Models for the study of cell proliferation in the developing gastrointestinal tract.

Which is the best system to study in order to obtain data for the human developmental situation? While the temporal pattern of differentiation and growth in some of these species is similar to human development, there are major differences in terms of absorption of immunoglobulins. Humans and rabbits receive virtually complete passive immunity in utero, while rats receive partial immunity, and other species such as the calf, sheep, and pig receive virtually their complete passive immunity during the postnatal period. The morphological similarity of the neonatal rat and mouse small intestine to human fetal small intestine between 10 and 22 weeks adds some validity to the limited use of this model. However, the pattern of gastrointestinal differentiation late in gestation; the dependence on the postnatal period for the transfer of passive immunity; and the delayed closure of the gut are limiting factors in the use of this model comparison to the human situation. Although the opossum provides a useful model for the study of gastrointestinal development in a species with an extremely dependent, underdeveloped newborn and an extremely delayed closure of the gut, this situation is not analagous to the human fetal and neonatal pattern of gastrointestinal development. While animals such as the calf, pig, and sheep are morphologically similar to the human condition and exhibit a relatively short postnatal period of functional immunoglobulin uptake they are almost completely dependent upon the early postnatal period for transfer of passive immunity.(ABSTRACT TRUNCATED AT 250 WORDS)