The obturaculum of Riftia pachyptila (Annelida, Vestimentifera): Ultrastructure and function of the obturacular muscles and extracellular matrix

The Vestimentifera have in their branchial plume a centralized structure, t he obturaculum, supporting the respiratory lamellae and enabling the worm t o close its tube. We have studied its structure in Riftia pachyptila, focus ing on the relations between its main parts: epidermis, muscles and extrace llular matrix (ECM). The epidermis is supported by a thick collagenous laye r, through which cell processes extend into the ECM. These processes contai n rough endoplasmic reticulum (RER), and they often surround the subepiderm al obturacular muscles. Each muscle is a bundle of 10-35 smooth muscle fibr es, embedded in the ECM The apical part, made of thin (and probably juvenil e) muscle fibres, is present in each muscle. The thick myofilaments range f rom 20 to 150 nm in diameter, and show a cross-banded periodicity of 12-14 nm similar to paramyosin. Each thick myofilament is surrounded by 12 to 18 thin myofilaments about 5-7 nm in diameter. The nucleus is located laterall y in the fibres, and generally surrounded by swollen cisterns of RER. The l arge obturacular matrix contains collagen fibrils, with a diameter of 30 nm and a striation every 64 nm. It also contains proteoglycans and cells, in groups of two or more, surrounded by a thin basal lamina, often lined with bundles of collagen fibrils. These cells often contain swollen cisterns of RER, suggesting they are equivalent to fibroblasts synthesizing the surroun ding collagen fibrils. We discuss whether the obturaculum presents the feat ures of a primitive cartilage, or those of a "catch" type connective tissue .