Capacitation, generally speaking, is the change sperm undergo in the female reproductive tract that enables them to penetrate and fertilize an egg.
This step is a biochemical event; the sperm moves normally and looks mature prior to capacitation. It is important to note that once the sperm reaches the egg, it does not mean that it is capable of fertilizing it immediately. In order to fertilize the egg, the sperm must undergo the process of capacitation in the reproductive tract where a number of enzymes and signaling molecules are involved. This process can take around 10 hours, which means that the fertilization time is approximately 24 hours.
Capacitation causes a series of changes in sperm’s biochemical composition and structure such that they develop increased motility and are prepared for interactions with the egg (or oocyte) at fertilization. Essentially, the sperm becomes able to break through the membrane (known as the zona pellucida) that surrounds the egg. Once sperm is able to traverse the zona pellucida, it can begin the process of fertilizing the egg.
It is also important to consider the two distinct types of fertilization when thinking about capacitation and how it occurs:
- For in vivo fertilization, or fertilization happening in living organisms, capacitation occurs after ejaculation when sperm leaves the vagina and enters later sections of female reproductive tract. The uterus aids in the steps of capacitation by secreting sterol-binding albumin, lipoproteins, and certain enzymes.
- For in vitro fertilization, or assistive reproductive technologies, capacitation occurs by incubating spermatozoa that have either undergone ejaculation or have been extracted from the epididymis and incubated outside the body. In this process, an ovum or ova is also removed from the woman’s ovaries and co-incubated with sperm for fertilization before being implanted into the female body.