The colostrum of many species will stimulate the growth of cells in vitro and colostrum ingestion may play an important rôle in directly promoting the growth of the gastrointestinal tract in the newborn animal (Berseth, Lichtenberger and Morriss, 1984). Most studies have suggested however, that growth-promoting activity declines to zero, or to negligible levels in post-colostrum milk (Shing and Klagsbrun, 1984; Cera, Mahan and Simmen, 1987). We have investigated the ability of sow's milk, collected up to 21 days of lactation, to stimulate the growth of epithelial cells in vitro. For these studies we use cells derived from the mucosa of the small intestine: RIE-1 cells (Blay and Brown, 1984) and a lung-derived cell line: CHV79 cells.

Milk was collected from three sows at each of the following times: 0 (colostrum), 7, 14, and 21 days post partum. The samples were defatted by centrifugation (30 000 g for 40 min), filter sterilized and diluted in phosphate buffered saline (PBS, pH 6.8). RIE-1 cells (a generous gift from K. Brown, Brabaham) or CHV79 cells were grown to confluence in Dulbecco's modified Eagle's medium (DME) containing foetal calf serum (FCS, 10%). The cultures were quiesced in a medium containing 0.5% FCS for 24 h and milk samples added in serum-free PBS for a further 24 h. [3H]-thymidine was added for the final 4 h before cells were harvested. Growth-promoting activity of milk samples are expressed as a stimulation index (SI) i.e. in relation to the amount of label incorporated after incubation with an equivalent volume of PBS. Fractionation of the growth-promoting activity of a pooled post-colostrum milk sample was performed on sephacryl S-200sf and S-300sf columns.