Introduction: Human Fertilization

Fertilization for humans, is the stage of sexual reproduction consisting of a fusion of male and female gametes in a single cell called zygote. It consists of a sequence of events located in the fallopian tubes and uterus that create fertilization:

1. Specific recognition: the sperm and ovum are identified as compatible in the same case. This recognition is made between the proteins composing the zona pellucida (surrounding the oocyte during its maturation) and receptors on the sperm membrane. It occurs when an acrosome reaction, which will "dissolve" the zona pellucida and allow passage of the male gametes, to the plasma membrane of the oocyte. In humans and other mammals with internal fertilization, there aa priori no problem of recognition, two different species can rarely copulating together. The experiments have nevertheless demonstrated that fertilization between two different species was not possible because of the difference in genomes between species. The mechanism of specific recognition is especially useful for animals with external fertilization, as some fish and amphibians: the female lays her eggs in the middle, and the male has to deposit his sperm.

2. Fusion of sperm and egg: to keep a 2n amount of genetic material in the zygote, a single sperm to fertilize the oocyte: the monospermous.

3. Resumption of meiosis to the oocyte: it was blocked in metaphase II before ovulation. It thus ends its second meiotic division and expels its second polar body. Once this step is complete, there are two in the oocyte nuclei, called pronuclei: the female pronucleus and male pronucleus (from sperm). We can then speak with egg instead of egg.

4. Amphimixis and trigger embryonic development: it is the merger of the two pronuclei (caryogamie). In reality, the two pronuclei do not fuse properly speaking, as one might imagine, but the genetic material is gathered on the equatorial plate during anaphase of the first cell division of the new zygote.

*image from here