THE FORMATION AND STRUCTURE OF THE GLOCHIDIAL CYST

Abstract

Mechanical stimulation of the larval glochidium induces non-selective, automatic closure upon the impinging gill or fin. The valves largely cleave the soft tissues encountered, but merely clasp such tough elements as blood vessels and fin rays which lie deeper. As a result, part of the glochidium is buried in host substance.The glochidium is then covered by host tissue which advances from all sides, primarily for the purpose of closing the wound. Encystment is not the result of cell proliferation. On the contrary, it is accomplished by a mass movement of cells from the adjoining regions, advancing by their own activities and directed over the exposed valves by thigmotaxis. A compensatory period of mitosis may appear subsequent to encystment, apparently to replace cells lost to the cyst by emigration.Fin cysts are largely epithelial in structure. Glochidia which attach to gill filaments lie embedded in cellular connective tissue, roofed over with an epithelial canopy.Shortly after encystment is completed the cyst becomes thinner, smoother and more symmetrical. Thereafter, and even until the time of rupture, there are no further significant morphological changes in the cyst. Special adaptations of the host tissues to care for the wants of the metamorphosing parasite are not developed.The glochidium is liberated partly through its own efforts, apparently ided somewhat by sloughing. Repair of the resulting defect in the host tissue is rapid and probably follows the general method utilized at encystment. This would involve an early redistribution of existing cellular elements, followed later by the formation of new cells to restore the tissue balance.