Changes of lectin-binding sites on the embryonic muscle cell surface in the developing ascidian, Halocynthia aurantium

Lectin-binding sites on the muscle cell surface of an ascidian embryo were studied using the ferritin labeling technique. The embryos at 4-cell, gastrula, late tail-bud, and larval stages were dissociated in the Ca²⁺- and Mg²⁺-free solution with or without collagenase. Dissociated cells and fragments were prefixed, reacted with ferritin-lectin conjugates and processed for electron microscopy. Lectins used were concanavalin A (ConA) and Ricinus communis agglutinin. Ferritin particles showing lectin-binding sites were found singly or in the form of clusters on the cell surface exposed directly to the conjugates. Most of the particles of both conjugates were distributed singly and sparsely on entire surface areas of the 4-cell stage cells, whereas they were rich in population and tended to form clusters when embryos reached the gastrula stage. At the succeeding stages tested, tagged ferritin, which was single or clustering particles, was less in number as compared with those at the former stage; on the surface facing neighboring muscle cells, in particular, the ferritin particles were much fewer than those in areas of notochordal and epithelial sides. It is suggested that the embryonic muscle cells of the ascidian show stage-specific changes of cell surface carbohydrates. They have high reactivity to both lectins around the gastrula stage and bring out the regional difference of both lectin-binding sites in the tail-bud stage, namely during the period of histogenesis.