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Development of the digestive system: comparative animal studies.

Although all the organs and tissues of the body grow and develop in an orderly fashion in relation to the body as a whole, many of them are capable of particularly rapid growth in response to the stimulus of additional demand. This may occur even after the body has ceased to grow, and examples are the heart, the lungs, the liver, the kidneys, the gastrointestinal tract and skeletal muscles. Hypertrophy of these organs in a mature animal or man is not brought about by an increase in the number of functional units, but by an increase in the size of those already there. The number of nephrons in the kidney, of alveoli in the lungs, and of fibers in muscle, for example, is fixed early in life and cannot be increased, though each pre-existing unit can be expanded considerably in size. It has been known for some time that the gastrointestinal tract may enlarge in response to an increased food intake, for example during lactation in the rat (1). The number of villi in the intestinal mucosa does not increase, but those that are there become longer, and cell proliferation speeds up (2). Some years ago I began to think about the response of the gastrointestinal tract to its first real test, when it has to deal with the colostrum that reaches it, sometimes within minutes, after the animal is born. The digestive tract has been growing and developing before birth, and in fact a great deal of detailed morphological work has been done on the development of the digestive tract of the human fetus. Some of this work was described at a Ross Conference held in 1976 on “Gastrointestinal Development and Neonatal Nutrition,” and at a Ciba Foundation Conference in 1979 on “Development of Mammalian Absorptive Processes” (3, 4, 5). The intestine has functioned in a small way before birth in that it has absorbed water and dissolved substances from the swallowed amniotic fluid. But colostrum and milk are very different from amniotic fluid, containing as they do nutrients never before encountered-milk proteins, fat and lactose, all of which require considerably more processing than water. The whole well-being of the newborn animal and baby depends on the gastrointestinal tract being functionally capable of digesting these nutrients and absorbing the products of digestion. Anyone who has tried to handle the intestine of a newborn rat must be astonished that so fragile an organ can deal with sufficient material to enable the animal to double its weight in less than a week.