PIRES, A; Dickinson College: Signals Mediating Rapid Loss of Cell Adhesion in Metamorphosis
Rapid morphological changes that occur during the abrupt metamorphoses of many taxa involve shedding of larval organs and migrations of populations of retained cells. Both processes require rapid modulation of cell adhesion. Veliger larvae of gastropods shed the larval velum during metamorphosis over a period of several hours (Phestilla sibogae) or 2-3 minutes (Crepidula fornicata). Velar abscission in C. fornicata entails sudden loss of adhesion within and among several types of epithelial cells, beginning with detachment of the preoral ciliated cells that line the margin of the velum. The shed cells maintain intracellular esterase activity as revealed by calcein dye and their plasma membranes exclude ethidium homodimer, indicating that they remain intact and metabolically active. Loss of velar cell adhesion can be induced in intact larvae and isolated velar lobes by 10-4 M H2O2, or by 10-5 M diethyldithiocarbamate, an inhibitor of superoxide dismutase. A burst of oxidative activity has been visualized in detaching preoral ciliated cells during natural metamorphosis of intact larvae, further suggesting a role for endogenous oxidative signaling. Calcium ionophore A23187 (10-6 M) also induces loss of velar cell adhesion in intact competent larvae as well as in isolated velar lobes, accompanied by an oxidative burst as in natural metamorphosis. Calcium signaling thus may mediate loss of cell adhesion in the velum. The velum is innervated by neurons that are immunopositive for tyrosine hydroxylase (a catecholamine-synthetic enzyme), 5-hydroxytryptamine, and FMRFamide. Terminals for all of these are concentrated in a marginal zone where the preoral ciliated cells detach at the beginning of velar abscission. Continuing investigations seek to connect efferent neural correlates of metamorphosis to mechanisms for modulation of cell adhesion.
