The Importance of Macrophages in the Removal of Fat from the Involuting Mammary Gland

Abstract

Summary The cells in the milk collected from cows and ewes at the cud of lactation and during involution were found to he comprised predominantly of polymorphs and mononuclear cells of 3 types—lymphocytes, macrophages and foamy cells. Most of the cells in the milk collected at last milking were polymorphs, but the proportion of mononuclear cells, particularly the foamy cells, increased as involution advanced. The presence of fat in the foamy cells was shown by staining with Oil Red O. Experiments were then carried out to elucidate the function and fate of the foamy cells in involuting glands. Colloidal carbon was infused into the glands of cows and ewes at last milking. Smears of milk samples collected at intervals from infused glands, showed clumps of carbon in the cytoplasm of foamy cells and also in macrophages, but rarely in polymorphs. Histological sections from glands infused with carbon showed carbonladen foamy cells in the alveolar and ductal lumens, in the interalveolar areas, in the lymphatics of the gland substance and in the regional lymph nodes. Large accumulations of foamy cells were found in the cortex of regional lymph nodes of ewes at 4 and 8 days, but not at 2 days, after last milking. These observations suggest that the foamy cells are macrophages which are responsible for the removal of fat from the involuting gland.