Embryonal morphogenesis of intestinal villi

Morphogenesis of intestinal villi begins in duodenum and passes in two stages. At first more actively proliferative and metabolic pseudostratified intestinal epithelium takes root into underlying mesenchyme, which loosens and transformates in connective tissue in this period. Together they form primary longitudinal folds of only beginner forming mucosa. Then these folds break up on typical intestinal villi. Morphogenesis of intestinal villi takes place in period of recanalization of duodenum, which, as morphogenesis of whole organ, is based on interconnections of processes, passing in its epithelial bud and surrounding mesenchyme. Intestinal epithelium exercises inductive influence on underlying mesenchyme, connecting with the process of cellular proliferation, always more intensive in epithelium. Intensive currents of epithelial metabolites erode mesenchyme. Its cells proliferites too, lose long branches and lie more and more tightly about epithelial bud of organ. Wide layer of compact mesenchyme limits external growth of epithelium, and it fills up intestinal lumen. Simultaneously capillaries narrow in thick mesenchyme, mitotic activity of epithelium and mesenchyme falls. Thin reticular, winding fibers and traces of weak sulfat glycosaminglicans appear in the mesenchyme about epithelium. Then nerve fibres and neuroblasts penetrate into intestinal wall, reticular fibers form network in mesenchyme about epithelium, capillaries widen and secondary, mitotic activity of intestinal epithelium rises. Epithelial “plugs” appear in lumen of duodenum, circular muscular layer - in external zone of its mesenchyme. After this mesenchyme mainly about epithelium produces hyaloronans and proteoglycans, bonding water. Considerable production of extracellular matrix and loosening of mesenchyme are accompanied such lowering of mitotic activity of its cells. It is beginning accelerated widening of lumen and recanalization of organ. Circular muscular layer levels external displacements of mesenchyme with different speed on perimeter of duodenum, causing different metabolic activity of intestinal epithelium on its perimeter. In result mesenchyme redistributes in thickness of duodenal wall on its perimeter: “surplus” of mesenchyme moves from sector of more active epithelium in sector of less active epithelium, causing its delay there with lag from locus of active epithelium. Thus apexes of primary longitudinal folds and then intestinal villi emerge in intestinal lumen.