An electron microscopic study of the formation and structure of the periostracum in Astarte (Bivalvia)

An electron microscopic study of the outer and middle folds of the mantle edge of Astarte castanea reveals the fine-structural details of the cells which are involved in the formation of the periostracum. There are four types of cells in the outer fold whereas there is only one type in the middle fold. The role of different cell types in the formation of the periostracum has been discussed. The periostracum originates in the intercellular space between the basal cell of the outer fold and the first cell of the middle fold. Even at the point of origin it consists of two outer membranous layers and the inner dark homogeneous layer. The fibrous translucent layer, which is the third layer of the fully mature periostracum, does not appear until the young periostracum has moved down the periostracal groove at the level of the third cell. As the periostracum matures, the two membranous layers remain remarkably constant in width, whereas the two inner layers, dark homogeneous and fibrous translucent, increase in thickness. Scanning electron microscopy and thin sectioning reveal that the fully mature periostraca of A. castanea and A. elliptica appear uniformly pitted, but the nature of the pits between the two species is different. The fibrils of the fibrous layer show either uniform or random arrangements, depending on the region. It is the innermost part of the fibrous layer of the periostracum that provides the nuclei for calcification.