The
zona pellucida (plural
zonae pellucidae, also
egg coat or
pellucid zone) is a
glycoprotein layer surrounding the
plasma membrane of
mammalian oocytes. It is a vital constitutive part of the oocyte. The zona pellucida first appears in unilaminar primary oocytes. It is secreted by both the oocyte and the follicular cells. The zona pellucida is surrounded by the
cumulus oophorus. The cumulus is composed of cells that care for the egg when it is emitted from the ovary.