Intro to embryology

 intro to EMBRYOLOGY - Fertilization 

Have u ever asked yourself how a baby or a fetus is made?  

 

The simple answer to this question is when a cell from the male, called the sperm, and a cell from the woman called the egg, fuse with each other to give us a big cell that will eventually be turned into a baby.

 

so in general the process of forming a baby inside the body of the female has 3 trimesters called the embryological periods, which are

1-  1 -Pre – Pre-embryonic period- it is also called the Germinal fertilization period and it lasts for 2 weeks

2-  2-  The embryonic period- where all the organs of the baby will initiate developing from the 3 germinal layers and it lasts for 6 weeks starting from week 3 until week 8

3-   3- Fetal period starting from the 9^th week until birth 

Now let's take it from the start:

As we said it all begins when the sperm fuse with the egg, this process is called fertilization, this fertilization happens in a special place in the woman's reproductive tract called the Ampullary region, as u can see in the photo below

 

So why the fertilization process is that important??

1-   To determine the sex of the baby

2-   To get back the full number of chromosomes which is 46, 23 from the father and 23 from the mother

3-   To initiate another important process called cleavage

 

After we know the reason why this process is important, we now need to know its phases, fertilization has 3-4 phases depending on different sources, so we will go with the 3 phases:

1-   Penetration of the corona radiate

2-   Penetration of the zona pelluicida

3-   Fusion of the oocyte(the egg) and the sperm and finally the zone reaction

 

U may not be familiar with these names so let’s break Them down:

First of all, let's talk about the oocyte, the oocyte has 2 protective barriers called the corona radiate and the zone pelluicida, see the pictures below,

so for the sperm to fuse with the oocyte completely, it must penetrate both these barriers, but for the sperm to penetrate these barriers it must undergo some changes which are the

1-   Capacitation – it is a physiological process that sperm must undergo to gain the ability to penetrate and fertilize an egg. This process occurs after ejaculation when sperm comes into contact with the female reproductive tract, see the pictures below.

 

2-   The acrosomal reaction - the acrosome reaction is a subsequent event that is facilitated by capacitation. During the acrosome reaction, enzymes are released from the acrosome (a cap-like structure over the sperm head) to help the sperm penetrate the zona pellucida. See the pictures to identify the structure of the sperm as well as both processes.

 

The last thing we need to mention is the ZONA REACTION, this is a very important process done by the oocyte to prevent something called polyspermy, polyspermy is a condition that
happens when 2 sperms fuse with the egg, which means that we will have 23+23+23 chromosomes inside the egg, this abnormal number of chromosomes will lead to the death of the egg = the fertilization process failed