Steps in Fertilization in Humans

The process of Fertilization can be broadly categorized into the following major processes.

Acrosomal Reaction

After being released into the female reproductive tract, the sperm undergoes various modifications, one of that is capacitation (several modifications occur at the sperm plasma membrane, which enables it to fuse with the female gamete.). Followed by the process of Capacitation, the Acrosomal reaction takes place when the sperm binds to the plasma membrane of the secondary oocyte. The head of the sperm contains enzymes that help it penetrate the outer layer of the egg cell, called the zona pellucida. Once the sperm has entered the egg cell, the zona pellucida undergoes a chemical change that prevents other sperm from entering and hence prevents the Polyspermy. Furthermore, Ca²⁺ ions play a crucial role in the formation of the acrosome; other than this, pH, temperature, and Mg²⁺ also play a key role in the accomplishment of the process.  

Cortical Reaction

It is one of the preventive measures that oocyte takes to prevent polyspermy. Once the fusion of the plasma membrane of the egg and sperm, the cortical reaction occurs on an immediate basis.  Cortical granules are modified forms of egg Golgi apparatus which contain several proteases and are stored between the egg plasma membrane and zona pellucida. Once after the fusion of the plasma membrane of the egg and sperm, the cortical granules get released inside the space, which results in the hardening of zona pellucida, which ultimately prevents the entry of other sperms. 

Sperm Entry

The sperm enters the secondary oocyte through a projection known as the cone of reception, which is formed while the sperm contacts the secondary oocyte. 

Karyogamy

The entry of sperm results in the release of a second meiotic arrest of the secondary oocyte. This results in the formation of a haploid ovum and a secondary polar body (going to degenerate).  Karyogamy is the fusion of both male and female pronuclei. The head of the sperm contains the male pronuclei, which is the only part of the sperm that enters into the cytoplasm, and the rest of the parts get degenerated. This fusion takes place inside the ovum cytoplasm. 

Egg Activation

When once sperm invade the ovum, then sperm activates the metabolism i.e., Protein synthesis and other cellular respiration.

Fertilization and implantation are the 2 important events in human reproduction, which is the biological process of producing new individuals from a union of male and female gametes. This complex process involves the fusion of gametes, the development of a zygote, and the growth and differentiation of the embryo, which ultimately leads to the formation of a fetus within the uterus.