Cell types differing in thick myofilament diameter in obliquely striated muscle of a polychaete annelid, Neanthes

The fine structure of the obliquely striated muscle cells of the longitudinal muscle of an annelid was investigated. The characteristics of myofilaments and cell components, such as sarcoplasmic reticular system (S.R.), T-systems and J-rods corresponded to those previously reported, but it was noted for the first time that the cells could be classified into two types with respect to the diameters of their thick myofilaments. In one type, the thick myofilaments were about 29 nm in diameter (A-type) and in the other they were about 41 nm in diameter (B-type). Most of the obliquely striated muscles described to date have been composed of a single type of cell, but we found two types of cell mixed together in the longitudinal muscle. The A-type cells with slender thick myofilaments were distributed mainly in the inner part of the muscle and the B-type cells with broader thick myofilaments were distributed in the outer part.