THE MAMMARY GLAND SPREADING FACTOR IN NORMAL PREGNANT ANIMALS1

Abstract

IN A number of species it has been shown that growth of the mammary gland occurs chiefly during the first half to two-thirds of pregnancy (Turner 1939 a and b, Folley, 1952). The duct system first and later the lobule-alveolar system penetrates into a fatty pad of connective tissue. It was suggested (Elliott and Turner, 1950) that the forward progress of the growing mammary gland would be facilitated if the cells of the endbuds of the ducts, and later the growing side-branches which form the alveoli, produced a spreading factor which acted to break down some component of the connective tissue, possibly the ground substance. A preliminary investigation (Elliott and Turner, 1950), showed that such a spreading factor was present in the mammary glands of pregnant rats and could be extracted and assayed. In the present report, the investigation has been extended to determine if a mammary gland spreading factor such as was found present in the rat glands was present in other species of experimental animals during pregnancy.