With procreation (from ahd. Giziogon "procure, manufacture"), fertilization , fertilization or rarely also procreation are understood the processes that lead to the formation of a zygote from an egg cell (oocyte). In humans, for example, two germ cells fuse together : the male sperm and the female egg.
If the offspring arise from unfertilized egg cells, we speak of a virgin generation or parthenogenesis .
The process of natural fertilization in humans
In the case of an ejaculation during vaginal intercourse , about three to five ml of sperm (seminal fluid) of the man enter the female vagina . This corresponds to around 200–600 million sperm. For most of the cycle, they die within ½ – 3 hours in the acidic environment of the vagina. As soon as estrogen-stimulated cervical mucus is formed during the follicle maturation phase, the sperm can survive in this cervical mucus in the cervical glands of the cervix for up to 5 days and wait for ovulation. On their way to the fallopian tube, the sperm can only be procreative when certain proteins in the sperm and on the sperm are removed by female enzymes. The process of capacitation lasts for a few hours, during which the sperm cannot fertilize an egg. Only a few hundred sperm reach the fallopian tube. These processes taking place in women and all subsequent processes discussed here are collectively referred to as conception .
Further development depends on whether there is a fertile egg cell in one of the fallopian tubes or whether ovulation is imminent. The sperm can stay alive in the fallopian tube for several days. When ovulation occurs, the sperm move towards the egg. Hormone-controlled processes or the influence of a substance released by the egg, for which the sperm have a scent receptor (OR1D2 and hOR17-4), are suspected of being able to find the egg . The sperm are likely to move towards the concentration gradient of a fragrance. Recent studies have shown that the bourgeonal fragrance , which is also present in the scent of lily of the valley, the above. Activated scent receptor in the sperm and thus leads to positive chemotaxis . However, other factors are likely to play a role as well. According to the latest findings, a control system of the sperm is activated by the female hormone progesterone , which serves the egg cells as an attracting messenger substance, which increases the calcium concentration in the sperm . The flapping pattern of their tail is changed so that the sperm can target the egg.
Docking on the egg cell
Egg cells are surrounded by a loose layer of follicular cells . The zona pellucida , a layer of various closely meshed glycoproteins, is also located above the plasma membrane of the egg cell . The sperm must pass through both in order to be able to fuse with the egg cell. Special glycoproteins of the zona pellucida bind to the sperm heads that hit it and trigger the fusion of the acrosome with the zona pellucida . The enzymes stored in the acrosome dissolve the zona pellucida and enable the sperm to reach the plasma membrane below. The acrosome reaction further activates and exposes proteins on the sperm that specifically bind to appropriate receptors on the egg's plasma membrane.
Upon contact, the sperm and egg now fuse and the entire sperm is drawn into the egg. At the same time, the membrane of the egg cell is depolarized, which prevents further fertilization with other sperm cells for a short time ( polyspermia ). The depolarization is replaced by the "slow block against polyspermia", in which the zona pellucida changes its structure through enzymes from the cortical granules and protects itself against further penetration by sperm.
Once the sperm has been absorbed into the egg cell, it ends its second meiosis or meiosis II. In doing so, it cuts off one last polar body . As before, it now only has the haploid single-chromatid chromosome set. The male and female haploid one-chromatid chromosomes transform into haploid two-chromatid chromosomes. This is how the male and female pronuclei are formed. These now combine to form a complete, double or diploid two-chromatid chromosome set. This completes the actual procreation process, a fertilized cell has formed that contains the genetic material of both parents.
The fertilized cell, known as the zygote, begins cell division one day after conception. It reaches the uterus after about three days in the 12 to 16 cell stage, where it is implanted in the uterine lining about five to six days after ovulation , which, according to the prevailing opinion, starts pregnancy . If the implantation was successful, the human chorionic gonadotropin (hCG hormone for short) is produced in increasing quantities, which means that a pre-period pregnancy test is possible from this day on.
Seasonal fluctuation in the fertility rates in humans
Up until around the 1960s, most societies in the northern hemisphere had verifiably more children - up to 10% above the annual mean - than in the other seasons. In the 1960s and 1970s, this changed comparatively abruptly: the maximum became less pronounced and extended into summer and sometimes even into autumn. An obvious explanation for this process is the increasing independence of humans from seasonal fluctuations in temperature and sunlight due to advancing technology and ever larger proportions of the population who have jobs that are independent of meteorological conditions.
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