The morphologic and immunologic development of the gastrointestinal system progresses rapidly during the third trimester, and therefore is immature in preterm neonates.
Human breast milk is ideally adapted to match the digestive capacities of the newborn infant and contains several factors for the protection and development of the neonatal gastrointestinal tract.
Respiratory distress syndrome is highly prevalent in preterm infants and may lead to bronchopulmonary dysplasia and chronic lung disease in older children.
Several circulatory changes occur immediately after birth to switch from the parallel fetal circulation to the serial circulation of the newborn infant.
The preterm neonate is at high risk for intraventricular hemorrhage (IVH) and those infants with IVH have a higher risk of mortality.
Embryology of the Gastrointestinal System
The entire respiratory and gastrointestinal system is derived from the endoderm after cephalocaudal and lateral folding of the yolk sac of the embryo. After folding, the primitive gut can be divided into three sections: the foregut, the midgut, and the hindgut. These sections can be distinguished not only by their morphologic pattern, but also by their gene expression patterns that give rise to a variety of organs of the gastrointestinal tract. The foregut extends from the oropharynx to the liver outgrowth and gives rise to the thyroid, esophagus, respiratory epithelium, stomach, liver, biliary tree, pancreas, and the proximal part of the duodenum. The midgut continues past the liver outgrowth to the transverse colon and develops into the small intestine and proximal colon. The hindgut extends from the transverse colon to the cloacal membrane and forms the remainder of the colon and rectum as well as the urogenital tract.
The respiratory epithelium appears as a bud of the esophagus around the fourth week of gestation and a tracheoesophageal septum develops to separate the foregut into ventral tracheal epithelium and dorsal esophageal epithelium. Failure of development of this tracheoesophageal septum leads to the formation of a spectrum of tracheoesophageal clefts and fistulae. The esophagus is initially short, but lengthens to its final extent by 7 weeks. After the esophageal lengthening is complete, the cuboidal epithelium is gradually replaced with squamous epithelium.
The stomach begins as a dilatation of the foregut at 5 weeks of gestation. Over the next several weeks, the stomach undergoes a variety of longitudinal and anterior–posterior axis rotations until it assumes its final position at 22 weeks. The gastric fundus is formed between 12 and 16 weeks of gestation followed by formation of the antrum and pylorus by 20 weeks gestation. Neural crest cell and vagal innervation begins at 7 weeks and continues until birth. The development of gastric glands begins around 10 weeks of gestation and continues until they reach an adult form by 17 weeks gestation. Parietal and chief cells appear between 11 and 12 weeks of gestation.