Fertilization Process- The Complete Info
Simple said, fertilization is the process of sperm reaching the egg leading to the formation of a fertilized egg (Zygote).
How does it begin?
Fertilization process begins with women’s menstrual cycle. Fertilization can take place around half of this period. At this point, when an egg cell is released or ovulated in fallopian tube, then fertilization takes place.
What happens during sexual intercourse?
During sexual intercourse, around 300 million sperms enters vagina and most of them die due to acidic environment. Nevertheless, few sperms survive because of the protecting elements present in the fluid surrounding them.
Afterwards, sperms will pass through the cervix, an opening in the uterus. Normally, this will be tightly closed but the cervix remains open for a few days while the women ovulate. The sperm move through the cervical mucus, which has thinned for easy navigation.
Once sperm enters the cervix, it moves towards uterus where millions will die while trying to enter the mucus. In the uterus, muscular uterine contractions help sperm to reach the egg. However, resident cells from the women’s immune system mistaken sperms for foreign fighters and kills thousands of them. Then, half of the eggs enter the fallopian tube while the other half moves towards the tube containing the unfertilized eggs.
What happens in fallopian tube?
Inside the fallopian tube, tiny cilia push the egg towards uterus. Here, the sperm must surge against this motion to reach the egg. In this process, few sperms get trapped in the cilia and die. During this journey, chemicals present in the reproductive tract cause the membranes covering the heads of the sperms to change. Due to this action, sperm becomes hyperactive and swims faster to reach egg. Only a few sperms of the 300 million sperm remain and the egg is covered with a layer of cells known as corona radiata. The sperm must penetrate through this layer to reach the outer layer of the egg, the zona pellucida.
When this happens, they combine with specialized sperms receptors, which make their acrosomes to release digestive enzymes. These enzymes make sperms to penetrate the layer. The sperm that reaches first will fertilize the egg and only a single sperm get attached to the egg cell membrane. After sometime, egg pulls the sperm inside. This action prevents other sperm from attaching to the egg. Then, the egg releases chemicals that makes sperms to not to contact the egg and creates an impenetrable fertilization membrane. Now, other sperms can’t attach to the zona pellucida.
How zygote is formed?
Meanwhile, firmly packed male genetic material spreads out in the egg and a new membrane surrounds the genetic material leading to the formation of male pronucleus. The genetic material spreads into 23 chromosomes. Whereas, the female genetic material formed by the fusion of the sperm with the egg completes dividing and forms female pronucleus containing 23 chromosomes. When both male and female pronucleus is formed, microtubules bring two sets of chromosomes together leading to fertilization process. At this point, a unique genetic code will be formed, which determines characteristics such as gender, hair, eye color, etc. This cell is called as Zygote.
Now, the cilia in the fallopian tube gently push zygote towards the uterus, where the cell will grow for the next nine months.