The epitheloid modified walls of helicine arteries are built of tightly arranged specialized smooth muscle cells (epitheloid cells). They are polygonal in shape, but do not branch out. The number of myofilaments is markedly reduced, whereas cell organelles are well developed (mitochondria, Golgi regions, rough endoplasmic reticulum). Myofilaments are gathered to bundles with no orientation as to their direction. Regular dense patches and attachment zones do occur. The cell surface is provided with caveolae (surface vesicles) and a basal lamina. Where epitheloid cells are joined together, they share a single basal lamina in common. In circumscribed regions basal lamina material is completely absent, and the cell membranes approach to form a 150Å gap (paired cells).
Endothelial cells are rich in cytoplasmic filaments and show only few transport vesicles. In particular areas a basal lamina is absent, and epitheloid cells and endothelial cells are joined together leaving a 200Å wide cleft.
Fluorescence histochemistry shows that helicine arteries are provided with an extremely dense network of adrenergic nerves located at the medio-adventitial border. Epitheloid cells, like ordinary vascular smooth muscle cells, show a postive ATPase reaction, but lack any histochemically demonstrable 5'-nucleotidase activity. Endothelial cells in helicine arteries react on unspecific alkaline phosphatase, while the endothelium of deep arteries and of the cavernous spaces does not.