MILK PROTEIN DROPLET FORMATION IN THE GOLGI APPARATUS OF THE C3H/Crgl MOUSE MAMMARY EPITHELIAL CELLS

Two kinds of droplets are revealed by the electron microscope in the milk, secretory alveoli, and epithelial cells of the lactating mammary gland of rats and mice (1-3). The larger droplets are fat and the smaller ones, measuring 30 to 400 m# in diameter, are primarily protein (4). The protein droplets first appear enclosed in vacuoles in the Golgi region; these vacuoles presumably pass through the apical cytoplasm, and open at the cell surface to release the protein droplets into the milk (3). In the course of previous studies (3, 4) it was observed that the flattened sacs and vacuoles of the Golgi apparatus in the lactating mammary gland cells of C3H Crgl mice usually contained fine particles, heretofore undescribed. Although these particles were much smaller than the milk protein droplets, it seemed possible that they wei e related to formation of the protein droplets. This report presents a description of these minute particles, and offers morphological evidence for their transition into the previously described milk protein droplets. MATERIALS AND METHODS Lactating mammary tissue was obtained from 3-to 4-month-old female C3H/Crgl mice. After 1 to 15 days of lactation these mice were anaesthetized with sodium pentobarbital administered intraperitoneally (5), and samples of the lactating gland were removed. Tissue was cut into blocks measuring less than 1 ram. and was fixed for 2 hours in 1 per cent osmium tetroxide, buffered at pH 7.4 with acetate veronal, and with added sucrose, 0.045 gm./ml. (6). The blocks were washed briefly in distilled water, dehydrated in a graded series of ethyl alcohols, and infiltrated with n-butyl methacrylate. Polymerization was accomplished at a temperature of 45°C. with the catalyst 2,4-dichlorobenzoyl peroxide. Thin sections were cut using a Porter-Blum microtome set at 50 mg and equipped with a plate glass knife. The thin sections were floated on a saturated solution of uranyl acetate according to the method reported by Watson (7). Treatment with uranyl acetate improved the contrast in electron micrographs, but did not otherwise alter morphology. RESULTS The cytomorphology and the structure of the Golgi apparatus of mammary cells of the lactating C3H/Crgl mice used in this study did not differ from that reported earlier (1-3). The Golgi apparatus was usually composed of several flattened membranous sacs oriented parallel to one another, and associated with numerous vesicles (Figs. 1 and 2). The Golgi membranes lacked the small ribonucleoprotein particles seen on the outer surfaces …