 the fully differentiated cells of the rat mammary parenchyma , the ductal epithelial , alveolar , and myoepithelial cells , are distinguished by their ultrastructure and by their accumulation of immunocytochemically detectable marker proteins . 
 the different cell types probably develop from primative ductal structures called terminal end buds , which are present in the developing rat mammary glands , and these structures contain relatively undifferentiated cells . 
 clonal epithelial stem cell lines , obtained from normal rat mammary glands or benign mammary tumors , differentiate under appropriate conditions along a pathway to droplet - cell / doming cultures of primative alveolarlike cells . under different culture conditions , the epithelial stem cells differentiate along a separate pathway to myoepitheliallike cells . 
 they accumulate some of the specific marker proteins of myoepithelial cells in vivo , including type iv collagen , laminin , and thy-1 antigen . 
 in addition , these myoepitheliallike cells in culture contain an abundance of a potential calcium - binding protein , p9ka , which also occurs in myoepithelial cells of histological sections from mammary glands . 
 the accumulation of type iv collagen , laminin , thy-1 , and p9ka occurs asynchronously along the pathway to the myoepitheliallike cells in vitro . 
 furthermore , the steady - state levels of these different marker proteins arise by alterations in the controls at the transcriptional , the posttranscriptional processing , and the translational stages of their production . 
 these results suggest a stepwise control of synthesis of myoepithelial cell marker proteins , and in the case of p9ka and thy-1 antigen , this altered control may arise through their possession of novel transcriptional promoters.imagesfigure 1 . 
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