Sequential Coccolith Morphogenesis in Hymenomonas carterae

SYNOPSIS. A careful re‐examination with refined technics of the ultrastructure of the formation of calcified scales (coccoliths) in the marine unicellular alga Hymenomonas carterae has yielded new and more detailed information about the structure and morphogenesis of these unique and complex Golgi‐elaborated organelles. The coccolith rim is formed from 2 distinct, alternating, anvil‐shaped elements, 13–16 each, fitted together with a “right‐handed” asymmetry. The coccolith is assembled in Golgi cisternae from 2 precursors, a single, scale‐like base and multiple granular elements called coccolithosomes. The association of scales and coccolithosomes and subsequent development to the mature coccolith occur in a characteristic sequential fashion within what is one of the better examples of a polarized Golgi. Morphogenesis involves a special cisternal membrane association with the base of the coccolith, the contribution of granular material by coccolithosomes to form the outer rim matrix, and the subsequent filling of the area enclosed by the matrix with an electron‐dense material, presumably CaCO3. A “microenvironment” model system for species specific shape‐determination of calcified elements is proposed.