Cambial zones in gills of Bivalvia

Localization and intensity of cell division in the gills of 9 species of Bivalvia were studied. In addition, deoxyribonucleic acid (DNA) synthesis in the gills of two species of mussels (Crenomytilus grayanus and Mytilus edulis) and of the scallop Mizuhopecten yessoensis was investigated by means of radioautography using thymidine-H³. In all bivalves examined, the distribution of branchial cell division was very uneven. Distinctly visible zones exhibited considerably higher mitotic and DNA-synthesizing activities than the remaining branchial epithelium. Topography of these zones appeared similar in all Filibranchia and Eulamellibranchia examined. Areas with enhanced cell division in the gills of Bivalvia are considered as cambial zones or points of growth, apparently serving as a source of growth and/or renewal of the entire organ.