The Process Of Fertilisation
Fertilisation or fertilization (also known as conception, fecundation and syngamy) is fusion of gametes to form a new organism of the same species.
The term "conception" commonly refers to fertilisation, but is sometimes defined as implantation or even "the point at which human life begins" and is thus a subject of semantic arguments within the abortion debate. Gastrulation is the point in development when the implanted blastocyst develops three germ layers, the endoderm, the exoderm and the mesoderm. It is at this point that the genetic code of the father becomes fully involved in the development of the embryo. Until this point in development, twinning is possible. Additionally, interspecies hybrids which have no chance of development survive until gastrulation. However this stance is not entirely warranted since human developmental biology literature refers to the "conceptus" and the medical literature refers to the "products of conception" as the post-implantation embryo and its surrounding membranes. The term "conception" is not usually used in scientific literature because of its variable definition and connotation.
Human fertilization is the union of a human egg and sperm, usually occurring in the ampulla of the fallopian tube.
There is a specific sequence of events that occur in fertilization:
- The sperm passes through the corona radiata, the outermost cell layer of the egg.
- The sperm breaks through the zona pellucida.
- This occurs with the aid of several enzymes possessed by the sperm that break down the proteins of the zona pellucida, the most important one being acrosin.
- When the sperm penetrates the zona pellucida, the Acrosome reaction occurs. This makes the egg impermeable to any other sperms and prevents fertilization by more than one sperm.
- The cell membranes of the egg and sperm fuse together.
- The female egg, also called a secondary oocyte at this stage, completes its second meiotic division. This results in a mature ovum.
- 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 male and female pronuclei fuse to form a new nucleus that is a combination of the genetic material from both the sperm and egg.