Calcification and Decalcification During Epithallial Cell Turnover in Coralline Red Algae

Epithallial cells of the coralline red algae are characterized by unusual structural specialization, which include deep invaginations of the distal cell surface, and by unique development, which culminates in senescence, shedding, and replacement of the cells. Electron microscopic study of epithelial cell differentiation in morphologically and taxonomically disparate species suggests that the unusual features of epithelial cell structure and development stem from the fact that these dynamics occur within a calcified matrix. Distal wall ingrowths begin to form on the initial cells, cells whose cleavage eventually gives rise distally to new epithelial cells. After the distal wall ingrowths form, the overlying crosswall becomes rich in organic material. For this organic wall material to be deposited into the existing crosswall, the wall must first be decalcified; therefore, the presence of abundant organic material in the crosswall provides a marker of localized decalcification. We propose that the location and time of origin of distal wall ingrowths indicate a connection between the ingrowths and two coordinated processes: localized secretion of wall material, and decalcification of the overlying cell wall in preparation for the movement of the young epithelial cell into a new location relative to the surrounding calcified matrix. The large plasmalemmal surface area associated with the distal wall ingrowths allows for a greater abundance of membrane‐associated components, such as proton pumps, that could drive localized cell wall decalcification.