Oolemma

The oolemma, sometimes called the oocyte plasma membrane, is the membrane that encloses the oocyte, the female gamete, in animals. It forms the boundary between the oocyte’s cytoplasm and the surrounding extracellular environment and lies beneath the zona pellucida in species where this gelatinous layer is present.

Structure and location: The oolemma is a lipid bilayer embedded with proteins and receptors. It is the outermost cellular membrane of the mature oocyte and participates in mediating interactions with sperm. Its molecular composition includes membrane proteins that facilitate recognition and fusion with sperm, as well as channels and transporters required for oocyte physiology.

Role in fertilization: The oolemma plays a central role in sperm binding and fusion. Sperm surface protein Izumo1 interacts with the oocyte receptor Juno on the oolemma, an essential step for successful fusion. Other oolemmal proteins, including certain tetraspanins such as CD9, contribute to the fusion process by organizing membrane microdomains. After the sperm binds and fuses with the oolemma, the egg undergoes activation, which involves calcium oscillations and downstream events that resume meiosis and begin embryonic development.

Post-fertilization events: Fusion of the sperm with the oolemma triggers cortical granule exocytosis and the zona reaction, reinforcing the block to polyspermy. The oolemma also participates in the rapid establishment of the zygotic cell membrane and supports early embryonic divisions as the embryo forms.

Terminology and scope: The term oolemma is widely used in vertebrate reproductive biology to denote the oocyte’s plasma membrane, distinct from surrounding layers like the zona pellucida. It is functionally analogous across many species, though specific molecular components can vary.