) and rectal atresia. In addition, the cloaca may persist.




Spleen


Although not a part of the digestive tract, the spleen is encountered during renal and adrenal surgery and its development will be discussed here. At 8 weeks, the mesenchyme on the left side of the mesogastrium enlarges and becomes covered with mesothelium. The mesothelium becomes peritoneum, and the mesenchyme differentiates into splenic tissue, first with the appearance of sinuses and later with hemopoietic tissue. Only after birth will splenic nodules form.


Accessory spleens are occasionally found but are rarely of surgical importance. They most often occur near the splenic hilum but may appear at a distance from the spleen.




Fascia of the intestinal organs


The primitive retroperitoneal tissue differentiates into three strata—(1) an outer stratum associated with the body wall, (2) a middle stratum about the urinary tract, and (3) an inner stratum consisting of a thin layer of connective tissue that develops as the supporting tissue for the mesothelium (see Fig. 12-43 B). The inner stratum lies just beneath the peritoneum and constitutes the adventitia of the several organs imbedded within it. Because the mesenteries are covered with peritoneum, their contained vessels and nerves are also within this stratum, as is the connective tissue over the spleen, pancreas, and liver.


Perirenal fascial layers


As the right and left colon rotate, their mesenteries come to lie parallel with the posterior body wall. When the peritoneum of the original left side of the mesentery fuses with the dorsal peritoneum of the body wall (the primary peritoneum), the colon becomes fixed over the entire kidney on the left, which also is covered by the fused mesoduodenum. On the right, it is adherent to the lower part of the kidney. The posterior fixation extends to the sigmoid on the left and to the end of the cecum on the right. Laterally, the free margin of the colonic mesentery ends with fixation to the primary peritoneum, indicated by the white line of Toldt. In fetal life, the recess between the margin of the colon and that of the posterior body wall is large, extending behind the kidney; the same configuration may persist into the adult state.


Retroperitoneal fusion-fascia


Colonic rotation and fixation results in multiple layers covering the left kidney, as shown in Fig. 6-6 A. As the descending colon is pushed to the left and posteriorly, the overlying so-called primary dorsal peritoneum , that surface of the posterior peritoneum that originally covered the kidney before colonic rotation, becomes fused with the overlying layers of colonic mesentery. Thus, the original right and left leaves of the mesocolon , now fused, form three layers if the fusion layer between is counted as one. Being fused, they do not possess a potential space, so that medial mobilization of the ascending or descending colon requires entering the plane behind this fusion fascia between it and the anterior lamina of the renal fascia. The lateral margin of the fusion is marked by the white line of Toldt .




FIGURE 6-6.


The resulting layers over the kidney are transversalis fascia, posterior pararenal space, posterior lamina of the renal fascia, perirenal space, and anterior lamina of the renal fascia ( Fig. 6-6 B).




Structure of the gastrointestinal tract


Peritoneal cavity


The colon divides the peritoneal cavity into two compartments, supracolic and infracolic. The infracolic compartment is further divided into abdominal and pelvic parts. The cavity is also divided laterally by the obliquely oriented mesentery of the small intestine into right supramesenteric and left inframesenteric compartments. In addition, the ascending and descending colon delineate right and left paracolic gutters.




Visceral peritoneum


Unlike the parietal peritoneum that, on the anterior portion, has somatic sensory nerves that register pain, the visceral peritoneum has only autonomic nerves that respond to distention. Its blood supply is that of the underlying bowel, through the celiac trunk and the superior and inferior mesenteric arteries.


Stomach


The stomach wall has four layers. The serosa covers the entire surface except for space for entry of vessels at the attachment of the lesser and greater omenta and at the attachment of the gastrophrenic and gastropancreatic folds. The muscularis has three layers: (1) superficial longitudinal fibers, (2) middle circular fibers that extend throughout the stomach, and (3) more sparse oblique fibers of the body and cardiac orifice ( Fig. 6-7 A). The submucosa overlies the mucosa , which is raised into longitudinally oriented rugae on contraction of the muscular coat ( Figs. 6-8 and 6-9 ).




FIGURE 6-7.



FIGURE 6-8.


Gastric fundic mucosa. The surface is covered by tall columnar mucus-secreting epithelial cells. Pits (also known as crypts or foveolae) punctuate the surface and are quite shallow in the fundus. Tightly packed, relatively straight gastric glands empty into the pits. The glands are lined by a mixture of pepsin-secreting chief cells, which stain purplish, and acid-secreting parietal cells, which stain light pink.



FIGURE 6-9.


Gastric antral mucosa. The pits are deeper than in the fundus. The glands are coiled and exclusively mucus-secreting, although occasional parietal cells may be present. The cells lining the glands contain bubbly, foamy-appearing cytoplasm with an appearance quite different from that of the cryptal and surface epithelial cells.


The cardia marks the junction with the esophagus . The stomach ends where it joins the duodenum ( Fig. 6-7 B). As a somewhat flattened J-shaped organ, the stomach has two borders: (1) the lesser curvature medially and above and (2) the greater curvature laterally and below. The fundus is that part lying above the cardiac orifice; the body extends to the notch at the angle of the J, the incisura angularis; and the antrum joins the narrower pyloric canal , the entrance to the duodenum ( Figs. 6-10 , 6-11 , and 6-12 ).




FIGURE 6-10.


Distal esophagus and stomach from an autopsy case, opened. The gastric body exhibits prominent rugal folds.

(Image courtesy of Dawn Dawson, M.D.)



FIGURE 6-11.


Distal stomach, portion of duodenum and head of pancreas, resected for pancreatic cancer (Whipple procedure). A stent via the ampulla of Vater marks the entry of the common bile duct and pancreatic ducts.

(Image courtesy of Christina Bagby, M.D.)



FIGURE 6-12.


Duodenal mucosa. The length of the villi is variable. Mononuclear cells are relatively abundant in the lamina propria. Lobular collections of tubuloalveolar Brunner ’s glands, lined by cells that are distinctively different from those that line the crypts, are present both above and below the muscle bundles of the muscularis mucosae; they are a distinctive finding in this portion of the small bowel.


Blood supply to the anterior aspect of the stomach and greater omentum


The short celiac trunk arising from the aorta branches into the left gastric, the splenic, and the common hepatic arteries ( Fig. 6-13 ).




FIGURE 6-13.


The small left gastric artery runs behind the omental bursa (lesser sac) to the esophagus and along the lesser curvature inside the lesser omentum.


The splenic artery gives off the left gastroepiploic artery just before it divides to enter the spleen.


The common hepatic artery provides the right gastric artery , which runs from the first part of the duodenum along the lesser curvature from right to left within the lesser omentum. It then anastomoses with the branches of the left gastric artery coming from the left. More distally, it divides into the superior pancreaticoduodenal artery and the gastroduodenal artery , which, in turn, gives off the right gastroepiploic artery that runs in the greater omentum along the greater curvature.


Veins accompanying the arteries carry venous drainage into the portal system.


Greater omentum


The greater omentum receives its blood supply from the right and left gastroepiploic vessels. The right gastroepiploic artery arises from the gastroduodenal artery (or rarely, from the superior mesenteric artery), and the left gastroepiploic artery is the last branch of the splenic artery . The right artery is lower than the left and in most cases is larger, supplying from two-thirds to three-quarters of the omentum. The right and left arteries form the gastroepiploic arterial arcade; however, in one tenth of cases, the arcade is incomplete on the left side. The arteries join by means of numerous collaterals and through the capillary network of the gastric wall. The major vessels to the omentum arising from the arcade are the right epiploic artery (from the right gastroepiploic artery), the middle epiploic artery (arising at the junction of the two gastroepiploic arteries), and the left epiploic artery (from the left gastroepiploic artery). In addition, an accessory epiploic artery leaves the arch immediately before the takeoff of both the right omental artery and the short epiploic arteries that fill in the spaces between the major vessels.


The right and left distal arterial arcades, which together form the lower arterial arcade , are formed by junction of the right and left epiploic arteries in the posterior reflection toward the inferior margin of the omentum. These arcades are inconstant, being composed of smaller vessels that cannot be depended on to supply the omentum if the gastroepiploic arcade is divided.


The omental veins are valved, are larger than the arteries, and usually run in pairs with them. The left gastroepiploic vein carries venous drainage from the posterior layer of the omentum into the portal system; the right gastroepiploic vein empties blood from the anterior layer into the superior mesenteric vein and then into the portal vein. The lymphatics follow the epiploic arteries.


Anterior and posterior vagal trunks carry parasympathetic stimuli from the esophageal plexus on the esophagus via the anterior branch of the left vagus nerve to supply the anterior surface of the stomach.


Blood supply to the posterior aspect of the stomach


The pancreas and the vessels beneath it are exposed as the stomach and omentum are elevated. On the right, the gastroduodenal artery takes off from the common hepatic artery ( Fig. 6-14 ) It branches to form the right gastroepiploic artery , which, in turn, divides into the epiploic arteries for the right side of the omentum. The left gastroepiploic artery arises from the splenic artery before that vessel branches to enter the spleen . It terminates in epiploic arteries for the left side of the omentum. The left gastric artery originates from the celiac trunk. The superior mesenteric vein and the splenic vein drain into the portal vein behind the pancreas.


Mar 11, 2019 | Posted by in UROLOGY | Comments Off on Gastrointestinal tract
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