Role of milk-borne factors in weaning and intestinal development.

To determine whether a factor appearing in the milk at the beginning of the 3rd postnatal week elicits the appearance of jejunal sucrase activity in the pups, pups were raised with dams that had been lactating for either a week less or a week more than their natural mothers. Pups were sacrificed at 15, 17, 19, 21 and 23 days of age for assay of jejunal sucrase and for measurement of the proportion of chow in the stomachs. Sucrase activity appeared at the same age in all pups (day 17); however, at days 21 and 23 the enzyme activity was higher in the pups cross-fostered to the longer-lactating dams. Correspondingly, the analysis of stomach contents showed that these pups had almost completed weaning by day 21 whereas the other pups were still taking 40-50% milk. It is concluded that neither the appearance of jejunal sucrase nor the onset of weaning is cued by the appearance of a factor in the milk at the beginning of the 3rd postnatal week. However, the lactational stage of the dam can affect the rate of completion of weaning and, secondarily, the terminal portion of the sucrase rise.