The development of an incredibly complex human organism begins with a single cell. The beginning of a new life is laid at the moment of the fusion of two small gametes - male and female, sperm and egg. However, for fertilization to occur, germ cells must go through a long process of formation and maturation.
The ovum, the female gamete, is laid and grows in the ovary, a specialized organ of the female reproductive system. There it is surrounded by an island of cells that form the ovarian follicle. The cell lives in the ovary, surrounded by follicular fluid and a membrane until the moment of ovulation.
Embryonic oogenesis
The development of female germ cells (ovogenesis, oogenesis) begins at the embryonic stage of the formation of the body of a future woman. At this time, the eggs are not yet as large and mature as they become at the time of possible fertilization. Now these are only the beginnings of future eggs - oogonia (or ovogonia). They are located in the deep layers of the ovary and multiply by mitosis.
Oogonia-they are ordinary cells that have a normal double set of chromosomes, like the rest of the cells of the body. Before they can give birth to a new person, they will have to lose half of this set.
When a girl is born, her ovaries already contain hundreds of thousands of primary oocytes, most of which will die before they even develop. How many eggs will survive to full maturity? Only 400 – 500 cells.
The development of eggs in the body of a mature woman
After the birth of a girl, new eggs are no longer laid (unlike spermatozoa, which in men are actively produced throughout their lives). How many cells were formed in the embryo, so many will take part in the performance of the reproductive function.
Before the onset of puberty, a woman lives without serious changes in the ovaries, the primary oocytes are calmly waiting in the wings. And then the first egg matures and a cycle is established.
Female menstrual cycle- this is a special form of organization of the maturation of germ cells. After it is finally formed, the period between its beginning and end will take approximately 28 days. The duration may vary depending on individual features body, but normally it is about a month, so the cycle is also called monthly.
Every month, several follicles with small eggs dormant inside them are activated and begin to develop. How much it starts to grow is an individual indicator for each woman. Oocytes of the first order divide in a special way (meiosis) and lose half of the chromosome set, becoming haploid, that is, containing a single set of chromosomes.
How many chromosomes does a person have? There are 46 of them in each cell of the body, connected in pairs. To maintain the constancy of the genome, gametes have only half of this number - 23 chromosomes. Merging, they form a full-fledged zygote.
The primary follicle is then called the secondary follicle, fluid accumulates in its cavity, forming a bubble.
How many stages does in its development to the state of the possibility of fertilizationfuture egg? These are steps like
- oogony, that is, a sex cell that forms on early stage embryo development;
- oocyte of the first order, when germ cells have completed mitotic divisions and have accumulated the necessary nutrients;
- oocyte of the second order, having a half set of chromosomes;
- mature oocyte.
During one cycle, the follicles are normal are activated in any one ovary (left or right). Then they alternate.
Of the several active follicles, only 1 – 2 reach the mature blister stage, the rest undergo reverse development. A large bubble of a mature follicle is visible on the surface of the ovary as a rounded bulge about a centimeter in diameter. The egg in it is already almost ripe, increased in volume due to the accumulated nutrients (yolk), almost ready for fertilization. There are only a few hours left before its release.
Preparing the body for pregnancy
While the egg is diligently maturing in the ovary, the female body is already planning future pregnancy. To ensure a soft landing for a single-celled embryo, it builds up an additional layer of mucous membrane in the uterus, abundantly supplying it with blood vessels.
If two sex cells meet in the sacral act of fertilization, a zygote will be implanted in this layer. If conception does not occur, the body after the end of the cycleyou have to get rid of the extra cells. At their separation are torn blood vessels. This is menstrual bleeding.
The release of a mature egg from the ovary
The vesicular follicle enlarges as much as possible, lifting the integumentary epithelium of the ovary. Now it is called the Graaffian vesicle and stores a completely mature sex cell. At a certain moment, the membranes break and the egg is released into the body cavity.
This critical moment release is called ovulation of the egg. The mature oocyte leaves the follicle that has kept it from external influences and starts an independent life.
But what is an egg to do in the body cavity? How can she meet the sperm? Nature, of course, took care of this: in abdominal cavity the egg lives only a few minutes, after which it is drawn in by the funnel of the fallopian tube, equipped with a fringed border.
The bursting follicle does not leave the stage, it still has an important role to play. In its place, the so-called corpus luteum is formed. This is a temporary endocrine gland that will oversee conception and slow down the development of other follicles by releasing the pregnancy hormone progesterone. If fertilization does not occur, after a few days the corpus luteum will collapse. If pregnancy occurs, this gland will work as long as it takes to form a full-fledged placenta, which will then take over its functions.
How long does an ovum live?
The most important question that worries all women who are planning a pregnancy or who want to protect themselves from it. After all, fertilization is normally possible only for a certain period of time, when the egg makes its journey through fallopian tube into the uterus. How long does this period last?
A mature female gamete is a rather large rounded cell, reaching 150 microns in diameter. She has a significant supply of protein-lipid nutrients in the cytoplasm, thanks to which she lives until she fuses with the spermatozoon and implants in the uterine wall. The cage is enough for 12 – 24 hours (sometimes a little longer) from the moment of exit from the follicle.
Thus, the egg lives after ovulation for about a day. And how long can a spermatozoon withstand in the female genital tract? 3–4 days on average (its lifespan may vary).
In order for conception to take place, the sperm must already be waiting for the female cell in the uterus or hurry to meet her.
Hormonal regulation of oogenesis
All processes in the body associated with the development of germ cells are strictly controlled endocrine system, which is headed by the hypothalamic-pituitary complex. The hypothalamus determines the program of action and implements it through the pituitary gland, whose tropic hormones directly affect the sex glands.
Follicle-stimulating and luteinizing hormones ensure the development of the egg and the timely rupture of the follicular membrane at the time of ovulation.
Menstrual cycle
The countdown of the cycle starts from the first day of menstruation. Bleeding of varying intensity can last from 3 to 7 days. After that, under the influence of estrogen, the uterine endothelium is gradually restored, so that by the middle of the cycle it will again be ready to receive the embryo.
At this time, another batch of eggs develops in one of the ovaries under the influence of follicle-stimulating hormone. A few hours before ovulation, the cells of the dominant follicle secrete a loading dose of estrogen, which causes a response release of luteinizing hormone. Under his influence follicular sheathruptures and the gamete is released from the ovary. An ovulated egg lives for another day, moving towards the uterus and waiting for a meeting with a sperm cell.
Progesterone in the corpus luteum temporarily stops the development of other eggs. If the meeting does not occur, the cycle will repeat itself.
The development of an egg is a complex and lengthy process. In the process of growth, the female gamete undergoes a series of metamorphoses, after which it loses half of the chromosome set and accumulates a significant amount of protein. All these transformations occur in the sex glands - the ovaries and are regulated by the endocrine system.
An egg for humans and many animal species is a valuable nutritious product. But its original meaning is to give life to a bird child, a chick. In this article, using the example of a chicken, we will consider how an egg is born, why each of them is needed. separate part and how the chick develops.
The structure of the egg.
What is an egg made of? The most famous, but not the only, parts of an egg are the yolk, white, and shell. Yolk- this is the future chicken itself, more precisely the chicken egg.
If this egg is fertilized, under favorable conditions, it can begin to develop into a chicken. However, for the development of a chicken, building material is needed for its tissues and organs. It receives this building material from egg squirrel through a network of special vessels formed at the very beginning of the development of the embryo. Shell allows the egg to maintain a stable shape and protects the future chicken from injury and the penetration of pathogens. Two layers help in this shell shell membranes. Between the shell and shell membranes is air chamber which provides the chick with oxygen.
In addition, upon closer examination of the contents raw egg can be considered white clots in protein - cords, which allow you to keep the yolk of the egg in a constant position relative to other parts of the egg. Approximately in the middle. This is necessary so that the embryo can receive nutrients from protein from all sides around it. However, with strong shaking, these flagella can come off and then the yolk begins to float freely around the egg. Naturally, the future chicken in the egg will no longer be able to develop normally.
But you can distinguish a fertilized egg by a small white spot in yolk. It is clearly visible in a raw egg or through an ovoscope. This germinal disc.
How is an egg formed?
The eggs, which then grow into the yolk, are born in the body of the chicken while still in its embryonic state. Those. at the little chick female there are already a huge number of eggs inside. However, they begin to ripen when the chicken reaches a certain, so to speak, childbearing age. Usually it is 5 - 6 months. From this age, every year during the life of a chicken for a certain period, one of the eggs matures approximately once a day. It grows to the size of the yolk and begins its movement along the chicken oviduct, becoming covered with new layers: first with several layers of protein, then with shell membranes and, finally, with the shell. The whole process takes 22 - 25 hours, and most of this time is spent on the formation of the shell.
Is a rooster needed in a flock?
A hen doesn't need a rooster to lay eggs.. The maturation of the eggs occurs after a certain period, regardless of whether her rooster tramples or not. But what really depends on it is the season, light intensity, temperature, feeding, age and health of the laying hen. For example, no one keeps roosters for laying hens at industrial poultry farms. But those hens whose eggs will be used for incubation are fertilized by artificial insemination with specially prepared seed from the manufacturer's rooster.
And yet, in the backyard it is better to have at least one rooster for 10-12 hens. Chickens will walk near him, and not scatter to neighboring yards. And they will not leave the yard, having heard the voice of a neighbor's rooster. Yes, and their egg can be used for incubation or incubation, for example, when breeding chickens.
An ovum (egg) is a female germ cell that is separated by a female or bisexual (at) individual and, after merging with a male germ cell (or during parthenogenesis), gives a new organism of the corresponding species. A fertilized egg - a zygote - is a qualitatively new cell, a unicellular embryo of a new, daughter organism. The ovum is a large, usually devoid of active mobility cell; it provides the embryo of the future organism with the bulk and, especially in oviparous animals, with nutrient material (yolk). The egg cell is hundreds of times, and in animals with yolk-rich eggs, it is millions of times more. The egg of most animals is surrounded by protective membranes.
In humans, eggs develop in the female sex gland - the ovary (see). In the process of development of female germ cells - oogenesis - three phases are distinguished: 1) the reproduction period, during which the primary, multiplying, give rise a large number oogonia (predecessors of the egg); 2) a period of growth, when there is an increase in the nucleus and cytoplasm and the accumulation of yolk inclusions, and 3) a period of maturation, which always includes two cell division, resulting in the formation of a mature egg with a halved number (reduction of chromosomes) and two or three reduction bodies.
In a human (female) fetus, the number of oogonia as a result of their mitotic division reaches 400,000. By the end, the oogonia cease, and they turn into first-order oocytes. Each oocyte is surrounded by follicular cells, together with which it forms a primordial (primary) follicle. Only a small part of the oocytes present in the ovary of a newborn girl matures and gives rise to an egg. The rupture of a mature follicle and the release of an egg from the ovary, which occur from a period, is called ovulation (see). The second division of maturation occurs already in the oviduct. Promotion of the egg through the oviduct to the uterus lasts 2-5 days. The human egg has a spherical shape, surrounded by a shiny membrane and a layer of follicular cells that form a radiant crown. Its diameter is on average 120-150 microns. The egg cell retains the ability to fertilize for 1-2 days, after which it dies and is destroyed. Development of a fertilized egg - see Pregnancy,.
Ovum (egg cell) - a female germ cell, or female gamete, separated during sexual reproduction by a female organism (in hermaphroditic organisms - by a bisexual individual) and giving rise after fertilization or without it (during parthenogenesis) to the body of a new, daughter generation. The eggs develop in the ovary. Their predecessors during oogenesis are oogonia.
Stopping to multiply, oogonia turn into oocytes of the first order, are surrounded by follicular cells that feed them, and enter a period of growth. As a result of two maturational divisions (the period of maturation), of which one is reduction (see Meiosis), the oocyte of the first order, separating two reduction bodies, first turns into an oocyte of the second order, and then into a mature egg cell with a haploid (unpaired) chromosome set . The release of an egg from the ovary into the female genital tract is called ovulation (see Menstrual Cycle). In mammals and humans, another oocyte of the first order ovulates in the metaphase of the first division of maturation, which occurs in the initial parts of the oviduct. During fertilization, the sperm enters the egg, their haploid sets of chromosomes combine, and a unicellular embryo is formed - a zygote with a diploid (double) set of chromosomes. By repeated mitotic division, the zygote gives rise to a multicellular embryo.
Unlike sperm (see. Sperm), which actively provides a meeting with the egg during fertilization (see) and therefore must; to be mobile and free from the supply of nutrients, the egg delivers to the embryo most of the educational cytoplasm and the yolk in the form of granules consisting of proteins and lipoids (lecithin, etc.), accumulated by the growing oocyte while still in the ovary (phase of "large growth"), in the egg two poles: the animal, in the area of which the separation of the reduction bodies occurs and where the nucleus is displaced, and the vegetative (the yolk is displaced in its direction). Because specific gravity the yolk is higher than the rest of the cytoplasm, the eggs of animals developing in water are suspended in the latter with the animal pole up. Depending on the number and distribution of yolk inclusions, there are: a) isolecithal cells - there is little yolk in them and it is evenly distributed throughout the cytoplasm (many coelenterates, echinoderms, lancelet); such eggs perform complete uniform crushing; b) alecithal cells devoid of yolk (the embryo feeds on the yolk cells surrounding the egg, for example, in flukes); c) telolecital cells - there is a lot of yolk and it is concentrated in the vegetative parts of the egg (amphibian); such eggs undergo complete but uneven cleavage (the animal parts of the egg cleave faster and form smaller cells). The oocytes of these types, which perform complete cleavage and are entirely spent on the formation of the body of the embryo, are called holoblastic. In the centrolecithal eggs of arthropods, the surface layer of the cytoplasm of the egg is free from yolk, and it is involved in crushing (partial surface crushing). In many animals (cephalopods, fish, reptiles, birds, echidnas, and platypuses), the ovules are of a sharply telolecithal type, overloaded with yolk, which makes up most of the mass of the egg; almost free of yolk and retains the ability to crush only a small germinal disc at the animal pole (partial discoidal crushing). Oocytes that perform partial (superficial or discoidal) crushing are called meroblastic; part of such an egg is used to build the embryo. Viviparous mammals that descend from ancestors with yolk-rich eggs (extinct forms of reptiles), in connection with the transition to supplying the developing embryo with nutrients at the expense of the mother's organism, during evolution again acquired microscopically small (in humans, 100-150 microns in diameter), poor yolk isolecithal eggs undergoing complete crushing.
In the cytoplasm of the eggs of a number of animals there is a preliminary structure, that is, areas used up for the formation of various specific sheets, organs and tissues of the embryo. In the processes of fertilization, the surface, cortical, layer of the cytoplasm of the egg, containing cortical granules, plays a role; after penetration of the sperm, the latter are pushed to the surface and, merging, form a fertilization membrane that prevents other sperm from penetrating the egg.
There are three types of egg shells: primary, produced by the egg itself (fertilization shell), secondary (produced by follicular cells) and tertiary (produced by the glands of the female genital tract, for example, protein shell and shell chicken egg). The egg of mammals and humans is surrounded by a transparent membrane (zona pellucida) produced by the epithelial cells of the follicle in which the oocyte develops in the ovary and, therefore, represents the secondary membrane. It is permeated with thin tubules, through which they pass to the cytoplasm of the egg cytoplasmic processes follicular cells. During ovulation, the egg leaves the ovary surrounded by a radiant crown (corona radiata) from follicular cells; as it moves along the oviduct, and especially when meeting with sperm, the egg loses the radiant crown (egg denudation).
The female body is full of secrets and peculiarities. Unlike men, the body of the fairer sex is subject to cyclical changes. This article will tell you about the features of the female body. You will find out how many eggs a woman has. Also find out Interesting Facts about the development of this gamete.
female body
The body of the fairer sex was created to reproduce offspring. It is in this process that the oocytes take the main part. It is worth saying that a male gamete is also needed for fertilization. Only when these two parts merge, an embryo is formed, which subsequently becomes an embryo.
How many eggs does a woman have?
The sex of the unborn child is laid in the womb at about 8 weeks of development. It is during this period that the ovaries and uterus begin to form in future girls. Up to the 20th week of development, the number of female follicles increases. It reaches its peak around the middle of pregnancy. How many eggs does a woman have at this moment? Scientists say that the number of gametes in this period is from 6 to 8 million.
Then the reverse transformation takes place. Female gametes begin to undergo selection. However, most of them disappear. By the time of birth, a girl's ovaries contain about one million follicles. However, not all of them will be able to bear offspring.
Beginning of puberty
How many eggs does a woman have who is fertile? Around the age of 12-17, a girl experiences her first menstruation. Thereafter, within one year, female cycle. It is from this moment that the fair sex can become a mother. A healthy woman has 10 to 20 ovulations per year. During this period, the fair sex has about 300 thousand eggs. This number is decreasing every month.
Diseases play an important role reproductive system. If a woman, for one reason or another, removes part of the ovary or an organ completely, then the number of gametes capable of fertilization is significantly reduced. Also the number of eggs big influence provide various hormonal preparations. So, taking oral contraceptives inhibits the maturation of follicles and stops ovulation. Is it possible to save some of the cells in this way? Doctors give an affirmative answer to this question.
According to statistics, by the age of 35, about 25 thousand cells remain in the ovaries of women. If, from the beginning of puberty, a representative of the weaker sex begins to take oral contraceptives, that is, there is a chance to increase this figure.
Ovulation
How many eggs does a woman mature each month? It all depends on the reproductive system and genetic predisposition.
In an average woman, at least 10 eggs should come out of the ovary per year. Physicians admit two or three not consecutive. There are also representatives of the fair sex, in whom more than one gamete can be secreted in one menstrual cycle. Most often, these are those ladies in whose family the birth of two or more children was observed at the same time.
Conception
How many women? Scientists call the time interval from several hours to a couple of days.
It is worth noting that spermatozoa are able to live in the female body for up to one week. That is why contact is not always needed during ovulation for fertilization. Male gametes can wait for follicle rupture in reproductive organ or As soon as ovulation occurs and the cell is released from the follicle, the sperm enters it.
Features of the structure of germ cells
How many chromosomes are in a human egg? Doctors have established the fact that every month a woman produces cells that are different in structure and content. So, normally the sex gamete contains 24 chromosomes. It is these eggs that are found in most cases. However, if ovulation occurred, as a result of which a gamete with a large set of chromosomes came out, then there is a possibility of having a child with pathological syndromes.
Conclusion
Now you know the main interesting facts about eggs and their number. Currently, physicians have developed laboratory research, which allow you to find out how many eggs are left in a woman's body. To do this, you need to pass tests for follicle-stimulating, luteinizing and The study is carried out using venous blood for several days. The data will allow you to assess the state of health reproductive function any woman. Keep the number of your germ cells under control and always be healthy. I wish you success!
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Keywords: egg formation Source (book):
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Process egg formation also called ovogenesis. An egg is formed in the ovary and oviduct - the main genital organs of the reproductive system of birds.
In the ovary, the formation of the egg takes place, having matured, the egg passes into the oviduct, where it is fertilized by the sperm. Then, in its movement along the oviduct, the fertilized egg collects numerous shells. This is how it goes egg formation.
Having released the egg, the ovary saturates it with spare nutrients, mainly lipids. Thus, both the egg itself and the future yolk are formed. Nature lays two ovaries for birds, but, as a rule, only one develops, usually it is the left one. The ovary is located in the abdominal cavity. In females that are in the process of reproduction, the ovary looks like a bunch of grapes with berries of different sizes, since the degree of maturity of the eggs is different. For example, an ovary domestic chicken is able to form up to three thousand eggs, but depending on external conditions, it can lay from one and a half dozen to several hundred eggs. The ovaries of wild birds produce fewer eggs, but their number is still greater than what is eventually laid in the nest. In some cases, the weight of the egg can be up to 25% of the weight of the female.
Process egg formation continues after the release of the egg through a special funnel into the oviduct. There, fertilization by spermatozoa occurs, and the development of the embryo begins - embryogenesis. Having moved to the glandular part of the oviduct, the egg is saturated from the glands of this section with albumins and various secrets, which will form protein eggs. As it moves into the uterus, other membranes are formed around the cell, which are already in the uterus dressed in a calcareous shell.
The uterus ends the glandular part of the oviduct. The shell is formed in the uterus due to calcium, which is washed out by the blood from the bones of the bird.
Time egg formation in domestic chicken it consists of the following segments: the egg is in the funnel of the oviduct for about 45 minutes, in the glandular part for 4-5 hours, and in the uterus - up to 20 hours. The movement of the egg is provided by peristalsis, it has a rotational-translational character, which ensures its uniform coverage with all the necessary shells.
The egg leaves the body of the female simultaneously with the formation of a bactericidal cuticle. It comes out blunt end forward.
Notably, the oviduct is also capable of storing spermatozoa. Chickens keep them up to 20 days, and turkeys up to 60. It is this feature that allows some species of birds to re-lay without mating.
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