The egg envelope is an extracellular matrix that surrounds the oocyte of vertebrate animals and plays essential roles in gamete recognition during the initial process of fertilization, including species-specific sperm-egg binding, induction of the acrosome reaction and prevention of polyspermy. These egg envelopes are mainly constructed of glycoproteins belonging to different subclasses of the zona pellucida (ZP) gene family. Although this extracellular matrix for all vertebrates is collectively referred to as the egg envelope, it is more specifically called the zona pellucida in mammals, the vitelline membrane in amphibia, and the chorion in fish. The eggs of birds have an equivalent structure, which is called the perivitelline membrane.
In the zona pellucida of mammals, there is a molecule that binds with a high degree of affinity to sperm of the same species, and that is called the sperm receptor. On the surface of the sperm, there is a ligand protein that recognizes zona pellucida protein and binds specifically to it. We have discovered that the perivitelline membrane of Japanese quail (Coturnix japonica) consists of several glycoproteins and molecules called ZP1 and ZP3 that interact with sperm and become activated. However, we still do not know the precise formulation of the ligand protein on the sperm surface.
In birds’ oviduct, there are structures called sperm storage tubules (SSTs) where sperm that enter the oviduct after copulation are stored alive for long periods. That is, once birds mate, the deposited sperm can be stored alive in the SST for long periods. Therefore, birds can continue to lay fertile eggs without repeated copulation. (The viable period of sperm thus stored is about 70 days for turkeys, about two weeks for chickens, and about 10 days for quail.) This phenomenon, peculiar to birds, has been known about for a long time, however, the answers to the following questions are still being investigated: How do sperm enter the SST? How do they survive in the SST for prolonged periods? How are they released from the SST to achieve fertilization? The molecular mechanisms involved are still barely understood.
Our laboratory is doing research to identify the sperm receptor in the perivitelline membrane, identify the sperm’s ligand protein on the sperm surface, clarify its expression control mechanism, and examine the formative mechanism in the perivitelline membrane in order to understand the fertilization mechanism of birds. We are paying particular attention to the sperm storage tubules and their function, especially because they are a structure peculiar to birds. We are examining how the sperm storage tubules attract the sperm and keep them alive and how they release them again.