Ultrastructure of the abdominal sense organ of the scallop Mizuchopecten yessoensis (Jay).

The sensory epithelium of the abdominal sense organ (ASO) of the scallop Mizuchopecten yessoensis is composed of three cell types, sensory cells, mucous cells, and multiciliated cells. Sensory cells bear a single long (up to 250 microm) cilium surrounded by an inner ring of nine modified microvilli and an outer ring of ordinary microvilli paired with modified microvilli. Sensory cells make up about 90% of the total number of cells in the sensory epithelium. Mucous cells, which are much wider than sensory cells, bear only ordinary microvilli on their apical surface. Rare multiciliated cells with short (4-6 microm) cilia are scattered in the periphery of the sensory epithelium sheet. All hairs, cilium, and microvilli of each sensory cell are interconnected by a fibrous network. Nine modified microvilli of a single cell are interconnected by prominent laterally running fibrous links. Membrane-associated electron-dense material of modified microvilli is connected to the ciliary membrane-associated electron-dense material by fine string-like links. These links mechanically bridge the space between the cilium and modified microvilli, as do mechanical links, described for the stereocilia and kinocilium of vertebrate vestibular and cochlear hair cells. The proximal portion of a sensory cilium is about 100 microm long and has a typical 9 x 2+2 axoneme arrangement. The distal portion of a cilium is approximately 2 times thinner than the proximal one and is filled with homogeneous electron-dense material. Along the distal portion, diffuse material associated with the external surface of the membrane is found. The rigidity of distal portion of a cilium is much less than that of the proximal one.