Wednesday, July 9, 2008

Fertilisation

So now that we have covered how the sperms and eggs prepare for fertilisation, let us go in depth on how fertilisation actually takes place. So firstly, sexual intercourse must take place and the male must ejaculate sperms into the female's vagina. As ovulation approaches, the cervical mucus becomes watery allowing the sperm to swim up the vagina through the cervix. Only about 2,000 sperm will be strong enough to reach the uterus and the fallopian tube.
So to begin, sperms must penetrate the cell membrane of the oocyte. To reach the oocyte, the sperm must pass through the corona radiata and the zona pellucida; two layers covering and protecting the oocyte from fertilization by more than one sperm. When the sperm reaches the zona pellucida, which is an extra-cellular matrix of glyco-proteins, a special complementary molecule on the surface of the sperm head then binds to a ZP3 glyco-protein in the zona pellucida. This binding triggers the acrosome to burst, releasing enzymes that help the sperm get through the zona pellucida.
When the sperm penetrates the zona pellucida, the cortical reaction occurs. Cortical granules inside the secondary oocyte fuses with the plasma membrane of the cell, causing enzymes inside these granules to be expelled by exocytosis to the zona pellucida. This in turn causes the glyco-proteins in the zona pellucida to cross-link with each other, making the whole matrix hard and impermeable to sperm. This prevents fertilization of an egg by more than one sperm.
So with the fusion of cell membranes of the sperm and opcyte, both the oocyte and the sperm also go through transformations, preparing for the fusion of their genetic material.
For the oocyte, it completes its second meiotic division. This results in a mature ovum. The nucleus of the oocyte is called a pronucleus in this process, to distinguish it from the nuclei that are the result of fertilization.
The sperm's tail and mitochondria degenerate with the formation of the male pronucleus. This is why all mitochondria in humans are of maternal origin.
The pronuclei migrate toward the center of the oocyte, rapidly replicating their DNA as they do so to prepare the new human for its first mitotic division.
The male and female pronuclei don't fuse, although their genetic material do so. Instead, their membranes dissolve, leaving no barriers between the male and female chromosomes. During this dissolution, a mitotic spindle forms around them to catch the chromosomes before they get lost in the egg cytoplasm. By subsequently performing a mitosis, the cell gathers genetic material from the male and female together. Thus, the first mitosis of the union of sperm and oocyte is the actual fusion of their chromosomes.
Each of the two daughter cells resulting from that mitosis have one replica of each chromatid that was replicated in the previous stage. Thus, they are genetically identical. These cells will continue to divide so the fetus can develop

I thank wikipedia for the information and pictures.

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