Structural and Molecular Investigations on the Egg Coat in Phallusia mammillata

Summary

Sperm that approach the ascidian egg encounter first the follicle cells (FCS) and increasing evidence points to a crucial role of these cells for several fertilization steps. The main objective of our study was to investigate the particular surface structure of the follicle cells as potential sperm receptor site. SEM and TEM observations reveal the free upper surface and the filopodia of the FCS covered with an extracellular matrix (ECM) exhibiting a regular surface ornamentation consisting of small ring-like structures, the so-called microannuli (MA). To investigate the ECM properties we used lectin binding to characterize the carbohydrate moieties. From the lectins tested only WGA showed affinity for the FCS surface by specifically binding to the MA. Fluorescence and electron microscopy using solubilized biotinylated sperm surface proteins (SSP) was used to evaluate the ability of the FCS to act as sperm receptors. Biotinylated SSP was found to bind exclusively to MA. These findings are discussed in context with the suggested multiple role of follicle cells in fertilization and previously characterized components of the egg vitelline coat that exhibit sperm-binding activity.