Primitive muscle cells of nematodes: Morphological aspects of platymyarian and shallow coelomyarian muscles in two plant parasitic nematodes, Trichodorus christiei and Longidorus elongatus

Abstract

Morphological aspects of somatic muscle cells in a platymyarian nematode, Trichodorus christiei Allen, and in a shallow coelomyarian nematode, Longidorus elongatus (de Man) Thorne and Swanger, were studied by electron microscopy. The muscle cells in both nematodes are arrayed longitudinally forming a single muscular layer just beneath the thin hypodermis. The contractile region, located in the outer portion of the cell, is composed of thin and thick myofilaments, approximately 8 nm and 22 nm in diameter, respectively. Mitochondria and the beta form of glycogen granules are present in the sarcoplasms, located in the inner portion of the cell. Very few small sarcoplasmic reticula are situated close to the electron dense Z plates. No clear evidence of the existence of a T-system in these muscles is observed. In the platymyarian muscle cells, uniform Z plates extend perpendicularly from the periphery of the cell to the contractile region, dividing it into two to seven sarcomeres. Each sarcomere consists of I, A, and H bands. The thin myofilaments presumably pass through the Z plates. The thin and thick myofilaments run in two oblique directions forming striated patterns. These findings indicate that this type of somatic muscle is able to perform simple, sluggish movements. In the shallow coelomyarian muscle cells, the Z plates irregularly divide the contractile region. The sarcomeres, as well as the I, A, and H bands, are more complicated than those of the platymyarian muscle. Aggregations of thin myofilaments appear around the Z plates. The myofilaments are oriented in many directions within a cell, presumably forming oblique striations. The findings indicate that this type of nematode muscle can move in a more complicated and more lively way than those of the platymyarian nematodes, but less so than those of the higher coelomyarian nematodes. The ultrastructure of these nematode muscles suggests that the shallow coelomyarian muscle is a more advanced type than the platymyarian muscle, but more primitive than the coelomyarian muscle. The nematode muscle ultrastructure may be useful not only in the study of phylogenetic relationships of nematodes, but also in the study of evolution of muscle cells.