Elektronenmikroskopische Beobachtungen an den Sehzellen des Blutegels,Hirudo medicinalis L.

Summary The visual cell of the leech (Hirudo medicinalis) contains a big vacuole filled with a moderate dense substance (vitreous body). The wall of the vacuole consists of a system of microvilli (brush border) merging into the substance of the vitreous body. The cytoplasmic zone underneath the brush border has a special structure consisting of flattened sacs and vesicles giving a radial striation to this cell region.The cytoplasm is filled with a great number of mitochondria and small vesicles. Bundles of fine filaments are striking features of the cytoplasm; in the periphery of the cell they are in close contact with the cell membrane (half desmosomes). Elements of the endoplasmic reticulum and free ribosomes may also be present. The cells regularly contain membrane-bounded inclusions having a dense granular or cristalline content.The visual fibers (processes of the visual cells) show a certain similarity to the unmyelinated nerve fibers of higher animals. They are ensheathed into sheath cells possibly of glial nature. Processes of these cells often surround the visual cells and are sometimes embedded in their cytoplasm. Both visual and sheath cells are covered with a homogeneous basement membrane.The possible role of the structures and the problems of conduction from the brush border onto the cell surface are discussed.