Molecular Regulation Of Pancreas Development

Fibroblast growth factor (FGF) and activin (a TGF-^ family member) produced by the notochord repress SHH expression in gut endoderm destined to form pancreas. As a result, expression of the pancreatic and duodenal homeobox 1 (PDX) gene, a master gene for pancreatic development, is upregulated. Although all of the downstream effectors of pancreas development have not been determined, it appears that expression of the paired homeobox genes PAX4 and 6 specify the endocrine cell lineage, such that cells expressing both genes become 3 (insulin), 6 (somatostatin), and 7 (pancreatic polypeptide) cells; whereas those expressing only PAX6 become a (glucagon) cells.

CLINICAL CORRELATES Pancreatic Abnormalities

The ventral pancreatic bud consists of two components that normally fuse and rotate around the duodenum so that they come to lie below the dorsal pancreatic bud. Occasionally, however, the right portion of the ventral bud migrates along its normal route, but the left migrates in the opposite direction. In this manner, the duodenum is surrounded by pancreatic tissue, and an annular pancreas is formed (Fig. 13.23). The malformation sometimes constricts the duodenum and causes complete obstruction.

Accessory pancreatic tissue may be anywhere from the distal end of the esophagus to the tip of the primary intestinal loop. Most frequently it lies in the mucosa of the stomach and in Meckel's diverticulum, where it may show all of the histological characteristics of the pancreas itself.

Midgut

In the 5-week-old embryo, the midgut is suspended from the dorsal abdominal wall by a short mesentery and communicates with the yolk sac by way of the vitelline duct or yolk stalk (Figs. 13.1 and 13.15). In the adult the midgut begins immediately distal to the entrance of the bile duct into the duodenum (Fig. 13.15) and terminates at the junction of the proximal two-thirds of the transverse colon with the distal third. Over its entire length the midgut is supplied by the superior mesenteric artery (Fig. 13.24).

Development The Coronary Blood Supply

Cloaca

Figure 13.24 Embryo during the sixth week of development, showing blood supply to the segments of the gut and formation and rotation of the primary intestinal loop. The superior mesenteric artery forms the axis of this rotation and supplies the midgut. The celiac and inferior mesenteric arteries supply the foregut and hindgut, respectively.

Cloaca

Figure 13.24 Embryo during the sixth week of development, showing blood supply to the segments of the gut and formation and rotation of the primary intestinal loop. The superior mesenteric artery forms the axis of this rotation and supplies the midgut. The celiac and inferior mesenteric arteries supply the foregut and hindgut, respectively.

Cephalic limb of primary intestinal loop

Vitelline duct

Cephalic limb of primary intestinal loop

Vitelline duct

Normal Rotation Midgut

Stomach

Duodenum

Superior mesenteric artery

Caudal limb of primary intestinal loop

Was this article helpful?

0 0
Invisible Viagara

Invisible Viagara

You are about to discover the "little-known" techniques, tricks and "mind tools" that will show you how to easily "program" your body and mind to produce an instant, rock-hard erection. Learn how to enjoy all of the control, confidence and satisfaction that comes from knowing you can always "rise to the challenge" ... and never have to deal with embarrassment, apologies, shyness or performance anxiety in the bedroom, ever again.

Get My Free Ebook


Post a comment