Ultrastructure of muscle cells in Acetabulostoma (Crustacea, Ostracoda) – mussel shrimp from the Ross Sea (Antarctica)

Abstract

This paper gives the first ultrastructural description of muscle cells in Acetabulostoma sp., a small (less than 1 mm in length) Antarctic ostracod. All muscle cells showed the same ultrastructural pattern: this consisted of mononucleated cells containing a single thick myofibril which was surrounded by numerous mitochondria. The nucleus and most cytoplasmic organelles were also located in the cell periphery. The myofibril showed the ultrastructural pattern of transversely striated muscle. Myofilaments formed well-defined sarcomeres, delimited by Z lines. The average sarcomere lengths were: total sarcomere 4.6 ± 0.9 μm, A band 2.86 ± 0.45 μm (0.178 ± 0.06 μm corresponded to the H band), and I band 1.72 ± 0.58 μm. The myofilament diameters were 5.8 ± 0.3 nm (thin myofilaments), and 14.8 ± 0.68 nm (thick myofilaments). In the same myofibril and in cross sections, some thick filaments appeared homogeneously dense whereas others showed a tubular appearance. Each thick filament was surrounded by 12 thin filaments, and the thin/thick filament ratio was 6/1. The sarcotubular system was well developed and consisted of both transverse and longitudinal components, both running within the myofibril among the myofilaments. The transverse components consisted of 20- to 22-nm-diameter tubules that originated from invaginations of the plasma membrane (T tubules). They penetrated radially throughout each A-I interband and branched inside the myofibril. The longitudinal components consisted of bundles of parallel-arranged sarcoplasmic reticulum saccules running parallel to the myofilaments. Each bundle comprised from three to six 30 to 35-nm-diameter saccules. At the levels of the A-I interbands, the saccules of some bundles fused together to form triads with the T tubules. In the triad, the distance between the sarcoplasmic reticulum tubules and the T tubules was 10–14 nm. The mitochondria were large, presented numerous transverse cristae, and were joined one to another by electron-dense plaques. The muscle cell tips ending beneath the cuticle were connected to a modified epidermal cell by a specialized intercellular junction of the fascia adherens type. This junction occurred at the level of a Z line, and displayed an irregular outline with numerous interdigitations that formed conical projections. The ultrastructural characteristics of the myofibrillar apparatus, the plentiful sarcotubular system and the abundance of mitochondria in Acetabulostoma muscles suggest that this muscle type is concurrently a developed model of both the fast-twitch type and the fatigue-resistant type.