Cell size and shape changes in the myoepithelium of the mammary gland during differentiation.

We have studied changes in myoepithelial cell size and shape during different stages of mouse mammary gland differentiation by using the fluorescent probe for actin NBD-phallacidin. Pieces of mammary tissue were fixed, mounted on slides, permeabilized with cold acetone (-20 degrees C), and then treated with nitrobenzoxadiazole-phallacidin. Myoepithelial cells lining ducts of glands at all stages of development are spindle-shaped structures oriented parallel to the long axis of the duct at the base of the luminal epithelium. In virgin animals, myoepithelial cells also occur as linear tracts oriented parallel to the long axis of small projections along the sides of ducts and terminal end buds. In early pregnancy, small stellate-shaped cells begin to appear around presumptive secretory units. By late pregnancy, larger star-shaped units of intense fluorescence appear at the base of alveoli. During lactation, both cell bodies and cell processes further enlarge as these interlacing stellate-shaped cells encompass the expanded alveoli. In regressing glands, cell size decreases and the processes appear to retract. Although alveoli are virtually absent in the multipartate resting gland, myoepithelial cells remain around lateral buds of ducts. These myoepithelial cells have two distinct shapes: small star-shaped cells capping the buds and spindle-shaped cells oriented parallel to the long axis of the buds. A comparison of myoepithelial cell shape in virgin mice and nulliparous women indicates a more developed cell in the human gland at this stage of development. Intact segments of mammary gland combined with NBD-phallacidin as a probe for actin provide an ideal system for future studies of the control of myoepithelial cell size and shape and their influence on cell functions.