[Ultrastructural study of the ureter smooth muscle of the cat]

Morphological specializations of smooth muscles from the middle part of the cat ureter have been examined by electron microscopy using thin sections and freeze-fracture replicas. It is notable that there were many gap junctions which were characteristic in being formed on the surface of blunt cytoplasmic projections protruded from each neighboring cells. In addition to gap junctions, there were two other different junctions, i.e, intermediate junctions and "focal attachments". The latter represents the cell attachment in which two apposing plasma membranes are attached to each other without the intervension of basal lamina in the narrow intercellular space. Caveolae were lined in rows which were intervened by dense band zones parallel to the long axis of the muscle fibre as seen by freeze-fracture replica. Gap junctions were noted to be localized within or along the caveolar zones, being elliptical in shape with the long axis parallel to muscle fibres. As observed in thin sections, gap junctions in freeze-fracture replicas were noted on the top of cytoplasmic protrusions. No membrane specializations were observed in freeze-fracture replicas which corresponded to the "focal attachment" or intermediate junction as seen in thin sections. As to the innervation, the autonomic nerve terminals were in abundance, closely apposed to the smooth muscles. In addition to autonomic nerve terminals, sensory nerve terminals were observed between muscle fibres, consisting of axon terminals and lamellar cells, the same structure as mechanoreceptive lamellar corpuscles known in other organs.