There are two main types of reproduction - asexual and sexual. Asexual reproduction occurs without the formation of gametes, and only one organism participates in it. In asexual reproduction, identical offspring are usually produced, and random mutations serve as the only source of genetic variability.

The beginning of a new life is conception. It occurs when a male reproductive cell, a spermatozoon, penetrates a female egg. The union of a sperm with an egg is called fertilization. The whole process of conception, both in man and in animals, is one of the greatest mysteries of nature.

The semen remains in the vagina near the opening of the cervix. Millions of tiny sperm are so weak that they can only live for a few minutes if they can't get into the uterus.

The spermatozoon has a tail that helps it to move. In general, spermatozoa are very similar to miniature tadpoles, moving their tail from side to side. After the sperm cells are near the cervix, they need to swim through the mucous barrier that closes the exit to the uterine cavity. Ten million spermatozoa die without being able to do this. And those that have penetrated the cervix move forward a few centimeters and enter the uterine cavity. There they reach two openings where the fallopian tubes begin. Swimming through the narrow passage of the fallopian tube, they can finally meet the egg. However, such a meeting can occur only within two to three days of each month. Usually once a month, a female egg, no larger than a pinhead, leaves the ovary. This is called ovulation. Ovulation usually occurs between two menstrual periods.

After leaving the ovary, the egg finds its way into the opening of the fallopian tube. Inside the fallopian tube, the egg travels very slowly towards the uterus. She is helped to move by tiny hairs that cover the fallopian tubes from the inside. These hairs are called cilia - they are so small that they can only be seen under a microscope. The journey through the fallopian tube, which is about 10 cm long, takes 3 to 5 days. During this time, the egg can meet the sperm there.

If this happens, then one of the spermatozoa is likely to penetrate into it and they will unite into one cell. This is called fertilization. When this happens, all other sperm cells die. Sometimes 2 - 3 eggs or more appear from the ovaries. They can also be fertilized. Then they will develop and then twins or triplets will be born. In addition, twins can appear as a result of the division of one fertilized cell into two.

However, the union of sperm and egg does not always lead to the birth of a new life. The fertilized cell can continue down the fallopian tube and into the cervix a few days later. And in order for pregnancy to occur, a fertilized egg must penetrate into the inner lining of the uterus, gain a foothold in it and begin to grow. All this is very similar to what happens to a grain planted in the ground. If the membrane is infected or blood circulation is disturbed in this part of the uterus, the egg will not be able to firmly gain a foothold there and begin to grow.

INTRAUTERINE DEVELOPMENT

As soon as the sperm enters the egg, the nuclei of these cells combine into one nucleus. The nucleus is the main part of any cell, including the egg and sperm. Under a microscope, it looks like a large dark spot. Inside the nucleus are the chromosomes, genes, and other vital elements of the cell. Fiber-like chromosomes contain genes that are responsible for the appearance and character of the unborn child, such as how he will look and how he will behave. It depends on the chromosomes and the gene which of the parents it will look like. They determine the color of the child's eyes; brown, green, blue. The future growth of the child - high or low - also depends on chromosomes and genes.

The most important thing that depends on the chromosomes of the sperm is the sex of the unborn child; a boy or girl will be born. The chromosomes of the egg cell have no effect on this.

Each normal germ cell contains 23 chromosomes. When a sperm cell and an egg unite, their chromosomes join and form pairs. 22 chromosomes combine, and 23 determines sex, about half of all sperm will have an X chromosome, and the other half will have a Y.

The gender of the baby is determined at the time of fertilization. It is determined by spermatozoa, which are of two types: half is female (x-chromosome), and the other half is masculine (y-chromosome). Eggs are only female, they always have the same chromosomes (x-chromosomes).
Sex cells are formed in the sex glands. Eggs are produced in the ovaries, and spermatozoa are produced in the male gonads (testes). They produce a huge number of spermatozoa, small mobile cells consisting of a head, neck and tail.

For the normal process of sperm formation, a temperature below body temperature is required. Therefore, the testicles are removed from the body cavity. The ducts of the testes, prostate, and seminal vesicles empty into the urethra, which runs inside the penis. Inside the female body, they can be in a viable state for 2-4 days.

The ovaries are paired organs located in the abdominal cavity. They produce eggs. The release of one mature egg into the body cavity is called ovulation and occurs on average once every 28 days. The mature egg released from the ovary enters the oviduct (fallopian tube), where it lives for only 1 day and can merge with the sperm that entered there through the vagina during intercourse.

If fertilization does not occur, then the mucous membrane of the uterus is rejected, this is accompanied by blood discharge.


If fertilization occurs, the resulting zygote immediately begins to divide, resulting in a lump consisting of small identical cells. Such an embryo descends through the fallopian tube into the uterus - a thick-walled sac-like organ, where it is introduced into the mucous membrane, rich in blood vessels.

2 days after entering the uterus, the embryonic period begins. During this period, the placenta is formed - through it there is a connection between the body of the mother and the fetus. From the moment the placenta is formed, the period begins. lasts 9 months, then childbirth occurs.


In humans, periods of accelerated growth alternate with its slowdown. The most active and rapid growth occurs in the first year of life (body weight increases by almost 3 times). Infancy (1st year of life)
The first month is considered the neonatal period. The posture of the newborn resembles the position of the fetus in the uterus. Most of the day he sleeps, waking up only at the time of feeding. Newborn care requires special attention. During the 1st year of life in the child's body there are many changes in the motor system.

At the end of the 1st month of life, he straightens his legs, at the 6th week he raises and holds his head; sits on the 6th month, at the end of the 1st year tries to take the first steps.

The psyche develops no less intensively during this period. At the second month, the child smiles, at 4 months he takes toys in his mouth, explores them, begins to distinguish between adults. Good physical and mental development of the child is determined by a rational regimen.

Early childhood (from 1 year to 3 years)

The child grows rapidly, eats the same food as adults, the desire for independent knowledge of the world, the craving for self-respect. The child walks well and masters various ways of manipulating objects. Appearing motor skills. During the game, the child imitates the actions of adults.

Preschool period (from 3 to 7 years)

Preschool children show great interest in the world around them. Curiosity, the period of questions - this is how you can call this period. The brain grows and forms and inner speech is formed. The child actively plays, talks to himself (formation of speech). Outdoor games form the muscular apparatus.

School period (from 7 to 17 years old)

All organs and systems are rebuilt. Difficult period of admission to school. The child masters writing, learns a lot about the world around him, learns the experience accumulated by many generations of people. Training accelerates the development of skills and abilities. Collective influence in public work, labor education, sports also creates conditions for the development of a harmonious personality. From the age of 11, a child is called a teenager. The restructuring of the body is associated with puberty. Development of the muscles of the back and chest. Weight gain, development of secondary sexual characteristics. In recent decades, in all economically developed countries, the pace of physical and sexual development of children, called acceleration, has accelerated.

Reproduction of their own kind in humans occurs through sexual reproduction, although the case of the formation of one-ovular twins, when the embryo is divided into two organisms in the early stages of crushing, should be considered as a variant of asexual reproduction. A new person is formed by the fusion of germ cells - gametes, formed in the sex glands: eggs - in the ovaries, sperm - in the testes.

male reproductive system

The composition of the male reproductive system includes the genital organs: internal - testes, vas deferens, prostate gland, seminal vesicles, and external - the penis and scrotum. Paired testicles (testicles) are located in the scrotum - a skin-muscular sac located outside the body. During the embryonic development of a boy, the testicles begin and develop in the lower abdomen and descend into the scrotum shortly before or shortly after birth. The location of the testicles outside the body cavity is due to the fact that the normal maturation of spermatozoa (spermatogenesis) occurs only with reduced. The testicles are 3-4 cm long and weigh about 20 g each. The testis consists of seminiferous tubules, in which, from the period of puberty and almost to the end of a man's life, spermatozoa are formed in huge quantities. On average, 1500 spermatozoa are produced per second, and their total number during the life of a man is about 8 * 10 11. Mature spermatozoa are expelled by smooth muscle contraction from the testis into the vas deferens, and then mixed with secretions from the prostate and seminal vesicles to form semen, or seminal fluid. Outside, the sperm enters through the urethra, which passes inside the penis.

female reproductive system

The female reproductive system includes internal (ovaries, oviducts, uterus and vagina) and external (large and small labia, clitoris) genitals. Paired ovaries are located in the abdominal cavity in the pelvic area. They are 3-4 cm long and weigh about 7 g each. The precursors of the eggs are laid in the body of the future girl during her embryonic development. By the time a girl is born, their number is several thousand pieces, and they are called first-order oocytes. Each oocyte is surrounded by epithelial cells that form a vesicle called a follicle. Further maturation of the eggs occurs precisely in the follicles, and only 350-400 eggs reach full maturity. The period during which a woman is able to reproduce lasts about 30 years, after which the activity of the ovaries stops. As the oocyte matures, the epithelium of the follicle grows and a cavity filled with fluid appears in it. Once every 28 days (on average), the wall of the ovary ruptures in the place where the follicle is attached to it from the inside, and the egg enters the abdominal cavity, from where it enters the oviduct (fallopian tube) through the fringed funnel. This process is called ovulation. The moment of ovulation is accompanied by an increase in temperature in the rectum by 0.5°C. Usually ovulation occurs alternately in the left, then in the right ovary.

At the same time, in place of the bursting follicle, a temporary endocrine gland develops - the corpus luteum, where the hormone progesterone is produced. If the egg is fertilized, that is, pregnancy occurs, then progesterone will ensure its flow. If pregnancy does not occur, then the corpus luteum on the 13-14th day after ovulation stops the release of progesterone and collapses. At this time, the mucous membrane of the uterus, which has grown under the action of progesterone, is rejected, while rather large blood vessels burst, menstruation begins.

After the egg has entered the oviduct, it begins to move towards the uterus due to the contraction of the smooth muscles of the oviduct, as well as the movement of the ciliary epithelium of its walls. In the oviduct, the final maturation of the egg occurs, and here it can be fertilized by a sperm cell. If fertilization does not occur, then the egg enters the uterine cavity, where it is destroyed.

Fertilization

Fertilization is the process of fusion of the egg and sperm, when their haploid nuclei merge and a zygote is formed - a cell with a diploid nucleus, which gives rise to a new organism. In humans, normal fertilization occurs in the upper third of the oviduct. The optimal time for fertilization is 12 hours after ovulation. With one ejection of spermatozoa (ejaculation), about 200 million spermatozoa enter the vagina, but much fewer of them penetrate the uterine cavity, and only a few hundred reach the egg descending towards them through the oviduct. Many spermatozoa surround the egg, and the surface of their heads comes into contact with its membranes. At the same time, spermatozoa secrete, which increases the permeability of the membranes of the egg. Finally, one sperm nucleus penetrates the cytoplasm of the egg, and a special membrane forms around it, preventing the penetration of other spermatozoa. The resulting zygote begins crushing in the oviduct.

development of the human fetus

Individual human development (ontogenesis) is divided into two major periods: embryonic and postembryonic. The embryonic period lasts from the moment of fertilization to the birth of a child and lasts about 280 days (40 weeks). Let's consider it in more detail. The crushing zygote moves along the oviduct, a blastula is formed, and on the sixth day after fertilization, the blastula enters the uterus. For about a day, the embryo is in the uterine cavity, and on the seventh day it is introduced into its wall - implantation occurs. In the same period, gastrulation occurs, i.e., germ layers are formed, and extra-embryonic organs are laid - allantois, yolk sac, amnion and chorion. From allantois and chorion, the placenta is subsequently formed, which connects the embryo with the vascular system of the mother's body. The yolk sac temporarily performs hematopoietic functions, in addition, primary germ cells are laid in its walls, which then move to the rudiments of the sex glands. The amnion is a protective bag filled with liquid. In it, the embryo develops all nine months, only at the beginning of childbirth does it break. On the 14-15th day, direct contact is formed between the chorionic villi and the vessels of the uterine mucosa - the formation of the placenta begins, which ends by the end of the 8th week of intrauterine development. The placenta is a disk, part of which is formed from the mucous membrane of the uterus (maternal part), and part - from the villous chorion (children's part). In the placenta, the blood of the mother and fetus does not mix, and the exchange occurs through the thinnest vascular epithelium. All the time of intrauterine development, the fetus receives nutrition, oxygen, hormones, etc. from the mother's blood, and in the opposite direction, metabolic products intended for excretion pass from the fetus to the mother's body. In addition, the placenta is a temporary endocrine gland: hormones necessary for the normal course of pregnancy are secreted from the chorion cells.

By the end of the 8th week, the laying of all organs ends, differentiation of all systems occurs: circulatory, digestive, nervous, excretory. During this period, the embryo has a mass of 5 g and a length of about 4 cm. From the 9th week to the 40th, when the pregnancy ends with childbirth, all fetal systems develop and grow. At the 5th month, the mother begins to feel the movements of the fetus, although it begins to move a little earlier. By the end of the embryonic period, i.e. to childbirth, the fetus has a mass of about 3 kg and a length of about 50 cm.

The process of childbirth is regulated by a number of hormones. During pregnancy, the level of estrogen in the mother's blood constantly rises. Estrogens increase the sensitivity of the uterus to oxytocin, which stimulates contractions of its muscles. Normally, after 40 weeks of pregnancy, the cervix relaxes, and the remaining muscles of the uterus, under the influence of oxytocin, begin to concentrically squeeze the amniotic sac, pushing the fetus out. At the same time, numerous mechanoreceptors of the cervix and vagina are stimulated, their excitation is transmitted to the brain and leads to an even stronger release of oxytocin. Thus, labor activity is maintained until the complete expulsion of the fetus and placenta. When a newborn makes the first cry in his life, his lungs fill with air, and he begins to breathe on his own. After that, the placenta is divided, and its children's part with the membranes of the fetus also comes out. This ends the embryonic period of human development.

Like all living organisms, humans need to reproduce in order to exist as a separate species. It would seem that there is nothing complicated in this process, but in reality everything is somewhat different. The fact is that conception can not be carried out every day, and the subsequent one is multi-stage and very complex.

Conception

In fact, spermatozoa can only occur on certain days. In each menstrual cycle of a woman, there are only 1-2 suitable days. Taking into account the fact that spermatozoa, before reaching the ampullar part of the fallopian tubes, where the fertilization process takes place, must go through a rather complicated path that promises death for the vast majority of them, the chance of conception is not so high.

After fertilization is nevertheless carried out, the resulting zygote is gradually directed towards the uterus. During this journey, the gradual growth of the zygote occurs. Approximately on the 10th day, the embryo enters the uterus, where it is introduced into its wall. After that, the embryo gradually grows and develops. It is worth noting that the first periods of development of a child in the womb are associated with great risks for him. So human reproduction is a rather complex multi-stage process.

Childbirth and their danger

One of the highlights is the birth of a child. It promises a large number of dangers to both the mother and the child. Fortunately, the development of modern medicine makes it possible to save the mother's life in almost 100% of cases, even in the most difficult situations. As for the child, everything is somewhat more complicated with him, but now there are already many different technical means that can help save the life of even the weakest baby. Thanks to and human development has become an easier process.

The danger of childbirth lies in the possibility of damage to the tissues of the mother and child, as well as in hypoxia (lack of oxygen). In order to avoid this, gynecologists use special techniques to help the child pass through the birth canal. In extreme cases, a caesarean section is performed.

Demographic Issues

Biology can tell about how a child develops in the womb and is born. Human reproduction on the scale of the whole species or a separate population is studied by demography. It makes it possible to identify emerging trends in the change in the population size and its sex and age composition. This allows you to take the necessary action.

Today, in many developed countries, there is a gradual decrease in the population. This is due to the fact that human reproduction here is subject to a large number of laws and restrictions that each individual person sets for himself. People in highly developed countries prefer to first resolve all issues related to their careers and only then think about children. As a result, human reproduction occurs predominantly between the ages of 30 and 35. In addition, the population of such countries prefers to "live for themselves" and have only 1 child.

So, modern human reproduction as a species has its own characteristics. Only time can show what trends will emerge in this area in the future.

spermatozoa) and male hormones are formed in the testes, which are located in a special leather pouch - scrotum. Through the vas deferens (tubes about 40 cm long), which flow into the urethra, spermatozoa are excreted from the body of a man.

In women, the reproductive system is located in the pelvic region and consists of the sex glands (ovaries), fallopian tubes, uterus and vagina. Female sex cells (ova) and sex hormones that affect the increase in mammary glands, voice timbre, etc., are formed in the ovaries. Through the uterine (fallopian) tubes, which are suitable for the ovaries, the mature egg moves to the uterus, the lower end of which opens into the vagina. In the uterus, a sac-shaped muscular organ, the fetus develops, which is completely protected from external influences. During pregnancy, the uterine cavity can increase 500 times.

Fertilization - the fusion of a sperm with an egg - occurs as a result of sexual intercourse. Erection (hardening and increase in the size of the male copulatory organ) occurs as a result of narrowing of the veins and dilation of the arteries of the penis. Friction under the influence of rhythmic movements activates sympathetic neurons, which in turn cause contractions of the smooth muscles of the urethra. Seminal fluid is pushed out of the man's body (ejaculation) and poured deep into the vagina. The sensations experienced during this are called orgasm. The spermatozoon reaches the fallopian tubes in 5 minutes due to the movements of the flagellum, as well as contractions of the uterus and tubes. It remains viable for several days. Fertilization occurs after a few hours (under the action of sperm enzymes, the outer membranes of the egg should be destroyed). Further, the fertilized egg begins to divide and enters the uterus through one of the fallopian tubes, where it is introduced into the mucous membrane and begins its development. The placenta is formed - an organ that ensures the supply of nutrients from the mother's body to the embryo.

The development of the embryo occurs quickly - after 7-8 weeks, the structure of its body is already different, although the dimensions are only 2.5 cm. From this period, it is already called a fetus and continues to be in the mother's body up to 38-40 weeks. This period ends with the birth of a child.

The gender of the baby is determined at the sixth week of pregnancy; before this period, in his body there are the rudiments of both the female and male reproductive systems. The presence of the Y chromosome in the sperm subsequently leads to the production of testosterone in the body of the fetus and the development of the male reproductive system. At the 12th week, the fetus has already formed all the main organs.

The signal for the onset of labor is, perhaps, the immunological rejection of the mature fetus by the mother's body. Contractions of the muscular walls of the uterus push the baby out. After the baby is born, the umbilical cord is cut. After a few tens of minutes, the uterus contracts sharply, the placenta separates from the walls of the uterus and exits through the vagina.

Flattening the baby's lungs reduces the resistance to blood flow through them. The valves between the atria, which allowed blood to flow along the “short path” (bypassing the lungs), close, and after a while grow together. Blood supply from now on occurs in two circles of blood circulation.

After giving birth, the child begins to exist already as a separate organism. For some time, the mother still feeds the child with her breast milk (this property brings humans closer to animals of the mammal class), but after about a year, the child usually completely switches to another food. In the first year, the weight of the child triples, the length - by half, the capacity of the stomach - ten times. All internal organs develop intensively, the first milk teeth appear. Puberty, at the end of which a person reaches the ability to reproduce, occurs at 8–17 years of age in girls and at 10–20 years of age in boys. It is accompanied not only by the appearance under the action of hormones of secondary sexual characteristics (the development of the mammary glands, the shape of the pelvis, the pitch of the voice, the distribution of the hairline, the volume of muscles, and others), but also the awareness of oneself as a bearer of a certain sex.