For a successful fertilization a mature spermatozoon, with a haploid set of chromosomes, and a mature oocyte, in which the second meiosis is arrested in the metaphase, are required. The union of the two gametes normally occurs in the ampullar part of the fallopian tube. The process in which he spermatozoon penetrates the oocyte is often designated as impregnation. This causes the oocyte to complete its second meiosis and also leads to the formation of a haploid set of chromosomes. The whole fertilization, up to the formation of the zygote (condition in which the united sets of chromosomes of the two pronuclei are in the metaphase of the first mitotic division) lasts roughly 24 hours (stage 1, ca. 1 day, 1). The goal is the replication of the diploid set of chromosomes and the determination of the chromosomal gender.
The mature spermatozoon consists of:
- a head with the acrosome cap and the nucleus
- the neck with two centriols,
- the mid piece with mitochondria,
- the principal piece,
- and the end piece.
The sperm cells go through a series of activation procedures during their ascent through the cervix, uterine cavity and tube, which is known as capacitation. Thereby certain macromolecules, which originate in the seminal fluid, are eliminated from the surface of the sperm cells. Further, the sperm cells are made more mobile and prepared for the acrosome reaction.
After a successful impregnation of the oocyte by a spermatozoon a cortical reaction of the oocyte is triggered, which leads to a polyspermy block. Further, the completion of the second meiosis (meiosis II) thereby is effected. This procedure leads to the expulsion of the second polar body. The haploid nucleic material of the spermatozoon and of the oocyte swells and forms the paternal and maternal pronuclei. They approach each other slowly and roughly 22 hours after impregnation the zygote arises in that the two sets of chromosomes move to a equatorial position within the spindle. The first cell division takes place with the mitotic spindle, which comes from the proximal centrosome of the spermatozoon. With the creation of two daughter cells the fertilization is complete. The preimplantation phase has thereby begun.