Egg

Fertilization and implantation are the foremost critical events within the reproduction process. In this process, both egg and sperm are fused together to make a zygote. Later, it gets implanted into the uterus and therefore the development of an organism.

Fertilization in Humans

When the fusion of male and female gametes occurs, it facilitates the development of a new organism. This process is known as fertilization in humans. 

Fertilization is that natural life process, which is administered by the fusion of gametes from female and male counterparts, which ends up with the formation of a zygote. In humans, within the fallopian tube, the process of fertilization takes place. 

During this process, semen comprising thousands of sperms are inseminated into the vagina of the female during coitus. An utmost important component for reproduction viz. sperm moves towards the uterus and enters the opening of the fallopian tube at the time of ejaculation from the male reproductive organ, penis. In this procedure, only a few sperms succeed in reaching the opening of the fallopian tube. 

The secondary oocyte is released from the mature Grafian follicle of the ovary and enters into the fallopian tube, where it is fertilized within 24 hours, after which it is released from the ovary.

Though surrounded by several sperms, the oocyte is fertilized by one sperm. The sperm enters the secondary oocyte and completes the meiosis-II procedure. After this procedure, the secondary oocyte is known as the egg. 

Both sperm and egg remain viable only for a  limited time period. Sperm is alive for 48-72 hours inside a female genital system, whereas the egg remains viable until twenty-four hours before it's fertilised or released.

Steps Involved in Fertilization in Humans

In human beings, the fertilization process takes place in many stages that involve both chemical and physical events. Different stages of fertilization in humans are as mentioned below - 

Acrosomal Reaction

The sperms go through the reaction called acrosomal and release certain chemicals known as sperm lysins that can be seen in acrosomes.

The plasma membrane of the secondary oocyte and the sperm are fused together so that the contents of the sperms can enter due to the acrosomal reactions. When the plasma membrane of the sperm binds with that of the secondary oocyte, the plasma membrane of the oocyte depolarizes. This prevents polyspermy.

Calcium ions have a very important role in this reaction of acrosomes. Optimum pH, temperature, and calcium and magnesium concentration are the main factors essential for acrosomal reactions. 

Cortical Reaction

Soon after the fusion of the plasma membranes, the oocyte shows cortical reactions. Cortical granules are found under the plasma membrane of the oocyte, which fuses with the plasma membrane and releases cortical enzymes between the zona pellucida and plasma membrane. The zone is hardened by the cortical enzymes that prevent polyspermy.

Sperm Entry

A projection referred to as the cone of reception is made by the secondary oocyte at the site of sperm contact. This cone of reception receives the sperm.

Karyogamy

After the entry of the sperm, the suspended second meiotic division is completed by the secondary oocyte. This gives rise to a haploid ovum and a second cell.

The head of the sperm containing the nucleus detaches from the whole sperm and is called male pronucleus. The tail and the second polar body degenerates. The nucleus of the ovum is known as female pronuclei.

The nuclear membranes degenerate when there is a male and female pronuclei fusion. This fusion of the chromosome of male and female gametes is called karyogamy. After the fertilization procedure of the ovum, the resulting structure is called the zygote. 

Activation of Eggs

The entry of sperm triggers the metabolism within the zygote. Consequently, protein synthesis and cellular respiration increase.

Implantation

Once fertilization happens, the cell starts to divide and multiply within 24 hours within the Fallopian tube. This detached multi-celled structure is called a zygote. When finally after 3-4 days it travels to the uterus, then it can be called an embryo.

After the embryo is developed, it undergoes various stages before getting attached to the endometrial layer of the uterus. This process of attachment is known as implantation.

Sex Determination

Fertilization is that the process during which a replacement cell is made when two gametes (sex cells) –sperm and ova fuse together. All genetic information is transferred from both the parents to their child and the gender of the child is determined which is known to be a completely unbiased procedure that takes place in the body on its own. 

The sperm of a father determines the sex of the child. If the sperm carries a Y chromosome, then it is determined that the child born will be a boy and if the sperm carries an X chromosome, then it is determined that the child born will be a girl.