Relationships of Size, Intracellular Location, and Time Required for Secretion of Milk Fat Droplets

Fat droplet development in rat mammary tissue was evaluated morphometrically in electron micrographs of 200 typical lactating cells.Droplets in the extreme basal area of the ceils averaged .6 /~m in diameter, increased to 1.3 /lm in the extreme apical region, and increased further to a totally protruding (but still connected to the cell) size of 3.5/am.The basal to apical growth was linear with a slope of .09.The expansion in droplet diameter during secretion also was linear (slope 1.06) and involved a 20-fold increase in volume.No droplets were observed coalescing.Masssive lipid accumulation in the apical region of the cell is implied both by the growth and numbers of fat droplets in this region.Observation with the light microscope on lactating ceils of three other species (rabbit, cow, and cat) also showed substantial volume increases as fat droplets were being secreted.In view of data on the time required to synthesize a milk fat droplet, droplet secretion may be a relatively slow process requiring up to several hours from the time the droplet first contacts the plasmalemma until it is completely clear of the cell.