Development of the gastrointestinal tract.

My aim in this review is to use the rat as a model system for a discussion of the functional development of the gastrointestinal tract. I should make it clear at the outset that since the rat is an altricial species, many of the enzymic changes that occur postnatally in the rat occur prenatally in other mammals, including humans (Grand et al. 1976; Lebenthal et al. 1983). Nevertheless, I believe that these studies in the rat. have posed some questions of broader importance and have pointed to some potential answers. Thus, the review is structured 90 as to emphasize the general lessons which have potential application in many species and in many organ systems. Because of space limitations, the review does not attempt to be comprehensive. To set the stage, it is important to realize that the process of spontaneous weaning in the rat is a precisely timed phenomenon which normally begins at 17 d of age and is complete by day 26 (Henning et al. 1979). Weaning constitutes a significant change in dietary composition which, in turn, necessitates changes in digestive function. The entire gastrointestinal tract of the rat is functionally immature at birth and during the first two postnatal weeks. There are extensive changes in digestive capacity during the third postnatal week, and the adult modes of digestion are fully operative by the end of the fourth week. These developmental changes in gastrointestinal function have been reviewed elsewhere in detail (Henning, 1981 ; Klein & McKenzie, 1983) and only those in the small intestine will be summarized here. In the small intestine of the rat, we find that the neonate has hydrolytic activities which are specific for, and largely restricted to, the components of maternal milk. This is demonstrated very nicely by a consideration of carbohydrate digestion. Milk is relatively low in total carbohydrates, and the carbohydrates present are those not geoerally found in adult diets. The major carbohydrate in the milk of most placental mammals is lactose (Jenness et al. 1964), and high activities of its disaccharidase, lactase (EC 3.2.1.23), are found in the intestinal mucosa of the sucking animals (Deren, 1968; Kretchmer, 1971). In the rat, lactase is detectable on day 18 of gestation, has maximal activity during the first week after birth, and then begins to decline, reaching adult values by the end of the fourth week (Doell & Kretchmer, 1962). Many other species, including the human, have lower lactase activity in the adult than in the newborn (Deren, 1968) and accordingly show an inability to utilize ingested lactose in the postweaning period (Kretchmer, 1971). Intestinal hydrolases that are involved in digestion of carbohydrate components of solid food are absent or low at birth, then appear or increase in activity, or both. Maltase (EC 3.2.1.20) has low activity during the first two postnatal weeks then