3 EMBRYOLOGY: EGG + SPERM= FERTILIZATION
Lets first look how egg can meet sperm?
-egg is maturing in ovary and on day 14 secondary oocyte is released from ovary-ovulation -fallopian tube picked up the egg where it is moved inside fallopian tube and it waits there the arrival of the sperm. It will wait for 24h -sperm enters vagina and travels up uterus and into the fallopian tube to fertilize egg, most of the sperm will dye off on this journey but one will make it -fertilization will occur when one of the sperms will enter the egg and its genetic material will combine with egg genetic material. -egg will start dividing and moving from fallopian tubes towards the uterus wall. The implantation happens in uterus wall and not in the fallopian tubes.
Name the 4 steps of fertilization:
1. Sperm binding 2. Acrosome reaction 3. Cortical reaction 4. Fertilization
Females contain egg cell:
Egg cell is round and huge comparing to the sperm. It actually can be visible to human eye. Egg cell contains DNA, outside of the egg you have thick coating called ZONA PELUCCIDO with is a thick layer of glycoproteins that is located on the outside of the egg. Sperm needs to pass through that layer to get inside. Outside you have plasma membrane. Inside egg cell you have 100,000-200,000 mitochondria present!!!
Step 1: SPERM BINDING
First thing that happens is that sperm comes with contact with secondary oocyte. Sperm will bind to secondary oocyte corona radiate and Zona Peluccido. They are located outside the egg. This is called: SPERM BINDING. This binding sets up reaction called ACROSOMAL REACTION.
Where exactly fertilization occurs?
In the widest part of the fallopian tube called: AMPULIA
Organisms that reproduced sexually must get their genes together somehow. So they package their genetic material into specializes cells called SEX CELLS
Male have Sperm sex cell. The main and only purpose of the sperm cell is to transfer male genetic material into the female sex cell-egg. So sperm cell has special features that allow it to do their job. It contains flagella for movement, pointed head that is packed with DNA enclosed in nuclear envelope and in the middle section of sperm there are mitochondria that produce ATP for movement. Sperm has 70-100 mitochondria and they are big to produce lots of energy. Also, sperm contains special enzymes called ACROSOME that are located in the head of sperm that are very important for fertilization.
What important event happens after acrosome reaction?
Meiosis II
What is BLOCK TO POLYSPERMY?
Polyspermy means multiple sperms. We don't want more then one sperm to inject its DNA into egg. Cortical granules dissolve all the sperm binding glycoproteins receptors other sperm can't get in.
Think to know is:
Sperm will enter the secondary oocyte in several steps/reactions
Step 2: ACROSOME Reaction
Step two: is the ACROSOME Reaction. The Acrosome enzymes that were sitting in the front of the sperm head will leak out into zona Peluccido and start to digest it. That allows out sperm head dive deeper toward the plasma membrane. As the sperm gets closer to plasma membrane it eventually touches the membrane. This sets another reaction: Cortical reaction
How do you end up with fraternal twins?
These are also called dizygotic. When two eggs are ovulated (released from the ovary) and picked up by fallopian tube and fertilized. Each zygote then will implant in the uterine wall and each will develop its own placenta. They are not genetical identical since they come from different eggs and different sperms.
How do you get identical twins?
These are also called monozygotic twins. During the journey from fallopian tubes to uterus egg will be dividing. If these cells separate you will end up with identical twins. Since you just split zygote. These twins will be genetically identical (2 boys or 2 girls). If the cells do not split completely in here you will end up with CONJOINED TWINS where two offsprings will be attached somewhere in the body.
Step 3: CORTICAL REACTION
Third step is CORTICAL REACTION. There are structures right under plasma membrane of the egg called CORTICAL GRENULES. They sit under plasma membrane and wait for sperm to bind to plasma membrane of the egg. When sperm binds to egg there is a cortical reaction in which a release of Ca²⁺ ions happens. The calcium depolarize the egg membrane and cortical granules are fused with membrane of the egg. Cortical granules contain enzymes that during this reaction are being pushed outside of the egg membrane. These enzymes digest Zona Peluccido and glycoproteins even more. So other sperms can't enter the egg. Since they don't have the attachments. Why this happens? This happens to block other sperms from binding to egg. Since the Zona Peluccido is removed they have no attachment and can't bind to egg. This is called: BLOCK TO POLYSPERMY.
Step 4: Plasma membrane on sperm fuses with plasma membrane on egg + Genetic material enters the egg. This is called FERTILIZATION.
When this happens genetic material DNA can enter the egg.
What are Cortical granules?
Within the oocyte, cortical granules are located along the cortex, the region furthest from the cell's center. Following fertilization, a signaling pathway induces the cortical granules to fuse with the oocyte's cell membrane and release their contents into the oocyte's extracellular matrix. This exocytosis of cortical granules is known as the cortical reaction.
Notice that secondary oocyte has two structures outside:
corona radiate and zona pelucciada Sperm must go through these two structures.