The<unk>  (plural<unk> e, also egg coat or<unk> zone) is a glycoprotein layer surrounding the plasma membrane of mammalian oocytes. It is a vital constitutive part of the oocyte. The<unk>  first appears in unilaminar primary oocytes. It is secreted by both the oocyte and the ovarian follicles. The<unk>  is surrounded by the corona radiata. The corona is composed of cells that care for the egg when it is emitted from the ovary. This structure binds spermatozoa, and is required to initiate the acrosome reaction. In the mouse (the best characterised mammalian system), the<unk>  glycoprotein,<unk> 3, is responsible for sperm binding, adhering to proteins on the sperm plasma membrane.<unk> 3 is then involved in the induction of the acrosome reaction, whereby a spermatozoon releases the contents of the acrosomal vesicle. The exact characterisation of what occurs in other species has become more complicated as further<unk>  proteins have been identified. In humans, five days after the fertilization, the blastocyst performs<unk>  hatching; the<unk>  degenerates and decomposes, to be replaced by the underlying layer of trophoblastic cells. The<unk>  is essential for oocyte growth and fertilization.
Zona pellucida
Cave of septum pellucidum
Sinus pericranii
External lamina
Tapetum lucidum
Infundibulum
Germinal matrix
Growth cone