25.3: Fertilization
During fertilization, an egg and sperm cell fuse to create a new diploid structure. In humans, the process occurs once the egg has been released from the ovary, and travels into the fallopian tubes. The process requires several key steps: 1) sperm present in the genital tract must locate the egg; 2) once there, sperm need to release enzymes to help them burrow through the protective zona pellucida of the egg; and 3) the membranes of a single sperm cell and egg must fuse, with the sperm releasing its contents—including its nucleus and centrosome—into the egg’s cytoplasm. If these steps are successful, the genetic material of the male and female gametes combine, and mitotic cell division commences, giving rise to a diploid embryo.
Calcium Signaling and Fertilization
The binding of the sperm and egg cell brings about various changes, among them the production of waves of calcium ions (Ca2+) pulsing through the egg cell. Such oscillations are initiated by sperm-egg fusion and result from both the release and uptake of endogenous Ca2+ in the endoplasmic reticulum of an egg cell and the simultaneous discharge and intake of such ions from the egg’s extracellular environment. Importantly, calcium signaling modifies the egg by causing vesicles, called cortical granules, that lay directly below its plasma membrane to release their contents into the open space beneath the zona pellucida. These contents include enzymes that cleave sperm-binding proteins, which change the surface of the zona pellucida, preventing additional sperm cells from entering; this process is a type of block to polyspermy, or fertilization by multiple male gametes.
In addition, Ca2+ waves also “activate” the egg cell. Interestingly, an egg is arrested in meiosis when it is released from the ovary, and only resumes the process upon fertilization. This revival is due, in part to fertilization-induced Ca2+ signaling, which activates kinases in the egg cell capable of jumpstarting division. Once meiosis restarts, the egg divides into two cells: a large mature egg (also called an ovum) containing most of the cytoplasm, and a smaller polar body that subsequently dissolves. Following the completion of meiosis, the egg contains a nucleus with one set of unduplicated chromosomes, termed the egg pronucleus.
Joining of the Pronuclei
As the egg’s pronucleus is forming, changes also occur in the genetic material of the sperm. Initially, the sperm’s chromatin is tightly packed; however, once the sperm’s nucleus enters the egg cell, its membrane dissolves, and the chromosomes begin to unravel. A new nuclear membrane is established around the loosely-packed sperm material, generating the sperm pronucleus. Interestingly, the sperm’s centrosome is also introduced into the egg cytoplasm upon fertilization, and this structure will become oriented between the egg and sperm pronuclei, forming microtubules that draw these structures towards one another. As they meet, the pronuclei lose their membranes, their chromosomes mingle, and the process of mitosis begins.
