Electron Microscopy of Milk Secretion in the Mammary Gland of the C3H/Crgl Mouse. I. Cytomorphology of the Prelactating and the Lactating Gland2

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Abstract
The cytology of milk secretion in the mammary gland of the mouse was examined by light and electron microscopy. Protein, fat, and glycoprotein, 3 of the major milk components, were identified by histochemical methods and shown to increase in quantity in the alveolar lumens through the successive stages of pregnancy and lactation. Protein and fat occurred as 2 different kinds of droplets which were readily identified in electron micrographs. The relative quantity of each kind of droplet observed at any given stage of pregnancy or lactation was positively correlated with the quantity observed by histochemical methods. The protein droplets appeared to be formed in the vacuolar spaces of the Golgi apparatus. The Golgi apparatus and the endoplasmic reticulum were markedly hypertrophied in lactating cells. The mode of origin of the fat droplets in cytologic terms remains unknown. The evidence indicates that protein and fat droplets are released into the alveolar lumen without the fragmentation of large masses of apical cytoplasm.