Fertilization in humans class 12th


You must have obviously heard of the term fertilization but sometimes you just know the word and if someone asked you what that means you would go blank. Just to not keep you blank and so that you can answer quickly I have a post here to answer your questions related to fertilization.

In this article I will be telling you what fertilization means, how it is taken place, stages, significance and everything else you will need to know in order to master this topic. Keep reading ahead to know more about fertilization.

What is fertilization ?

Let's keep the definition simple. Fertilization is fusion of the haploid male gamete which is the spermatozoon and the haploid female gamete which is the ovum which further forms diploid zygote. If you don't know what diploid and haploid means don't worry, I've got your back. Haploid means having a single set of unpaired chromosome and diploid means having two complete sets of chromosomes.

Fertilization takes place in the upper region of the fallopian tube which is the ampulla. Hence, the process of fertilization is internal. Fertilization is basically the beginning of pregnancy.

How does fertilization takes place ? 

Fertilization is an easy process, our books have just made them seem complicated until you actually concentrate on it. Some people find it hard to understand biology but I will make sure I'll explain it to you in simple sentences and in an easy way. 

We all know in order to fertilize an egg the sperms have to play a role in it. During the time of coitus, the semen from the male reproductive system is discharged into the vagina of the female and this is called insemination

Related article: Male reproductive system

Once the semen has entered into the vagina it keeps going ahead to the female reproductive system. These sperms swim in all possible directions in order to reach the ovum like they are all a part of a race. Some find their way up to the cervix and then the fallopian tube where the ovum is present. 

Stages of fertilization

The ovum doesn't always be present in the fallopian tube but during the time of ovulation the ovum leaves the ovary into the fallopian tube. When the sperms meet the ovum in the fallopian tube the process of fertilization starts. It takes three days for the ovum to reach the uterus through the fallopian tube. Hence, you can say that the time of fertility in a woman occurs only 1-2 days after ovulation. 

Now that the sperm has reached the ovum here starts the process and there are a lot of changes happening. There are many sperms trying to get into one ovum but in the end only one sperm can fertilize that ovum. Only one sperm enters in the ovum by the help of the enzymes present in the acrosome

One sperm has now entered in the ovum through the zona pellucida and then enters the cytoplasm of the ovum. Before the sperm enters in the ovum it gets detached meaning it gets separated from the body and middle piece. Since now the sperm has made full entry inside the ovum some changes are going to take place. 

The vitelline membrane turns into fertilization membrane which opens sodium channels in the cell membrane which causes a change within the cell which stops the entry of more than one sperms. This confirms monospermy

After the sperm has entered inside the ovum, second maturation division takes place which forms second polar body which further breaks into small parts which do not take part in development. Inside the ovum, the two haploid gametes which is of the male and female fuses together and becomes diploid. Mitochondria and centrioles are received from the sperms. 

Now, the sperms are going to be activated by second meiotic division. The nuclei of the sperm and ovum swell and become the pronuclei of the male and female after separation of head from the body. 

Since the sperm is now activated and both the female and male gametes became pronuclei, it is now time for fusion of the pronuclei. They don't fuse right away but instead they first approach each other. The membranes rupture during the first cleavage and paternal (father) and maternal (mother) nuclear materials are intermixed before the division of the fertilized ovum. This is known as zygote. The intermixing of the nuclear materials from the parents is known as amphimixis

Zygote

After the fusion takes place, diploid chromosome number is obtained i.e. 46. The middle piece and the head of the sperm introduces centrioles which is necessary for further cell division. With this you can say that the process of fertilization has now been completed and the fertilized ovum will now be implanted in the endometrium.

Implantation:

When the zygote starts moving towards the uterus there are mitotic divisions taking place. Implantation takes place about seven days after fertilization. The zygote passes from the isthmus of the oviduct called cleavage towards the uterus which further forms 2, 4, 8, 16 daughter cells called the blastomeres. An embryo with 8 to 16 blastomeres is known as morula. This morula divides and transforms into a blastocyst

The blastomere is in the blastocyst is made of the 2 following layers:
  1. Trophoblast - This is the outer layer which is attached to the endometrium
  2. Inner cell mass - This is the inner group of cells which are attached to the trophoblast which gets differentiated as the embryo. 

The blastocyst goes down in the pit which is formed by endometrium and then is completed buried in it. After going in, the blastocyst forms villi around it which is used for nourishment. After totally going into the uterine cavity the embryo looses its villi from the free surface. 

Implantation

Implantation is regulated by the ovarian hormones which are oestrogen and progesterone which are produced by the ovary. It is when you can say there is a confirmed case of pregnancy. 

Significance of fertilization:

If it wasn't fertilization then how would we be here enjoying our lives ? Apart from just existence fertilizaton also plays some very important role to humankind, which are as follows:
  1. Fertilization evokes the egg to complete its maturation. 
  2. Fertilization activates the ovum so that a new individual is developed by repeated mitotic division. 
  3. Fertilization restores the diploid number of chromosomes in the zygote. 
  4. During the process of fertilization the characters of the both the parents is combined. This helps in evolution of new species and variations. 
  5. Helps in determination of the child. 
  6. The vitelline membrane which turns into fertilization membrane stops the entry of more than one sperm. 
  7. Due to the process of fertilization centrioles are introduced in the ovum which are missing. 
  8. Once the process of meiosis II is completed, secondary oocyte stops the process of oogenesis. 

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