Postnatal growth of gut and muscle: competitors or collaborators.

Although growth and functional maturation are essentially continuous from conception to the attainment of sexual maturity, birth and weaning are particularly critical milestones in life. Birth is associated with a change in the route of nutrient delivery to the organism and weaning requires the attainment of full locomotor function as the organism is called upon to actively seek its own food. Weaning is associated also with major changes in the composition of the diet, changes which in their turn demand intestinal adaptation. Given this, the tissues responsible for the assimilation of nutrients and the tissues responsible for mobility must grow and develop rapidly during the immediate postnatal period. Table 1 summarizes information on the growth rates of the body, intestinal tract, liver, and the leg muscles of the infant rat. Although the rat, as an altricial species, represents one extreme of the interspecific spectrum of chemical and functional maturation at birth, the information illustrates two general points. First, that as a reflection of the chemical maturation of the body, the rate of accretion of body protein exceeds that of the body mass as a whole. Second, that gut and muscle grow during the sucking period with rate constants that are significantly greater than that of body protein as a whole. We have included values for the liver to show that organs that have been of major physiological significance to the fetus tend to grow during the suckling period at a slower rate than that of the body protein mass as a whole.