The follicle cells of styela plicata (ascidiacea, tunicata): a sem study

The morphological aspect of the follicle cells of Styela plicata eggs is described by means of scanning electron microscope investigations. The follicular layer is made of spaced, cylindrical box-like cells which are arranged hexagonally. They adhere to the egg through a complex network of membrane extensions making an overall thin layer on the vitelline coat. The walls of the follicle cells are plentifully provided with microvilli, filopodia and lamellipodia, which allow a connection among the cells. At their apical end lies a large vacuole containing a granule, probably involved in secretion. At insemination the majority of spermatozoa is distributed on the apical membrane of the follicle cells. The membrane often breaks after sperm-egg impact and the granule is therefore displaced. By means of the present investigations it is once again suggested a role played by follicle cells in ascidian eggs at fertilization.