Egg-Coat and Zona Pellucida Proteins of Chicken as a Typical Species of Aves

Abstract

Birds are oviparous vertebrates in terrestrial animals. Birds' eggs accumulate mass of egg yolk during the egg development and are accordingly much larger than the eggs of viviparous vertebrates. Despite such difference in size and contents, the birds' eggs are surrounded with the egg-coat morphologically and compositionally resembling the mammalian egg-coat, zona pellucida. On the other hand, there are some differences in part between the two egg-coats, though relationships of such structural differences to any biological roles specific for the extracellular matrix of birds' eggs are not fully understood. In birds, unlike mammals, ZP proteins constituting the egg-coat are highly conserved and therefore those of chicken are described as a representative of birds. The egg-coat ZP proteins, ZP1, ZP3, and ZPD as the majors, accumulate and form the matrix by self-assembly around the egg rapidly growing in the ovarian follicle, in which ZP1 is from liver and both ZP3 and ZPD are from follicular granulosa cells. Although details of the egg-coat-sperm interaction on fertilization remain to be investigated, the lytic degradation process of egg-coat matrix for the sperm penetration has become to be clarified gradually. ZP1 is the primary target of sperm acrosin, and the limited cleavage in the specific region leading to the loss of intermolecular cross-linkages is crucial for the lysis of egg-coat matrix. Possible roles of the ZP1 with the additional sequence characteristic to birds are discussed from a viewpoint of giving both robustness and elastomeric nature to the egg-coat matrix for the birds' eggs.

Keywords: Chicken egg-coat; Germinal disc; Lytic degradation; Pro-rich repeat; ZP protein; ZPAX; ZPD.