The ultrastructure of normal human mammary cells cultured from post‐weaning breast fluids is described. Cells from confluent monolayers in two week old cultures were studied. The epithelial nature of these cells was established by the demonstration of a well developed system of cell‐to‐cell interdigitation and numerous desmosomes. These cells also share with breast epithelial cells in vivo, polarity, with blunt short microvilli on the apical surface and an oriented arrangement of organelles in the basal and apical portions of the cells. The Golgi apparatus, which is the most highly developed organelle, is localized in the apical pole and contains substantial quantities of secretory material in the cisternae and vesicles. A variegated palisade of finely granular material mixed with tonofilaments is seen in the basal portion of the cells; many of these tonofilaments end in the terminal web of the desmosomes. The regular occurrence of these cells in breast fluids during the terminal phases of lactation suggests that their separation is a part of normal breast involution.
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