Helper Function of Follicle Cells in Sperm-Egg Interactions of the Ascidian, Ciona intestinalis: (Ciona gametes/fertilization/follicle cells/soy bean agglutinin/DAPI).

Studies were made on the involvement in sperm-egg interactions of follicle cells of Ciona intestinalis, which are tall, vacuolated cells attached to the outer surface of the egg vitelline coat. The basal surface of the follicle cells is polygonal. The borders between cells could easily be observed by the binding of fluorescent SBA (soy bean agglutinin), a lectin recognizing N-acetylgalactosamine (GaINAc) residues. At fertilization many spermatozoa aggregate along these polygonal borders of cells on the vitelline coat, through which they entered the perivitelline space. The removal of follicle cells was sometimes associated with loss of SBA-binding sites, and in such cases the sperm did not show a hexagonal pattern of aggregation, but became dispersed all over the vitelline coat. Removal of the follicle sometimes delayed fertilization. Examination of sections of gametes stained with DAPI, a fluorescent dye staining DNA, showed that removal of the follicle reduced the number of spermatozoa bound to the vitelline coat and, more especially, the number of spermatozoa penetrating through the vitelline coat. The blockage of GalNAc residues on the vitelline coat with SBA did not appreciably affect the time course of fertilization or the number of sperm associated with eggs. These findings are discussed in relation to the role of follicle cells in facilitating sperm aggregation on the vitelline coat and their penetration through it.