Centrioles in the mammary epithelium of the rat

Through serial thin-section analysis of rat mammary epithelial cells, the number of centrioles per cell and their intracellular location were determined. In all developmental stages (e.g. virginal, pregnant, lactation, involution), each epithelial cell contained a single centriole that was located in the apical region. Centrioles were 200-220 (mean = 210) nm in transverse section, and exhibited the typical 9 X 3 'pinwheel' configuration of microtubules. In longitudinal section, centrioles were 330-380 (mean = 360) nm in length. Each centriole was surrounded by a homogeneous pericentriolar matrix. During mitosis in pregnant animals, centrioles were paired at the nuclear poles and oriented at right angles (90 degrees) to each other. At the completion of mitosis a single diplosome (pair of centrioles) was associated with each interphase nucleus. Because all postmitotic cells contained only a single centriole, it was assumed that one of the two diplosomal centrioles had disintegrated. There appeared to be a correlation between centriole location and cell polarity. When centrioles were located near the apical plasma membrane, epithelial cells exhibited polarity. However, when centrioles were associated with the nuclear poles during mitosis, epithelial cells were typically apolar. These observations suggest that centrioles may function as determinants in cell polarity.