Fertilization in mammals

Fertilization is the meeting and fusion of a sperm and an egg. In mammals it takes place inside the female body, in the ampulla, the widest part of the oviduct (the fallopian tube in humans). It is not a single event but a sequence: sperm must become competent to fertilize, pass through the layers that surround the egg, recognise and fuse with the egg membrane, activate the egg, and make sure that no second sperm gets in.

The journals behind this archive followed each of these steps across many species, from cattle, sheep, pigs and buffalo to laboratory rodents, often in the context of in vitro fertilization. The sections below trace what is now known and where the gaps remain.

Getting ready: the female tract and capacitation

Freshly ejaculated sperm cannot fertilize straight away. They first undergo capacitation, a change that makes them capable of fertilizing an egg still enclosed in its coat. In the body this takes place in the female tract and lasts hours. Sperm stored in the lower part of the oviduct, the isthmus, are thought to be released a few at a time towards the ampulla. Despite decades of work, the molecular basis of capacitation is still not fully understood: one long-accepted marker, a particular pattern of protein phosphorylation, turned out not to be essential in some experiments.

Crossing the egg’s coats

The mammalian egg is surrounded by two barriers. The outer one is the cumulus, a cloud of cells held in a sticky matrix. The inner one is the zona pellucida, a thick coat of glycoproteins. To reach the egg, sperm must pass through both, and to cross the zona they need the acrosome reaction, the release of the enzyme-filled cap on the sperm head.

For many years the standard view was that contact with the zona triggered the acrosome reaction. Experiments published in 2011, using mouse sperm engineered to make the reaction visible under the microscope, found that most of the sperm that went on to fertilize cumulus-enclosed eggs had begun the reaction before reaching the zona. Ryuzo Yanagimachi, who has studied mammalian fertilization for more than 60 years, noted in a 2022 review that there is still no general agreement on the time, site and cause of the acrosome reaction in the body. Why the cumulus helps fertilization, and how exactly sperm pass through the zona, are listed among the field’s unsolved questions.

These findings matter partly because so much of the classic work on fertilization was done in the laboratory dish, often with the cumulus removed to make sperm–zona interactions easier to observe. Results obtained under such conditions do not always match what happens in the oviduct, and researchers now pay closer attention to the environment in which fertilization naturally takes place.

Recognition and fusion: Izumo1 and Juno

The molecules that let sperm and egg recognise each other were identified only recently. In 2005 researchers in Japan described Izumo, a protein on the sperm surface. Male mice lacking it were healthy but sterile: their sperm could penetrate the zona pellucida but could not fuse with the egg. Human sperm carry the same protein.

The partner on the egg remained unknown until 2014, when a team in the United Kingdom showed that a protein previously classified as folate receptor 4 binds Izumo1. They renamed it Juno. Female mice without Juno are infertile, and the Izumo1–Juno interaction is conserved in several mammals, including humans. Neither protein appears to cause membrane fusion directly; both behave as adhesion molecules, so the actual fusion machinery is still being sought.

Waking the egg: calcium and PLCζ

Once the sperm has fused, the egg is activated by a series of repeated rises in calcium inside the cell. These calcium oscillations are essential for further development. A long-standing hypothesis held that the sperm delivers a soluble “sperm factor” that sets them off. In 2002 researchers identified a sperm-specific enzyme, phospholipase C zeta (PLCζ), that triggers calcium oscillations in mouse eggs indistinguishable from those seen at fertilization. The amount present in a single sperm was enough to support development to the blastocyst stage. PLCζ is the best-studied candidate for the sperm’s activating signal, but other molecules have been proposed and the question is not considered fully closed.

One sperm only: blocks to polyspermy

Fertilization by more than one sperm, called polyspermy, is a danger the egg must guard against. Mammalian eggs have two defences:

• the zona reaction, in which granules just below the egg surface release enzymes that partly break down zona proteins, so that further sperm can no longer bind or pass;
• a membrane block at the egg surface. The rapid shedding of Juno from the egg membrane after fertilization has been suggested as one way this block works.

From the laboratory to the clinic

Knowledge of fertilization underpins assisted reproduction, described on the page on IVF and assisted reproduction. In intracytoplasmic sperm injection (ICSI), an embryologist injects a single sperm directly into the egg, bypassing capacitation, the egg coats and membrane fusion. The UK fertility regulator, the HFEA, describes ICSI as the most common and successful treatment for male infertility. It also states that there is currently no scientific evidence to support using ICSI when infertility is not related to sperm, and that ICSI carries slightly more risks than some other fertility treatments.

What is settled and what is not

The overall sequence of mammalian fertilization is well established, as are the roles of Izumo1 and Juno in sperm–egg binding and of calcium oscillations in egg activation. Open questions include the exact timing and trigger of the acrosome reaction in the body, how sperm penetrate the zona, which molecules actually fuse the two membranes, and the full set of factors the sperm uses to activate the egg. This page summarises peer-reviewed research and regulator information and is not medical advice.