Cytoskeleton-related structures in tetrahymena thermophila: microfilaments at the apical and division-furrow rings.

A ring consisting of microfilaments was found in the apical region of Tetrahymena thermophila wild-type strain B and janus mutant. This ring, about 0.4 micrometer wide and 0.2 micrometer thick, is located at the bases of the anterior, non-ciliated basal bodies of the apical ciliary couplets. The apical ring is made of fine filaments showing a banded pattern, the distance between bands depending on the fixation procedure and ranging from 30-200 nm. The bands are made of small beads fastened to the filaments. The microfilaments of the apical ring are attached to the bases of the basel bodies. No connection with the cell membrane was found. In dividing cells in the incipient furrow region of filamentous band originates from the epiplasmic fibrogranular meshwork. This contractile ring is about 0.4 micrometer wide and 0.8 micrometer thick. It is formed by circumferentially aligned microfibrils. During constriction the contractile ring remains associated with the epiplasmic layer, which in turn adheres to the inner alveolar membrane. The microfilaments of both the apical and the division-furrow rings have diameters ranging from about 3.8-7.I nm.