Striated anchoring fibrils‐anchoring plaque complexes and their relation to hemidesmosomes of myoepithelial and secretory cells in mammary glands of lactating rats

Striated anchoring fibrils (SAF) are associated with the basement membrane underlying myoepithelial and acinar cells of mammary glands. Their proximal extremities are inserted in electron‐dense areas of the lamina densa, the anchoring plaques seen facing the hemidesmosomes of both myoepithelial and acinar cells. In the case of myoepithelial cells, the hemidesmosomes show a thick cytoplasmic plaque applied to the basal plasma membrane in which cytoplasmic filaments are inserted. Facing this plaque but on the extracellular aspect and at a short distance of 5–10 nm, there is a thin layer of electron‐dense nodular material called the subcell membrane plate, which is connected to the plasma membrane by short filamentous bridges. Between this subcell membrane plate and the anchoring plaque, there is an abundance of fine anchoring filaments crossing the lamina lucida. Such anchoring filaments are less abundant in the lamina lucida outside the hemidesmosomal areas. In the case of acinar cells, the cytoplasmic plaques of the hemidesmosomes are thin and the associated cytoplasmic filaments less conspicuous. No distinct subcell membrane plate is seen on the extracellular aspect of the plasma membrane facing the cytoplasmic plaque of the hemidesmosomes. However, in this area numerous anchoring filaments cross the lamina lucida between the plasma membrane and the SAF‐anchoring plaque complex. The abundance, in these cells, of hemidesomomes and their association with SAF‐anchoring plaque complexes seen in the basement membrane must constitute a strong attachment for both myoepithelial and acinar cells and bind them to the underlying collagen fibrils, thus preventing their detachment from the connective tissue during the contractions of myoepithelial cells during milk ejection. © 1993 Wiley‐Liss, Inc.