A human embryo in the first week of life is a "loose idea" for a future existence. If the sperm does not penetrate into the egg and cell division does not take place within a strictly defined time, the woman will get another chance for fertilization in the next menstrual cycle.
Certainlyspermdoesn't have an easy life. When it enters a woman's body, a murderous race awaits him. It begins with about 300 million sperm, but only 200 reach the final stage. Only the strongest can overcome the arduous uphill journey to the fallopian tubes located in the upper part of the genital tract. The winning sperm finally meetsoocyte , but his problems don't stop there. It must break through the radiant wreath that surrounds it, i.e. the outer envelope. The transparent cover is another obstacle. It will only get inside the egg when it carries a channel in it. Although several sperm can cross the first barrier, usually only one gets inside the egg. That is why haste is indicated here. The moment one of the sperm enters the egg cell, its shell hardens and none of its colleagues trampling on it has no chance.
X, Y chromosomes
determining the sex of the childWhen a sperm cell penetrates inside an egg, its cell membrane fuses with the egg's cell membrane. Only the head with the cell nucleus penetrates inside. There are 23 chromosomes in both the egg and the egg, so when the two nuclei have merged, they also mix, in the new formation - called the zygote - there are 23 pairs of chromosomes. Due to the chromosomes, at the time of conception, the decision about the sex of the baby is also made. If the big race is won by the sperm with the X chromosome in its set, there will be a girl; if Y, the future man grows in the womb of a woman.
Zygote turns into morula
Afterfertilizationthe new life does not have any significant energy reserves, so the embryo travels down the fallopian tube towards the uterine cavity in search of a fertile substrate to feed it. At the same time, the zygote divides continuously in the cleavage process. The division of the zygote into the first two cells takes place after about 30 hours after fertilization. After 40 - 50 there are 4 of them, after 60 hours 7 - 8. The zygote does not change its size at the same time, because it is limited by the sheath that remembers the sperm times.clear. Therefore, the young cells formed inside, called blastomers, are getting smaller. After 3-4 days, there are already 12-16 blastomeres inside. Then the zygote changes its name. It resembles a mulberry fruit, so it was called a morula.
ImportantWhen blastocyst cells do not yet have a specific specialization, they are called embryonic stem cells or primordial germ cells. Each of them retains the ability to divide and, under the right conditions and after obtaining appropriate signals, can develop into a mature cell with a specific structure and functions.
The embryo places itself in the uterine wall
As the morula moves towards the uterine cavity, fluid from the fallopian tubes passes through the disappearing transparent sheath. It accumulates between the blastomers until it finally fills the entire space inside the morula. This creates a single cavity, called the blastula cavity, and the morula changes its name and becomes a blastocyst. The cells arrange themselves in the outer layer (from which the placenta will be formed) and the inner mass of cells forming the embryonic node. It is from there that a man will develop. About 3-4 days after fertilization, the blastocyst falls out of the fallopian tube and after several dozen hours, about 5-6 days, when the transparent sheath disappears, it settles somewhere inside the uterine cavity. The lining of the uterus, or the endometrium, is an ideal place for growth. The blastocyst clings to it, a process known as implantation. Once the embryo has established itself in the wall of the uterus, the cells inside it begin the arduous process of specialization. They arrange in the so-called germ layers. The nervous system, skin and hair will develop from the ectodermal leaf; from endodermal - digestive system with liver, pancreas and thyroid; the mesodermal leaf will form the skeleton, connective tissue, circulatory system, urogenital system and most of the muscles.
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