Sequence of insemination and pregnancy

Fertilisation of the egg describes the process that takes place when a mature egg and a mature sperm (spermatozoon) meet. From a purely medical point of view, this involves a union of two simple (haploid) gametes to form a diploid egg (zygote) in the ampullary part of the fallopian tube. Three stages can be distinguished in the fertilisation of the egg.

Once the egg has been fertilised, the phase of early development begins after fertilisation. This lasts one to three weeks and includes the migration of the egg from the tube towards the uterus. Furthermore, the implantation of the egg in the endometrium (endometrium) follows with the beginning of neurulation.

After early development, the embryonic period begins. It extends from the fourth to the eighth week of pregnancy and is important for determining the position of organs and the basic shape of the body. In the ninth to the thirty-eighth week of pregnancy, organ growth and sexual maturation take place in the foetal period.

In the first stage, the spermatozoon penetrates the outermost envelope (corona radiata) of the egg cell. – In the second stage, the inner sheath (zona pellucida) is dissolved enzymatically by the acrosome. – In the last stage, the plasma membranes of the two gametes fuse together.

Requirements for fertilization

In order for fertilisation (fertilisation) to take place, the sperm of the man must overcome many obstacles to reach the fallopian tube. The acidic environment in the vagina already reduces the 300-500 million sperm initially present in the ejaculate to 300,000. It is only in the cervix that better conditions for sperm are found, since an alkaline environment prevails there as in the ejaculate.

The longitudinal threads in the cervical mucus are conducive to the further movement of the sperm. Spermatozoa with malformations get caught in this latticework of threads. A further part of the sperm is temporarily stored in indentations (crypts) of the cervical canal and later slowly released again.

In the lining of the uterus, further sperm are sorted out, leaving about 500-800 sperm in the area of the tube (fallopian tube). As the sperm make their way through the female genital tract, they go through the final stage of their maturation (capacitation). During this process, the splitting enzymes in the sperm head in the area of the acrosome are activated in order to penetrate the egg cell from the outside.

If one of the hundreds of sperm succeeds in penetrating the egg cell completely with its two sheaths, the other sperm are blocked from entering the egg cell. The cortical reaction of the egg cell causes the outer layer to harden, thus preventing polyspermy. Multiple sperm penetration leads to non-viable eggs.

As soon as the cell membranes of the two germ cells fuse together, the pronucleus with the haploid one-chromatide chromosomes are formed in the sperm and the egg cell. While the secondary oocyte completes its 2nd maturation division, the spermatozoon transforms into the pronucleus by separating from the sperm tail. The coming together of both haploid nuclei is called conjugation. The nucleus resulting from the conjugation is the zygote.