Easter celebration is very tightly linked to eggs and therefore we will take this opportunity to have a closer look at the zona pellucida proteins, some of the most important genes in the human oocyte.

The Zona Pellucida is a specialized, semi-transparent extracellular matrix that surrounds the plasma membrane of the oocyte, the mammalian egg. This jelly-like mesh of glycoproteins is like a soft eggshell with three major functions, to ensure species-specific fertilization, to prevent fertilization by multiple sperm by hardening upon entry of the first sperm, and to protect the embryo on the journey through the fallopian tube and prevent premature implantation.

An important group of proteins involved in executing these functions are the four human zona pellucida glycoproteins. ZP1 gives the matrix its structural integrity and thickness, while ZP3 is the primary sperm receptor that upon binding triggers the sperm to release digestive enzymes that enables penetration of the matrix (the acrosome reaction). In humans also ZP4 is involved in this process. ZP2 is involved in the structural changes and hardening of the zona pellucida. It binds to the sperm to keep it attached while penetrating the matrix and following fertilization ZP2 is enzymatically cleaved. This triggers ZP2 oligomerization that effectively cross-links the zona filaments and hardens the zona pellucida making it physically impenetrable to sperm.

In the Human Protein Atlas all ZPs except for ZP1 are detected in follicle cells in the ovary using IHC, and shown to be specifically expressed in oocytes according to single cell RNAseq. ZP1 on the other hand is not detected in tissues and show highest expression in late spermatids with some expression in oocytes in single cell RNAseq data.

Explore the ZP genes in the Atlas