Comparative anatomy, pre- and postnatal changes during the development and maturation of the small intestine:...

The gastrointestinal (GI) system absorbs nutrients and xenobiotics, excretes waste, and performs immunologic and endocrine functions. The subdivisions of the mature gut and the complexity of their corrugated, absorptive luminal surfaces differ greatly among mammals. Regardless, the embryonic gut tube in all mammalian species arises when cephalocaudal folding incorporates the roof of the yolk sac into the embryo. The gut tube quickly lengthens and bulges into the umbilical cord. Upon reentry into the abdominal cavity, the gut tube begins to differentiate-a process that continues until well into the lactation period. Differentiation of the small intestine involves (1) increasing the absorptive surface area of the lumen; (2) establishing mechanisms to control the pH of luminal contents; (3) forming a hierarchical vascular system for distribution of absorbed nutrients; (4) developing a complex enteric nervous system to control motility; (5) providing a system for replenishment of cells; and (6) contributing to the immunity of the organism. Because the length of gestation varies among species typically used in safety tests and is much shorter than human gestation, the state of GI maturation at the time of parturition differs significantly. Differences in GI maturation can contribute to species differences in the rate and extent of absorption; these differences must be considered when designing and interpreting pharmacological/toxicological studies and extrapolating safety test results to humans.