Human fertilization

Human fertilization is the union of a human egg and sperm, usually occurring in the ampulla of the uterine tube. It is also the initiation of prenatal development. Scientists discovered the dynamics of human fertilization in the nineteenth century.

Fertilization (also known as conception, fecundation and syngamy), is the fusion of gametes to produce a new organism. The process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo.

It is when first of all the acrosome at the head tip produces enzymes, which cuts through the outer jelly coat of the egg. After that has happened, the sperm plasma fuses with the egg’s plasma membrane. Finally, the Head disconnects with the body, and the egg can now travel down the Fallopian tube to reach the womb, where the baby grows.

Fertilization may or may not involve sexual intercourse. In vitro fertilisation (IVF) is a process by which egg cells are fertilized by sperm outside the womb, in vitro. Sperm is propelled through the female reproductive tract by flagellation and may get through the jelly coat through a process called sperm activation. The oocyte and sperm fuse once the sperm is through the corona radiata and the zona pellucida; two layers covering and protecting the oocyte from fertilization by more than one sperm.

Corona radiata
The egg binds the sperm through the corona radiata, a layer of follicle cells on the outside of the secondary oocyte. Fertilization is when the nuclei of a sperm and an egg fuse. The successful fusion of gametes form a new organism..

Acrosome reaction
The acrosome reaction must occur to mobilise enzymes within the head of the spermatozoon to degrade the zona pellucida.

Zona pellucida
The sperm then reaches the zona pellucida, which is an extra-cellular matrix of glycoproteins. A special complementary molecule on the surface of the sperm head then binds to a ZP2 glycoprotein in the zona pellucida. This binding triggers the acrosome to burst, releasing enzymes that help the sperm get through the zona pellucida.

Some sperm cells consume their acrosome prematurely on the surface of the egg cell, facilitating for other surrounding sperm cells, having on average 50% genome similarity, to penetrate the egg cell. It may be regarded as a mechanism of kin selection.

Recent studies have shown that the egg is not passive during this process.

Cortical reaction
Once the sperm cells find their way past the zona pellucida, the cortical reaction occurs: cortical granules inside the secondary oocyte fuse 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.

Fusion
After the sperm enters the cytoplasm of the oocyte, the cortical reaction takes place, preventing other sperm from fertilizing the same egg. The oocyte now undergoes its second meiotic division producing the haploid ovum and releasing a polar body. The sperm nucleus then fuses with the ovum, enabling fusion of their genetic material.

Cell membranes
The cell membranes of the secondary oocyte and sperm fuse together.

Transformations
Both the oocyte and the sperm go through transformations, as a reaction to the fusion of cell membranes, preparing for the fusion of their genetic material.

The oocyte now 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

Replication
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.

Mitosis
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 (which includes pulling of chromatids towards centrioles in anaphase) 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.

In other words, the sperm and oocyte don't fuse into one cell, but into two identical cells.