An electron microscopic study of two types of muscle fibers in the pharyngeal pad of Crussian carp, Carassius carassius.

In the pharyngeal pad on the roof of the anterior pharynx of Carassius carassius two types of thin striated muscle fibers (1.5-10 micron in diameter) were found. No pattern was discerned in the orientation of muscle fibers which form a loose tissue as a whole. One of them (Z fiber) is characterized by position of triads at the level of Z lines, and in the other type (A-I fiber) they were seen at the junction of the A and I bands. Three types of intermyofibrous junction are noted between muscle fibers of the same type or between those of different types. The first type possesses ultrastructural features such as a uniform intercellular space about 90 nm which contains the basal lamina, a dense mat of the filamentous material on the sarcoplasmic aspect of the cell membrane, and a connection of myofibrils with the dense layer by thin I band filaments. The second type resembles the previous type but is distinguished from it by the lack of myofibrillar association. The third type is the nexus or gap junction. Intermyofibrous junctions of the second type are most frequently encountered (82%). Those of the first type are less frequently seen (15%), whereas the third type junctions are far less frequently seen (3%). Nerve endings at the neuromuscular junction of both types of muscle fiber contain numerous small clear vesicles suggesting their cholinergic nature.