Membrane modifications in the apical endocytic complex of ileal epithelial cells.

Ileal lining cells of the suckling rat possess an "apical endocytic complex" capable of sequestering intact protein from the intestinal lumen. The complex consists of a network of invaginations of the apical plasma membrane, a number of subjacent small vesicles, and a giant supranuclear vacuole. The first two components initially incorporate material from the intestinal lumen and then transfer it to the giant vacuole where it is stored. Their limiting membrane displays striking structural modifications when viewed in various planes of section. Its lumenal dense leaflet appears discontinuous and consists of an ordered array of minute discrete plaques. A dense particle approximately 70 A in diameter is centered over each plaque. The particles are arranged in a two-dimensional square lattice with center-to-center spacing of approximately 120 A.