How does fertilization occur? Fertilization is an amazing process

Fertilization is a process that is the fusion of male and female reproductive cells. It would seem that there is nothing so complicated in it, but in reality the situation is somewhat different. Fertilization is not a one-time process.

What are the characteristics of germ cells, and why are they needed?

Female gametes are called eggs. They are quite large in size when compared to other cells. At the same time, oddly enough, in terms of the amount of genetic material, such a cell is twice as inferior to ordinary autosomes (not germ cells). This is due to the fact that eggs are formed due to meiosis, and not mitosis, like the others. Such a mechanism for creating gametes is simply necessary, since it allows the fusion of male and female germ cells to ensure a normal diploid (double) set of chromosomes. So future baby receives traits from both father and mother.

How do the egg and sperm “meet”?

Men's and women's are quite small. Most often they find each other in the ampullary part of the fallopian tubes. This matter is not as simple as it might seem at first glance. This is due to the fact that, in comparison with the lumen of the uterine germ cells, they are negligibly small. It is for this reason that a mechanism is needed for the egg and sperm to find each other. It is called "chemotaxis" (directed movement). Thanks to it, fertilization of a cell with a male gamete is relatively simple.

What happens after fertilization?

After the sperm and egg fuse to form a zygote, embryo development begins. Subsequently, the zygote gradually moves along and after 7-10 days enters the uterine cavity. There, the embryo implants into its wall and begins to receive nutrition from the mother’s body. Over the next few weeks it remains unformed and increases in size quite slowly. Gradually, the body is divided into 3 main tissues - mesoderm, ectoderm and endoderm. Each of them participates in the formation various systems organs of the embryo. During this period, it is very important not to do any harm. developing organism, since this can lead to significant defects in its formation. So, although fertilization is a very difficult process, everything that happens in the future is even more important.

When is it difficult?

Several conditions must be met for successful fertilization to occur. This is very important to know. Today, one of the most common problems is: With this pathology, fertilization becomes impossible. This is due to the fact that the sperm is simply not able to reach the egg.

also in last decades There is a decrease in the concentration of germ cells in sperm and a decrease in its total volume. All this leads to the fact that there are simply too few sperm to reach the ampullary part of the fallopian tubes.

Fertilization is the process of formation of the first stage of an embryo (zygote) as a result of the fusion of a female reproductive cell and a male sperm.

Fallopian tubes (fallopian tubes, oviducts) are the place where fertilization occurs for normal natural pregnancy. When a couple has sexual intercourse, the man ejaculates sperm into the woman's vagina. Up to 150 million sperm can be released in one ejaculation. Sperm are sent to meet the egg in fallopian tube, but they need to get there as quickly as possible, because female cage will die 12 - 48 after leaving the ovary. Although the sperm themselves can survive up to 5 days and wait while in the fallopian tube.

Fertilization can also occur in the upper part of the uterus, or even outside the genital tract in the case of inflammatory disease in the pelvic area. This will create, which is a serious pathology, life threatening women.

How does fertilization occur?

It all starts with a woman's menstrual cycle, which prepares her body for conception. And approximately halfway through the cycle, the egg reaches the ovulation stage and exits into the fallopian tube, where fertilization occurs.

Sperm follow signals from the cervix and vagina and reach the opening in the uterus. Then they make their way through the mucus of the cervix and rise up the uterine lining.

How many sperm survive?

The journey of sperm from the vagina to the egg is only 15 centimeters in length and takes only 20 minutes, but a very small number of sperm will be able to complete it. Of the 150-200 million that come out with the ejaculate, only 2 million will end up in the cervix. The rest will be killed acidic environment inside the vagina or lost in the "backflow" of sperm.

Of the approximately 2 million sperm that reach the cervix, only about 1 million make it into the uterus itself. The rest are stopped by church mucus or swim into dead-end channels inside the walls of the cervix.

Of the approximately 1 million sperm that enter the uterus, only about 10,000 make it into the uterus. top part organ. The rest are attacked and absorbed by white blood cells, which begin to attack as soon as the sperm enters the protective perimeter.

Of the approximately 10,000 that make it to the far end of the uterus, only about 5,000 move in the right direction. The rest are headed down the wrong fallopian tube, and unless both of a woman's ovaries released eggs at the same time (a rare event), those sperm are out of luck.

Of the remaining 5,000 that enter the tortuous space at the junction of the uterus and the tube, only about 1,000 pass on. The rest end up in the mucus lining the joint. Of these, only 200 reach the egg. The rest stick to the mucous membrane of the oviduct or simply die.

Of the last 200 sperm that reach and surround the egg, only 1 enters and fertilizes it. The rest are repelled from the surface of the cell, as its surface changes as a result of a special cortical reaction.

The process of conception in stages

Find out how fertilization occurs. It is very difficult to determine the exact day when you can get pregnant. The best thing your doctor can do is start counting from the first day of your menstrual cycle. This is approximately 2 weeks before you can actually become pregnant.

Fertilization - This is the process of fusion of germ cells. As a result of fertilization, a diploid cell is formed - a zygote, this First stage development of a new organism. Fertilization is preceded by the release of reproductive products, i.e. insemination. There are two types of insemination:

1) external. Sexual products are released into external environment(in many freshwater and marine animals);

2) internal. The male secretes sexual products into the female's reproductive tract (in mammals, humans).

Fertilization consists of three successive stages: the rapprochement of gametes, activation of the egg, fusion of gametes (syngamy), and acrosomal reaction.

Gamete convergence

It is caused by a combination of factors that increase the likelihood of gametes meeting: sexual activity of males and females, coordinated in time, appropriate sexual behavior, excess sperm production, large egg sizes. The leading factor is the release of gamones by gametes (specific substances that promote the rapprochement and fusion of germ cells). The egg releases gynogamons, which determine the directional movement of sperm towards it (chemotaxis), and the sperm secrete androgamones.

For mammals, the length of time gametes stay in the female genital tract is also important. This is necessary for sperm to acquire fertilizing ability (so-called capacitation occurs, i.e., the ability to undergo an acrosomal reaction).

Acrosome reaction

The acrosome reaction is the release of proteolytic enzymes (mainly hyaluronidase) that are contained in the acrosome of the sperm. Under their influence, the membranes of the egg dissolve in the place largest accumulation sperm. Outside is a section of the cytoplasm of the egg (the so-called fertilization tubercle), to which only one of the sperm attaches. After this, the plasma membranes of the egg and sperm merge, a cytoplasmic bridge is formed, and the cytoplasms of both germ cells merge. Next, the nucleus and centriole of the sperm penetrate into the cytoplasm of the egg, and its membrane is integrated into the membrane of the egg. The tail part of the sperm is separated and resorbed, without playing any significant role in the further development of the embryo.

Activation of the egg

Activation of the egg occurs naturally as a result of its contact with the sperm. Occurs cortical reactiontion, which protects the egg from polyspermy, i.e. penetration of more than one sperm into it. It lies in the fact that detachment and hardening of the vitelline membrane occurs under the influence of specific enzymes released from the cortical granules.

Metabolism changes in the egg, the need for oxygen increases, active synthesis begins nutrients. The activation of the egg ends with the beginning of the translational stage of protein biosynthesis (since m-RNA, t-RNA, ribosomes and energy in the form of macroergs were stored in oogenesis).

Fusion of gametes

In most mammals, at the time the egg meets the sperm, it is in metaphase II, since the process of meiosis in it is blocked by specific factor. In three genera of mammals (horses, dogs and foxes), the block occurs at the stage diakinesis. This block is removed only after the sperm nucleus penetrates the egg. While meiosis completes in the egg, the nucleus of the sperm that penetrates it takes on a different appearance - first an interphase and then a prophase nucleus. The sperm nucleus turns into the male pronucleus: the amount of DNA in it doubles, the set of chromosomes in it corresponds to n2c (contains a haploid set of reduplicated chromosomes).

After completion of meiosis, the nucleus turns into the female pronucleus and also contains an amount of hereditary material corresponding to n2c.

Both pronuclei undergo complex movements inside the future zygote, come closer and merge, forming a synkaryon (contains diploid set chromosomes) with a common metaphase plate. Then a common membrane is formed and a zygote appears. The first mitotic division of the zygote leads to the formation of the first two embryonic cells (blastomeres), each of which carries a diploid set of chromosomes 2n2c.

The birth of a new life is a very interesting process. Just two cells, calmly existing separately from each other, merging together, show the world a miracle. The process of conceiving a child is practically no different from how it happens in other forms of mammals, but over the long nine months he needs to go through a more difficult path. Today we want to go a little deeper and look in detail, step by step, at what actually happens in a woman’s body. early pregnancy, when she still does not even suspect that she will soon become a mother. The process of conceiving a child is not the act of love itself between a man and a woman, but what happens after that.

The miracle begins

It’s quite difficult to call it anything else. Indeed, a true miracle that happens most often at night, while parents are fast asleep. The process begins with the release of seminal fluid into the woman’s vagina. In a relatively small amount of a nutrient solution, which also helps reduce the acidity of the vagina, there are about 3 million sperm. Each of them carries an X or Y chromosome, and depending on which of them manages to penetrate the egg first, a boy or a girl will be formed.

But we got ahead of ourselves a little. The process has not yet begun. Spermatozoa capable of fertilization simply entered the body. Now their long and difficult journey to their goal begins. They can remain active for 9 days, and if during this time they are lucky enough to meet a mature egg on the way, then their life will not be lived in vain.

Fertilization

It is so secretive that we can only learn about it from films or books. The woman herself is not able to feel the movement of sperm and their fusion with the egg. Many people mistakenly believe that the entire sacrament is performed in the womb. This is not true. Sperm actually enter this large, muscular pouch through the vagina. However, their journey does not end there. Next they have to pass through the fallopian tubes, where the main prize, a mature egg, may await. For about 24 hours, she retains the ability to fertilize, and, obeying the laid down program, moves towards the uterus. If along this path it is not met and fertilized by a sperm, the egg will reach its goal and be destroyed. Menstruation will begin, after which the cycle will repeat.

In the fallopian tubes

These are paired organs that connect the uterine cavity with abdominal cavity. They are equipped with ciliated epithelium, the cilia of which are directed inward. Of course, for a reason. They are necessary to push the egg towards the uterus. This is where the mature egg is surrounded by sperm. Each of them strives to penetrate her, but only one succeeds, in rare cases two, then there is a chance that twins will be born. It may also be possible that two eggs are released at the same time. In this case, they can both be fertilized, and the mother also bears two children, but they will not be alike, like two peas in a pod.

Surprisingly, the head of the sperm penetrates inside the egg; it contains a special enzyme that slightly dissolves the wall and allows fusion to occur. After this, the tail falls off. It has completed its motor function and is no longer needed. At this moment, the surface of the egg changes. Now it is already a zygote, and other sperm are not allowed to enter it.

The beginning of a long journey

Next we need to take a closer look at how the process of conceiving a child occurs day by day. What's the big picture now? The nuclei of the egg and sperm are combined into one. A zygote is formed. After about a day and a half, division begins. While still in it, it is already turning into a complex complex of cells. And 48 hours after fusion, the zygote completes the first stage of division. Now these are two cells called blastomeres. They are still very large in size, but gradually the number increases and the cells decrease. In this case, the zygote remains the same size. Fallopian tubes are not at all appropriate place for intensive growth.

Quick complication

Every pregnant woman becomes very interested in the process of conceiving a child day by day. How the birth of a new life occurs at a moment when no one has any idea about this miracle. So, the third day. No, not yet pregnancy, it begins its countdown from the moment in the uterus. It is still the third day of its formation. Now the zygote contains from six to eight blastomeres. At this stage, the embryo already has its personal genome.

Day four, the zygote begins to move into the uterus

All this time was not wasted. Now the zygote already consists of 16 cells, and their chaotic accumulation begins to take on a certain shape. In other words, the process is underway compaction of the embryo. Soon the zygote will forever leave the place where the formation of the future little man began. However, this particular period is marked by a certain danger. The fact is that if for some reason the fertilized egg cannot descend into the prepared womb, it may be implanted directly in the fallopian tube.

Since the process of conceiving a child takes place in the fallopian tube, nature provides for the modest size of the zygote, which it retains until it enters the uterus, where there is room for growth and development. However adhesions can lead to a narrowing of the tubes, and the egg simply has no options how to penetrate the wall where it stopped. What happens next? It continues to divide and increase in size, which will lead to rupture fallopian tube. If not accepted emergency measures, then the consequences can be very sad.

Fifth-seventh day

The step-by-step process of conceiving a new life is always very interesting for expectant mother. How amazing it is to watch with awe the changes that take place inside you! But if we talk about this stage, then the woman is often still unaware of her interesting position. And inside the uterus the embryo is already moving freely and looking for a home for final implantation.

Having determined a suitable place for itself, the egg attaches to the uterus. Now her long journey is over, there will be a cozy home here for the next 38-40 weeks. By the way, at this stage the corpus luteum serves as a source of nutrition for her, which means that the woman’s lifestyle does not have any impact on the development process.

The diameter of the embryo now does not exceed 0.5 mm. Literally after a few days, that is, on the 9-10th day of its existence, the zygote is completely immersed in the wall of the uterus. This period lasts about 40 hours and is called implantation.

Feeling of a woman

If this is your first pregnancy, then most likely you will write off all these symptoms as simple malaise. However, women who already have children are much more sensitive to the signals that the body sends. These days you may encounter At first glance, it looks like menstruation, with which it is often confused. However, the bleeding is not profuse and most often goes away on the first day. Sometimes brown discharge continue for 10-14 days, but this is already a signal to see a doctor.

It is now that hCG begins to be produced. Because of it, a woman feels nausea in the morning, weakness and drowsiness. Almost everyone notes that the breasts fill up and become very sensitive. In addition, there is a feeling of slight heaviness in the lower abdomen.

We looked at the process of conceiving a child step by step, day by day. The characteristics of each organism may make some adjustments, but this is the general framework for the birth of a new life. Moreover, note that all this happens even before you notice the first symptoms or notice a delay.

If pregnancy is very desirable

Some await the appearance of the second line on the test with fear, others with hope and trepidation. How can you speed up the process of conceiving a child if you have been ready for motherhood for a long time, but it just doesn’t work out? It is very useful to track the onset of ovulation. Does this mean taking your temperature every morning and drawing graphs for months? In general, yes, but today there are special tests that show the onset of favorable period for conception. The appearance of two stripes indicates that ovulation has occurred. The egg has left the ovaries and is ready to meet the sperm.

A woman’s body must be full of strength and energy, otherwise it simply will not take on such responsibility as bearing a baby. Therefore, it is very important to eat right, eat a lot of vegetables and fruits, herbs, and take vitamins. Concerning traditional medicine, then she also offers a number of ways. For example: tea made from linden flowers improves the functioning of the ovaries, an infusion of carrot seeds is useful for a man, as it improves the quality of sperm. An infusion of boron uterus relieves inflammatory processes and improves the menstrual cycle. Additionally, it is recommended to lie down for 10-20 minutes after sexual contact with your legs raised up. This will allow sperm to enter the uterus faster. And, of course, think about your baby, because thoughts are material.

Fertilization is the process of combining male and female gametes, which leads to the formation of a zygote and the subsequent development of a new organism. During the process of fertilization, a diploid set of chromosomes is established in the zygote, which determines the outstanding biological significance this process.

Depending on the species of organisms in animals that reproduce sexually, external and internal fertilization are distinguished.

External fertilization occurs in the environment into which male and female reproductive cells enter. For example, fertilization in fish is external. The male (milk) and female (caviar) reproductive cells secreted by them enter the water, where they “meet” and unite. Fertilization data sea ​​urchins indicate that within 2 seconds after the contact of sperm and egg, changes occur in the electrical properties of the plasma membrane of the egg. The fusion of the contents of the gametes occurs after 7 seconds.

Internal Fertilization is ensured by the transfer of sperm from the male body to the female body as a result of sexual intercourse. Such fertilization occurs in mammals, and the central point here is the outcome of the meeting between the germ cells. It is believed that the nuclear contents of only one sperm penetrate into the egg of these animals. As for the cytoplasm of the sperm, in some animals it enters the egg in small quantities, in others it does not enter the egg at all.

In humans, fertilization occurs in the upper part of the fallopian tube, and in fertilization, as in other mammals, only one sperm participates, the nuclear contents of which enter the egg. Sometimes there may be not one, but two or more eggs in the fallopian tube, as a result of which the birth of twins, triplets, etc. is possible. For example, in the 18th century. A case was registered in Russia of the birth of 16 twins, 7 triplets and 4 quadruplets (69 children in total) by one mother (the wife of the peasant Fyodor Vasilyev).

As a result of fertilization, the diploid set of chromosomes is restored in the fertilized egg. Eggs are capable of fertilization within about 24 hours after ovulation, while the fertilizing ability of sperm lasts up to 48 hours.

Much remains unclear about the mechanisms of fertilization. It is assumed that the penetration of nuclear material into the egg by only one of many sperm is associated with changes in the electrical properties of the plasma membrane of the egg. There are two hypotheses regarding the reasons for sperm activation of egg metabolism. Some researchers believe that the binding of sperm to external receptors on the surface of cells is a signal that enters the egg through the membrane and activates inositol triphosphate and calcium ions there. Others believe that sperm contain a special initiating factor.

A fertilized egg gives rise to a zygote; the development of organisms through the formation of zygotes is called zygogenesis. Experimental developments in recent years have shown that fertilization of eggs from mammals, including humans, is possible in vitro, after which the embryos developed in vitro can be implanted into a woman’s uterus, where they can develop further. To date, numerous cases of the birth of “test tube” children have been known. It has also been established that not only spermatozoa, but also spermatids are capable of fertilizing a human egg. Finally, it is possible to fertilize eggs (artificially deprived of nuclei) of mammals with the nuclei of their somatic cells.

Unlike zygogenesis, many animal organisms are capable of reproduction in natural conditions through parthenogenesis (from the Greek parthenos - virgin and genesis - birth). There are obligate and facultative parthenogenesis. Obligate parthenogenesis is the reproduction of organisms from an unfertilized egg. This parthenogenesis serves as a means of reproduction for more than 90 species of animals, including some vertebrates. An example of obligate parthenogenesis is the reproduction of the Caucasian rock lizard, represented only by female individuals. On the contrary, facultative parthenogenesis means that eggs are able to develop both without fertilization and after fertilization. Facultative parthenogenesis, in turn, is female and male. Female parthenogenesis is common in bees, ants, and rotifers, in which males develop from unfertilized eggs. Male parthenogenesis occurs in some isogamous algae.

In plants, there are also cases where an embryo develops from an unfertilized egg. As noted above, this phenomenon is called apomix. It is very widely found in many angiosperms, including cultivated ones, such as beets, cotton, flax, tobacco and others.

Along with natural parthenogenesis, artificial (induced) parthenogenesis is distinguished, which can be caused by irritation of eggs using physical or chemical factors, which leads to the activation of eggs and, as a consequence, to the development of unfertilized eggs. Artificial parthenogenesis has been observed in animals belonging to many systematic groups - echinoderms, worms, mollusks and even some mammals.

There is a known form of parthenogenesis, called androgenesis (from the Greek andros - man, genesis - birth). If the nucleus in an egg is inactivated and after that several sperm penetrate into it, then from such an egg as a result of the fusion of male (sperm) nuclei it develops male body. The experiments of V.L. Astaurov (1904-1974), who showed androgenesis on the silkworm, are widely known. These experiments consisted of the following. In the eggs of one species of silkworm (Bombyx mandarina), the nuclei were inactivated using high temperature, and then such eggs were fertilized with sperm of a silkworm of another species (B. mori). Having penetrated the eggs, the latter merged with each other, which gave rise to new organisms, which in their properties turned out to be paternal organisms (B. mori). Crosses of these organisms with B. mori females produced offspring belonging to B. mori.

The role of parthenogenesis and its forms in nature is small, since it does not provide broad adaptive capabilities of organisms. However, its use has practical significance. In particular, B.L. Astaurov developed a method for obtaining parthenogenetic offspring from the silkworm, which is widely used in the industrial production of silk.

In contrast to zygogenesis and parthenogenesis, there is gynogenesis (from the Greek gyne - woman), which is pseudogamy, which consists in the fact that the sperm meets the egg and activates it, but the sperm nucleus does not merge with the nucleus of the egg. In this case, the enabling offspring consists only of females. U individual species roundworms, fish and amphibians, gynogenesis serves as a normal form of reproduction, producing offspring consisting only of females. Gynogenesis can also be induced artificially with the help of factors that can destroy cell nuclei (radiation, temperature, etc.). In particular, cases of artificial gynogenesis have been described in the silkworm and in some species of fish and amphibians. Obtaining such forms may have some practical significance in the case of economically useful species.

As noted above, fertilization in flowering plants (angiosperms) has a significant distinctive feature in the form of double fertilization (S. G. Navashin, 1896), which boils down to the fact that in the embryo sac a haploid egg and a diploid central cell are fertilized by sperm, in resulting in the formation of a diploid embryo and a triploid cell that develops into endosperm cells

Parthenogenesis, androgenesis and gynogenesis are forms of disorders of sexual reproduction. It is assumed that these forms arose during evolution as a result of particular evolutionary adaptations.