Ultrastructural Aspects of Spicule Formation in the Solitary Ascidian Herdmania momus (Urochordata, Ascidiacea)

The solitary stolidobranch ascidian Herdmania momus contains numerous calcium carbonate spicules in its tunic and body tissues. The slender body spicules form inside complex sheaths in the body wall and branchial basket, where they remain for the life of the animal. The much smaller tunic spicules form inside the tunic blood vessels and then migrate to the tunic surface, where they become anchored by their spiny base. This paper is an ultrastructural investigation of the formation of the body spicules; the tunic spicules, which apparently form quite differently, will be the focus of a future study. The body spicules are composed of rows of closely packed acicular spines which form completely extracellularly. The spine tips are covered by flattened, highly pseudopodial sclerocytes bound together by tightly interdigitating cell processes. The basal regions of contiguous spines are covered by very thin sclerocyte cell processes. An organic matrix is present within the spines; its exact nature is not clear. A very dense extracellular inter‐spine matrix is located between the spine tips and the contiguous basal regions. Presclerocytes within the sheaths between the spicules are probably responsible for formation of the extracellular structures of the sheaths. The presclerocytes appear to aggregate and transform into sclerocytes at the apical end of the spicule. New spines are added at the apical end of the spicule as well as between larger spines. Comparisons are made between body spicule formation in H. momus and skeletogenesis in echinoids.