ULTRASTRUCTURE OF MUSCLE-CELLS IN ACETABULOSTOMA (CRUSTACEA, OSTRACODA) - MUSSEL SHRIMP FROM THE ROSS SEA (ANTARCTICA)

This paper gives the first ultrastructural description of muscle cells in Acetabulostoma sp., a small (less than 1 mm in length) Antarctic o stracod. All muscle cells showed the same ultrastructural pattern: thi s consisted of mononucleated cells containing a single thick myofibril which was surrounded by numerous mitochondria. The nucleus and most c ytoplasmic organelles were also located in the cell periphery. The myo fibril showed the ultrastructural pattern of transversely striated mus cle. Myofilaments formed well-defined sarcomeres, delimited by Z lines . The average sarcomere lengths were: total sarcomere 4.6 +/- 0.9 mu m , A band 2.86 +/- 0.45 mu m (0.178 +/- 0.06 mu m corresponded to the H band), and I band 1.72 +/- 0.58 mu m The myofilament diameters were 5 .8 +/- 0.3 nm (thin myofilaments), and 14.8 +/- 0.68 nm (thick myofila ments). In the same myofibril and in cross sections, some thick filame nts appeared homogeneously dense whereas others showed a tubular appea rance. Each thick filament was surrounded by 12 thin filaments, and th e 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 transv erse components consisted of 20- to 22-nm-diameter tubules that origin ated from invaginations of the plasma membrane (T tubules). They penet rated radially throughout each A-I interband and branched inside the m yofibril. The longitudinal components consisted of bundles of parallel -arranged sarcoplasmic reticulum saccules running parallel to the myof ilaments. 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 b undles fused together to form triads with the T tubules. In the triad, the distance between the sarcoplasmic reticulum tubules and the T tub ules was 10-14 nm. The mitochondria were large, presented numerous tra nsverse cristae, and were joined one to another by electron-dense plaq ues. 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 l ine, and displayed an irregular outline with numerous interdigitations that formed conical projections. The ultrastructural characteristics of the myofibrillar apparatus, the plentiful sarcotubular system and t he abundance of mitochondria in Acetabulostoma muscles suggest that th is muscle type is concurrently a developed model of both the fast-twit ch type and the fatigue-resistant type.