Physiological Development of GI Tract

Birth brings about several challenges for the neonate, who has been moved from an environment of parenteral nutrition to enteral nutrition. The demands on the gut now require effective sucking and swallowing, gastric emptying and secretions, hepatobiliary and pancreatic secretions and enterocyte function with the development of appropriate enzymes for the uptake and assimilation of nutrients.

The small bowel is ∼270 cm long at birth in a term neonate and grows to an adult length of 450-550 cm by 4 year of age. The mucosa of the small intestine is composed of villi, which are finger-like projections of the mucosa into the bowel lumen that significantly expand the absorptive surface area. The mucosal surface is further expanded by a brush border containing digestive enzymes and transport mechanisms for monosaccharides, amino acids, dipeptides and tripeptides, and fats.

The cells of the villi originate in adjacent crypts and become functional as they migrate from the crypt up the villus. The small bowel mucosa is completely renewed in 4-5 days, providing a mechanism for rapid repair after injury, but in young infants or malnourished children, the process may be delayed. Crypt cells also secrete fluid and electrolytes. The villi are present by 8 week of gestation in the duodenum and by 11 week in the ileum.

Disaccharidase activities are measurable at 12 week, but lactase activity does not reach maximal levels until 36 wk. Even premature infants usually tolerate lactose-containing formulas because of carbohydrate salvage by colonic bacteria. In children of African and Asian ethnicity, lactase levels may begin to fall at 4 year of age, leading to intolerance to mammalian milk. Mechanisms to digest

The colon is a 75-100 cm sacculated tube formed by 3 strips of longitudinal muscle called taenia coli that traverse its length and fold the mucosa into haustra. Haustra and taenia appear by the 12th week of gestation. The most common motor activity in the colon is nonpropulsive rhythmic segmentation that acts to mix the chyme and expose the contents to the colonic mucosa. Mass movement within the colon typically occurs after a meal.

The colon extracts additional water and electrolytes from the luminal contents to render the stools partially or completely solid. The colon also acts to scavenge by-products of bacterial degradation of carbohydrates. Stool is stored in the rectum until distention triggers a defecation reflex that, when assisted by voluntary relaxation of the external sphincter, permits evacuation.

Enteral feeding is a key environmental trigger to gastrointestinal development. Oral feeding stimulates structural changes and enhances enzyme activity and metabolism. With birth and the onset of milk feeds, the neonatal intestine is exposed to a variety of nutrients, hormones, and growth factors that modulate further development.