The development of the human body begins from the very first day of fertilization of an egg by a sperm. The stages of embryogenesis are counted from the moment the cell begins to develop, which subsequently forms an embryo, and from it a full-fledged embryo appears.
The development of the embryo fully begins only from the second week after fertilization, and starting from the 10th week, the fetal period is already underway in the mother’s body.
First stage of zygote
Absolutely all somatic cells of the human body have a double set of chromosomes, and only sex gametes contain a single set. This leads to the fact that after fertilization and the fusion of male and female germ cells, the set of chromosomes is restored and becomes double again. The resulting cell is called a “zygote”.
The characteristics of embryogenesis are such that the development of the zygote is also divided into several stages. Initially, the newly formed cell begins to divide into new cells of different sizes, called morulae. The intercellular fluid is also distributed unequally. A feature of this stage of embryogenesis is that the morulae formed as a result of division do not grow in size, but only increase in number.
Second phase
When cell division ends, a blastula is formed. It is a single-layer embryo the size of an egg. The blastula already carries all the necessary DNA information and contains cells of unequal size. This happens already on the 7th day after fertilization.
After this, the single-layer embryo goes through the gastrulation stage, which is the movement of existing cells into several germ layers - layers. First, 2 of them are formed, and then a third appears between them. During this period, the blastula forms a new cavity called the primary mouth. The previously existing cavity completely disappears. Gastrulation allows the future embryo to clearly distribute cells for the further formation of all organs and systems.
From the first formed outer layer, all skin, connective tissues and the nervous system are formed in the future. The lower layer, formed by the second, becomes the basis for the formation of the respiratory organs and excretory system. The last, middle cellular layer is the basis for the skeleton, circulatory system, muscles and other internal organs.
The layers in the scientific community are called accordingly:
- ectoderm;
- endoderm;
- mesoderm.
Third stage
After all of the listed stages of embryogenesis have been completed, the embryo begins to grow in size. In a short time, it begins to look like a cylindrical organism with a clear distribution into the head and tail ends. The growth of the finished embryo continues until the 20th day after fertilization. At this time, the plate previously formed from cells, the predecessor of the nervous system, is transformed into a tube, which later represents the spinal cord. Other nerve endings gradually grow from it, filling the entire embryo. Initially, the processes are divided into dorsal and abdominal. Also at this time, the cells are distributed for further division between muscle tissue, skin and internal organs, which are formed from all cell layers.
Extraembryonic development
All initial stages of embryogenesis occur in parallel with the development of extraembryonic parts, which will subsequently provide nutrition to the embryo and fetus and support vital functions.
When the embryo has already fully formed and exited the tubes, the embryo is attached to the uterus. This process is very important, since the future functioning of the fetus depends on the correct development of the placenta. It is at this stage that embryo transfer occurs during IVF.
The process begins with the formation of a nodule around the embryo, which is a double layer of cells:
- embryoplast;
- trophoblast.
The latter is the outer shell, therefore it is responsible for the effectiveness of attachment of the embryo to the walls of the uterus. With its help, the embryo penetrates the mucous membranes of the female organ, implanting directly into their thickness. Only reliable attachment of the embryo to the uterus gives rise to the next stage of development - the formation of a child's place. The development of the placenta occurs in parallel with its separation from the litter. The process is ensured by the presence of a trunk fold, which, as it were, pushes the walls away from the body of the embryo. At this stage of embryo development, the only connection with the placenta is the umbilical peduncle, which subsequently forms a cord and provides nutrition to the baby for the rest of the intrauterine period of his life.
Interestingly, the early stages of embryogenesis in the area of the umbilical stalk also have a vitelline duct and a yolk sac. In non-placental animals, birds and reptiles, this sac is the yolk of the egg, through which the embryo receives nutrients during its formation. In humans, although this organ is formed, it has no influence on the further embryonic development of the body, and over time it is simply reduced.
The umbilical cord contains blood vessels that carry blood from the embryo to the placenta and back. Thus, the embryo receives nutrients from the mother and removes metabolic products. This part of the connection is formed from the allantois or part of the urinary sac.
The embryo developing inside the placenta is protected by two membranes. In the internal cavity there is a protein liquid, which is an aqueous shell. The baby swims in it until it is born. This sac is called amnion, and its filling is called amniotic fluid. All are enclosed in another shell - the chorion. It has a villous surface and provides the embryo with breathing and protection.
Step-by-step review
In order to analyze human embryogenesis in more detail in a language understandable to most, it is necessary to start with its definition.
So, this phenomenon represents the intrauterine development of the fetus from the day of its fertilization until birth. This process begins only after 1 week after fertilization, when the cells have already finished dividing and the finished embryo moves into the uterine cavity. It is at this time that the first critical period begins, since its implantation should be as comfortable as possible for both the mother’s body and the embryo itself.
This process is carried out in 2 stages:
- tight attachment;
- penetration into the thickness of the uterus.
The embryo can attach to any part of the uterus except the lower part. It is important to understand that this entire process takes at least 40 hours, since only gradual actions can ensure complete safety and comfort for both organisms. After attachment, the attachment site of the embryo gradually fills with blood and becomes overgrown, after which the most important period of development of the future person begins - embryonic.
First organs
The embryo attached to the uterus already has organs that somewhat resemble a head and tail. The very first protective organ to develop after successful attachment of the embryo is the chorion. To more accurately imagine what it is, we can draw an analogy with the thin protective film of a chicken egg, which is located directly under the shell and separates it from the protein.
After this process, organs are formed that provide further nutrition to the crumbs. Already after the second week of pregnancy, the appearance of an allantois, or umbilical cord, can be observed.
Third week
The transfer of embryos to the fetal stage is carried out only upon completion of its formation, but already in the third week one can notice the appearance of clear outlines of future limbs. It is during this period that the body of the embryo separates, the torso fold becomes noticeable, the head stands out and, most importantly, the future baby’s own heart begins to beat.
Power change
This period of development is also marked by another important stage. Starting from the third week of life, the embryo stops receiving nutrition according to the old system. The fact is that the reserves of the egg are depleted by this moment, and for further development the embryo needs to receive the substances necessary for further formation from the mother’s blood. At this point, to ensure the efficiency of the entire process, the allantois begins to transform into the umbilical cord and placenta. It is these organs that will provide the fetus with nutrition and release it from waste products throughout the remaining intrauterine time.
Fourth week
At this time, it is already possible to clearly determine the future limbs and even the places of the eye sockets. Externally, the embryo changes slightly, since the main emphasis of development is on the formation of internal organs.
Sixth week of pregnancy
At this time, the expectant mother should pay special attention to her own health, since during this period the thymus gland of her unborn baby is being formed. It is this organ that will be responsible for the functioning of the immune system throughout life. It is very important to understand that the mother’s health will determine the ability of her child to withstand external stimuli throughout his independent life. You should not only pay attention to the prevention of infections, but also protect yourself from nervous situations, monitor your emotional state and the environment.
Eighth seven-day period
Only from this time threshold can the expectant mother find out the gender of her child. Exclusively at the 8th week, the sexual characteristics of the fetus and the production of hormones begin to develop. Of course, you can find out the gender if the child himself wants it and turns to the right side during an ultrasound.
The final stage
Starting from the 9th week, fertilization ends and begins. By this point, a healthy baby should already have all its organs formed - they just have to grow. At this time, the child’s body weight is actively gaining, his muscle tone increases, and the hematopoietic organs are actively developing; the fetus begins to move chaotically. Interestingly, the cerebellum is usually not yet formed at this point, so coordination of fetal movements occurs over time.
Dangers during development
Different stages of embryogenesis have their own weaknesses. To understand this, you need to consider them in more detail. Thus, in some periods of human embryogenesis, it is sensitive to infectious diseases of the mother, and in others, to chemical or radiation waves from the external environment. If problems arise during such a critical period, the risk of the fetus developing birth defects will increase.
To avoid this phenomenon, you should know all stages of embryo development and the dangers of each of them. Thus, the period of blastula is particularly sensitive to all external and internal stimuli. At this time, most of the fertilized cells die, but since this stage passes through the first 2, most women do not even know about it. The total number of embryos dying at this time is 40%. at the moment it is very dangerous, since there is a risk of rejection of the embryo by the mother’s body. Therefore, during this period you need to take care of yourself as much as possible.
The transfer of embryos into the uterine cavity marks the beginning of the period of greatest vulnerability of the embryo. At this time, the risk of rejection is no longer so great, but from the 20th to the 70th days of pregnancy, all vital organs are formed; with any negative effects on the mother’s body at this time, the likelihood of the unborn baby developing congenital health abnormalities increases.
Usually, by the end of the 70th day, all organs are already formed, but there are also cases of delayed development. In such situations, with the beginning of the fertile period, a danger appears for these organs. Otherwise, the fetus is already fully formed and begins to actively increase in size.
If you want your unborn child to be born without any pathologies, then monitor your health both before and after the moment of conception. Lead the right lifestyle. And then no problems should arise.
Between the 18th and 25th days from the day of conception (3-4 weeks of pregnancy), the baby's heart begins to beat. By the 20th day, the foundations of the nervous system are formed. After five and a half weeks, the child moves his head, and at six weeks - his whole body, like an already born child. But the woman will feel these movements much later, at 16-20 weeks. At 43 days it is already possible to take an encephalogram of the brain. At 9-10 weeks of pregnancy, the baby already moves his eyeballs, swallows, moves his tongue, hiccups, is awake and sleeps. At 11 weeks - sucks his thumb, reacts to sounds, external noise can wake him up. By 11-12 weeks, nails appear, and by 16 weeks, eyelashes. From the 10-11th week of pregnancy, all body systems are functioning in the child.---------
At the end of the 40s. XX century, there was no scientific knowledge about the embryo. At that time they thought that an unborn child, a fetus, was something in the womb. And to consider him or not to consider him a human being, a unique person of his kind, was a matter of faith. Attitudes towards the unborn child changed in the 70s, when science began studying the fetus and the results of this study became the property of medicine. Discoveries in this area have become possible thanks to the introduction of the latest technologies, such as ultrasound, electronic monitoring of the embryonic heart, radiation immunochemistry and others. Real-time ultrasound introscopy, i.e. Capturing images of a baby in motion has been a clinical research method since 1976. The ultrasound device is so precise that it can detect even the tiny heart valves that open and close during heart contractions. This advanced equipment allows parents to see their child before birth. Thanks to the technologies, devices and equipment used by modern medicine, we are convinced that the unborn child is a human being, another member of the human community, no different from other people.
It is believed that the embryo is part of the mother's body. This is not true for many reasons. Firstly, he is genetically different from his mother. Secondly, the placenta does not grow into the wall of the uterus - there is a placental barrier that prevents most of the mother’s diseases from penetrating through it; infection of the child, as a rule, can only occur from the moment of birth. The mother's blood cannot penetrate inside the embryo; in its composition and group, in the genetics of each cell of its body, the embryo is different from the mother. The mother warms him, protects him, removes carbon dioxide and provides oxygen and the building blocks from which his proteins will be composed. But he will put them in each of his cells according to his own unique genetic program.
Professor, Department of Embryology, Faculty of Biology, Moscow State University,
Doctor of Biological Sciences D.V. Popov
In France, the life of a child begins to be protected by state laws 10 weeks after conception, in Denmark - after 12 weeks, in Sweden - after 20, in many countries life is legally protected only after birth. Nobel Prize winner James Watson proposed protecting the life of a child three days after birth... When does human life actually begin? Who to believe? Or maybe French children begin to be human 10 weeks after conception, little Danes - after 12 weeks, Swedes - after 20 weeks, and James Watson's child will become human only three days after birth?..
Today this is an indisputably established scientific fact: human life begins at the very moment when two sex cells meet and unite: male and female, and as a result of this union one cell is formed. And so, in this one microscopically small cell lies the entire future of a person: his gender, blood type, even the color of his eyes and hair - all this is in this cell and will only develop and come to light in the future. All that is needed to form an adult from this small cell is food, oxygen and time. This is all. Each such cell - an embryo - is already a unique and inimitable person. There has never been another such thing in world history; and no matter how many centuries or millennia this story continues, there will never be another like it.
How does the Orthodox Church look at this issue? Church laws (canons) have always protected the human being already in the mother’s womb: “Whoever deliberately destroyed a conceived fetus in the womb is subject to condemnation as for murder,” writes St. Basil the Great (canon 2). As we see, there is not a word about the age of the fetus: it doesn’t matter when it was conceived - even 10 weeks ago, even yesterday, even an hour ago, even a minute. All the holy fathers who expressed their opinions on this subject are unanimous in this. Let us name among them, in addition to St. Basil the Great, such pillars of Orthodoxy as St. Gregory the Theologian, St. John Chrysostom, St. Ephraim the Syrian, and St. Maximus the Confessor.
From the church point of view, human life does not begin with birth and does not end with death. These two milestones limit only one of the stages of human life. This stage is preceded by intrauterine life and this stage is followed by afterlife.
The fastest development and rapid growth of a child occurs immediately after implantation (introduction) of the embryo into the wall of the uterus.
Scientists have calculated that if the growth and development of a child throughout pregnancy proceeded as intensively as in the first 7 weeks, by the time of birth the weight of the newborn would be 14 tons - this is the weight of two elephants...
At 10-11 weeks (by the middle of the third month of pregnancy), all organ systems of the human body are fully formed. From this time on, in France, the life of a child begins to be protected by law. The laws of other countries allow killing this already formed little man. Why? On the basis that, although it has formed, it has not yet had time to fully develop?.. But it will continue to develop after birth for no less than 12-14 years. If it is possible to kill for this reason: insufficient development, then, together with intrauterine babies, let us allow the killing of all children before the end of adolescence. Or let's introduce gradation into the Criminal Code: for the murder of a three-year-old child, a shorter sentence is given than for the murder of a ten-year-old - because he is less developed... Savagery? And this is not savagery: for the murder of a newborn baby you will be sent to prison, and for the murder of an intrauterine baby you will receive paid sick leave, although both the fetus and the baby are people only potentially. We feel disgust for that mother who, after the birth of her child, secretly throws it into the trash can, and we bring flowers to another who throws her child into the trash bin in public, in the medical ward, with the help of doctors.
An American magazine talked about a woman who gave birth to a 5-month-old baby and begged doctors to save his life. The entire hospital was put on its feet so that the child would survive. The most experienced doctors worked for the rescue, large amounts of money were allocated... And in the same hospital, on the same day and hour - only in a different room - another woman aborts her 5-month-old child. In one room, a 5-month-old baby is perceived as a person, and in the next room, the same 5-month-old baby is regarded as a piece of meat.
At one feminist demonstration for the legalization of abortion, women chanted: “All laws away from our bodies!” These women obviously believe that abortion is their purely personal matter, and no one has the right to interfere in such matters. Such women need to be told: - No, abortion is not your personal matter. This matter would be your personal if it concerned only your personality. But it also concerns another personality - the personality of your child... Imagine that a bandit attacked you and wants to kill you. A policeman runs up, tries to protect you, and the bandit tells him: “Come on, get away! Killing her or not killing her is my personal business. You have no right to interfere in my personal affairs.” Will you, following the bandit, persuade the policeman to step back and not interfere?.. Why, when you are killed, do you cry out for help and recognize the need for state laws to protect your life, and when you kill - “all laws go away” and “no one has the right to interfere"?.. Your life needs to be protected, but the life of a defenseless baby shouldn’t be protected? We must protect his life too!
There are people who believe that abortion still cannot be equated with ordinary murder - because the intrauterine baby is not a “person” at all, and is not even viable.
Personality or non-personality is, of course, a matter of faith, decided depending on a person’s general philosophical and religious beliefs. For an atheist and materialist, he is really not a person. But with a materialistic, vague understanding of personality, it is generally difficult to find out when a person becomes one - some become one at 5 years old, others at 25, and others will live their whole lives and never become so... The Christian faith answers this question unequivocally: without a doubt, personality! How can there not be a personality when this intrauterine baby already has its own individual and immortal soul?! Yes, mommy is an immortal soul! Which, unlike the calf, you have no power to dismember or kill. And who will stand at the Last Judgment before the Almighty next to your, also immortal soul. “Whoever died in his mother’s womb and did not enter into life, he (the Judge) will make him an adult at the same instant in which he will return life to the dead (in the general Resurrection)... Those who have not seen each other here will see each other there, and the mother will know what it is - her son, and the son will know that this is his mother..." (St. Ephraim the Syrian. Ed. 1900, part 4. p. 105, "On the fear of God and the final judgment"). Perhaps only then will this unfortunate mother fully understand the horror of what she has done. “The fornicator who destroyed the fetus conceived in her womb so that he would not see this world, He (the Judge) will not allow him to see the new age,” writes St. Ephraim the Syrian further. “Just as she did not allow him (her child) to enjoy life and light in this age, so He (God) will deprive her of life and light in the next age. Since she decided to spew out her fetus from her womb prematurely in order to hide it in the darkness of the earth, then she, like the dead fruit of the womb, will be spewed out into utter darkness . Such is the reward for fornicators and fornicators who encroach on the lives of their children."
As for “viability”, it should be clarified: what is meant by the term “viability”. If we understand the ability to exist independently and not dependent on anyone, then a child is clearly not capable of such an existence even after birth. And he won’t be able to do it for a long time. Try leaving a two-year-old or even a five-year-old child to his own devices - will he live independently, without outside help, for at least a week?..
priest Alexander Zakharov
from book "
MOSCOW, November 29 - RIA Novosti. For the first time, molecular biologists from Britain have been able to turn stem cells into a miniature analogue of placental tissue, the study of which will help reduce the number of miscarriages and improve the health of the child. The results of the first experiments with it were presented in the journal Nature.
Over the past two decades, biologists have learned to turn stem cells into tissues of bones, muscles, skin and the nervous system. Such tissues can become “spare parts” in case of damage to the body or a cure for a number of degenerative diseases. For example, cultures of “stem” neurons could become a panacea for treating Alzheimer’s and Parkinson’s diseases, and other versions of them could help restore lost limbs or organs.
In particular, in April 2012, scientists were able to turn stem cells into hair follicles and successfully transplanted them onto the back of the head of hairless mice. Three years ago, Japanese scientists collected complete copies of various organs, such as kidneys or liver, from stem cells, and also grew a rat's leg and “connected” it to the body of a rodent.
As biologists explain, the placenta consists of two types of cells, syncytiotrophoblasts and cytotrophoblasts. The former are responsible for the formation of a special barrier separating the circulatory system of the fetus from the mother’s body, and the latter are responsible for organizing the metabolism between them.
Scientists have long been interested in how both groups of placental cells arise and how disturbances in their work can be associated with miscarriages, various complications and the death of the embryo even before it attaches to the wall of the uterus.
British researchers have taken the first step towards obtaining these answers. They selected a special “cocktail” of various hormones and signaling molecules that causes stem cells contained inside special “villi” in the placenta to turn into full-fledged miniature copies of this organ.
To do this, scientists analyzed which genes were most active inside the placenta at 6-9 weeks of pregnancy, and isolated substances associated with the “maturation” of stem cells. By combining them with a mixture of molecules from an already formed placenta, Turco and her colleagues obtained a drug that causes these “blanks” to turn into full-fledged placenta analogues that remain stable for almost an indefinitely long time.
These miniature organs, according to scientists, have all the important features of a placenta. They contain both types of cells, have “villi” inside them that are involved in the exchange of gases and nutrients, and they produce all the critical hormones and signaling molecules.
Biologists hope that experiments with these mini-organs will help not only reveal the secrets of infertility and the causes of miscarriages, but also understand why some rare pathogens, for example, the Zika virus, can penetrate through it.
The study of the development of the human body from the formation of a single-celled zygote, or fertilized egg, until the birth of a child. Embryonic (intrauterine) human development lasts approximately 265–270 days. During this time, more than 200 million cells are formed from the original one cell, and the size of the embryo increases from microscopic to half a meter.
In general, the development of a human embryo can be divided into three stages. The first is the period from fertilization of the egg to the end of the second week of intrauterine life, when the developing embryo (embryo) implants into the wall of the uterus and begins to receive nutrition from the mother. The second stage lasts from the third to the end of the eighth week. During this time, all the main organs are formed and the embryo acquires the features of a human body. At the end of the second stage of development, it is already called a fruit. The length of the third stage, sometimes called fetal (from the Latin fetus - fetus), is from the third month to birth. At this final stage, the specialization of organ systems is completed and the fetus gradually acquires the ability to exist independently.
GENIT CELLS AND FERTILIZATION
In humans, a mature reproductive cell (gamete) is a sperm in a man, an ovum (egg) in a woman. Before gametes fuse to form a zygote, these sex cells must form, mature, and then meet.
Human germ cells are similar in structure to the gametes of most animals. The fundamental difference between gametes and other cells of the body, called somatic cells, is that a gamete contains only half the number of chromosomes of a somatic cell. There are 23 of them in human germ cells. During the process of fertilization, each germ cell brings its 23 chromosomes into the zygote, and thus the zygote has 46 chromosomes, i.e., a double set of them, as is inherent in all human somatic cells. See also CELL.
While similar in their main structural characteristics to somatic cells, the sperm and egg are at the same time highly specialized for their role in reproduction. A sperm is a small and very motile cell (see SPERM). The egg, on the contrary, is immobile and much larger (almost 100,000 times) than the sperm. Most of its volume is made up of cytoplasm, which contains reserves of nutrients necessary for the embryo during the initial period of development (see EGG).
For fertilization, the egg and sperm must reach maturity. Moreover, the egg must be fertilized within 12 hours after leaving the ovary, otherwise it dies. Human sperm lives longer, about a day. Moving quickly with the help of its whip-shaped tail, the sperm reaches the duct connected to the uterus - the fallopian tube, where the egg enters from the ovary. This usually takes less than an hour after copulation. Fertilization is believed to occur in the upper third of the fallopian tube.
Despite the fact that the ejaculate normally contains millions of sperm, only one penetrates the egg, activating a chain of processes leading to the development of the embryo. Due to the fact that the entire sperm penetrates the egg, the man brings to the offspring, in addition to nuclear material, a certain amount of cytoplasmic material, including the centrosome, a small structure necessary for the cell division of the zygote. The sperm also determines the sex of the offspring. The culmination of fertilization is considered to be the moment of fusion of the sperm nucleus with the nucleus of the egg.
CRUSHING AND IMPLANTATION
After fertilization, the zygote gradually descends through the fallopian tube into the uterine cavity. During this period, over a period of about three days, the zygote goes through a stage of cell division known as cleavage. During fragmentation, the number of cells increases, but their total volume does not change, since each daughter cell is smaller than the original one. The first cleavage occurs approximately 30 hours after fertilization and produces two completely identical daughter cells. The second cleavage occurs 10 hours after the first and leads to the formation of a four-cell stage. Approximately 50–60 hours after fertilization, the so-called stage is reached. morula - a ball of 16 or more cells.
As cleavage continues, the outer cells of the morula divide faster than the inner cells, resulting in the outer cell layer (trophoblast) being separated from the inner cluster of cells (the so-called inner cell mass), maintaining connection with them in only one place. A cavity, the blastocoel, is formed between the layers, which is gradually filled with liquid. At this stage, which occurs three to four days after fertilization, cleavage ends and the embryo is called a blastocyst, or blastula. During the first days of development, the embryo receives nutrition and oxygen from the secretions of the fallopian tube.
Approximately five to six days after fertilization, when the blastula is already in the uterus, the trophoblast forms finger-like villi, which, moving vigorously, begin to penetrate the uterine tissue. At the same time, apparently, the blastula stimulates the production of enzymes that promote partial digestion of the uterine lining (endometrium). Around day 9–10, the embryo implants (grows) into the wall of the uterus and is completely surrounded by its cells; With implantation of the embryo, the menstrual cycle stops.
In addition to its role in implantation, the trophoblast is also involved in the formation of the chorion, the primary membrane surrounding the embryo. In turn, the chorion contributes to the formation of the placenta, a membrane with a spongy structure, through which the embryo subsequently receives nutrition and removes metabolic products.
EMBRYONAL GERM LAYERS
The embryo develops from the inner cell mass of the blastula. As fluid pressure increases within the blastocoel, the cells of the inner cell mass become compact and form the germinal shield, or blastoderm. The embryonic shield is divided into two layers. One of them becomes the source of the three primary germ layers: ectoderm, endoderm and mesoderm. The process of separation of first two and then the third germ layer (the so-called gastrulation) marks the transformation of the blastula into the gastrula.
The germ layers initially differ only in location: the ectoderm is the outermost layer, the endoderm is the inner layer, and the mesoderm is intermediate. The formation of the three germ layers is completed approximately a week after fertilization.
Gradually, step by step, each germ layer gives rise to certain tissues and organs. Thus, the ectoderm forms the outer layer of the skin and its derivatives (appendages) - hair, nails, skin glands, lining of the mouth, nose and anus - as well as the entire nervous system and sensory organ receptors, such as the retina. From the endoderm are formed: lungs; the lining (mucosa) of the entire digestive tract except the mouth and anus; some organs and glands adjacent to this tract, such as the liver, pancreas, thymus, thyroid and parathyroid glands; lining of the bladder and urethra. Mesoderm is the source of the circulatory system, excretory, reproductive, hematopoietic and immune systems, as well as muscle tissue, all types of supporting trophic tissue (skeletal, cartilaginous, loose connective tissue, etc.) and the inner layers of the skin (dermis). Fully developed organs usually consist of several types of tissues and are therefore related by their origin to different germ layers. For this reason, it is possible to trace the participation of one or another germ layer only in the process of tissue formation.
EXTRAGEMONY MEMBRANES
The development of the embryo is accompanied by the formation of several membranes that surround it and are rejected at birth. The outermost of them is the already mentioned chorion, a derivative of trophoblast. It is connected to the embryo by a body stalk of connective tissue derived from the mesoderm. Over time, the stalk lengthens and forms the umbilical cord (umbilical cord), connecting the embryo to the placenta.
The placenta develops as a specialized outgrowth of the membranes. The chorionic villi pierce the endothelium of the blood vessels of the uterine mucosa and plunge into the blood lacunae filled with the mother’s blood. Thus, the blood of the fetus is separated from the blood of the mother only by the thin outer membrane of the chorion and the walls of the capillaries of the embryo itself, i.e., direct mixing of the blood of the mother and the fetus does not occur. Nutrients, oxygen and metabolic products diffuse through the placenta. At birth, the placenta is discarded as an afterbirth and its functions are transferred to the digestive system, lungs and kidneys.
Within the chorion, the embryo is contained in a sac called the amnion, which is formed from the embryonic ectoderm and mesoderm. The amniotic sac is filled with fluid that moisturizes the embryo, protects it from shocks and keeps it in a state close to weightlessness.
Another additional shell is the allantois, a derivative of endoderm and mesoderm. This is the storage place for excretory products; it connects with the chorion in the bodily stalk and promotes respiration of the embryo.
The embryo has another temporary structure - the so-called. yolk sac. Over a period of time, the yolk sac supplies the embryo with nutrients by diffusion from the maternal tissues; Later, progenitor (stem) blood cells are formed here. The yolk sac is the primary site of hematopoiesis in the embryo; subsequently this function passes first to the liver and then to the bone marrow.
EMBRYO DEVELOPMENT
During the formation of extraembryonic membranes, the organs and systems of the embryo continue to develop. At certain moments, one part of the cells of the germ layers begins to divide faster than the other, groups of cells migrate, and cell layers change their spatial configuration and location in the embryo. During certain periods, the growth of some types of cells is very active and they increase in size, while others grow slowly or stop growing altogether.
The nervous system is the first to develop after implantation. During the second week of development, the ectodermal cells of the posterior side of the germinal shield rapidly increase in number, causing the formation of a bulge above the shield - the primitive streak. Then a groove is formed on it, in the front of which a small pit appears. In front of this fossa, the cells quickly divide and form the head process, the precursor of the so-called. dorsal string, or chord. As the notochord elongates, it forms an axis in the embryo that provides the basis for the symmetrical structure of the human body. Above the notochord is the neural plate, from which the central nervous system is formed. Around the 18th day, the mesoderm along the edges of the notochord begins to form dorsal segments (somites), paired formations from which the deep layers of skin, skeletal muscles and vertebrae develop.
After three weeks of development, the average length of the embryo is only slightly more than 2 mm from crown to tail. Nevertheless, the rudiments of the notochord and nervous system, as well as eyes and ears, are already present. There is already an S-shaped heart, pulsating and pumping blood.
After the fourth week, the length of the embryo is approximately 5 mm, the body is C-shaped. The heart, which forms the largest bulge on the inside of the body's curve, begins to subdivide into chambers. Three primary areas of the brain (brain vesicles), as well as the visual, auditory and olfactory nerves are formed. The digestive system is formed, including the stomach, liver, pancreas and intestines. The structuring of the spinal cord begins, and small paired limb rudiments can be seen.
A four-week human embryo already has gill arches that resemble the gill arches of a fish embryo. They soon disappear, but their temporary appearance is one example of the similarity of the structure of the human embryo with other organisms (see also EMBRYOLOGY).
At five weeks of age, the embryo has a tail and the developing arms and legs resemble stumps. Muscles and ossification centers begin to develop. The head is the largest part: the brain is already represented by five brain vesicles (cavities with fluid); there are also bulging eyes with lenses and pigmented retinas.
In the period from the fifth to the eighth week, the actual embryonic period of intrauterine development ends. During this time, the embryo grows from 5 mm to approximately 30 mm and begins to resemble a person. His appearance changes as follows: 1) the curvature of the back decreases, the tail becomes less noticeable, partly due to reduction, partly because it is hidden by the developing buttocks; 2) the head straightens, the outer parts of the eyes, ears and nose appear on the developing face; 3) the arms are different from the legs, you can already see the fingers and toes; 4) the umbilical cord is fully defined, the area of its attachment on the abdomen of the embryo becomes smaller; 5) in the abdominal area, the liver grows greatly, becoming as convex as the heart, and both of these organs form a lumpy profile of the middle part of the body until the eighth week; at the same time, the intestines become noticeable in the abdominal cavity, which makes the stomach more rounded; 6) the neck becomes more recognizable mainly due to the fact that the heart moves lower, as well as due to the disappearance of the gill arches; 7) external genitalia appear, although they have not yet fully acquired their final appearance.
By the end of the eighth week, almost all internal organs are well formed, and the nerves and muscles are so developed that the embryo can produce spontaneous movements. From this time until birth, the main changes in the fetus are associated with growth and further specialization.
COMPLETION OF FETAL DEVELOPMENT
During the last seven months of development, the weight of the fetus increases from 1 g to approximately 3.5 kg, and the length increases from 30 mm to approximately 51 cm. The size of the baby at the time of birth can vary significantly depending on heredity, nutrition and health.
During fetal development, not only its size and weight, but also body proportions change greatly. For example, in a two-month-old fetus, the head is almost half the length of the body. In the remaining months it continues to grow, but more slowly, so that by the time of birth it is only a quarter of the body's length. The neck and limbs become longer, while the legs grow faster than the arms. Other external changes are associated with the development of the external genitalia, the growth of body hair and nails; the skin becomes smoother due to the deposition of subcutaneous fat.
One of the most significant internal changes is associated with the replacement of cartilage by bone cells during the formation of a mature skeleton. The processes of many nerve cells are covered with myelin (a protein-lipid complex). The process of myelination, together with the formation of connections between nerves and muscles, leads to increased mobility of the fetus in the uterus. These movements are well felt by the mother after about the fourth month. After the sixth month, the fetus rotates in the uterus so that its head is down and rests on the cervix.
By the seventh month, the fetus is completely covered with vernix, a whitish fatty mass that disappears after birth. It is more difficult for a child born prematurely to survive during this period. As a rule, the closer the birth is to normal, the greater the chance of survival for the baby, since in the last weeks of pregnancy the fetus receives temporary protection from certain diseases due to antibodies coming from the mother's blood. Although childbirth marks the end of the intrauterine period, human biological development continues during childhood and adolescence.
DAMAGING EFFECTS ON THE FET
Birth defects can result from a variety of causes, such as disease, genetic abnormalities, and numerous harmful substances that affect the fetus and the mother. Children with birth defects may be disabled for life due to physical or mental disabilities. Growing knowledge about the vulnerability of the fetus, especially in the first three months when its organs are forming, has now led to increased attention to the antenatal period.
Diseases. One of the most common causes of birth defects is the viral disease rubella. If a mother contracts rubella in the first three months of pregnancy, it can lead to irreparable abnormalities in the development of the fetus. Young children are sometimes given the rubella vaccine to reduce the chance that pregnant women who come into contact with them will get the disease. See also RUBELLA.
Sexually transmitted diseases are also potentially dangerous. Syphilis can be transmitted from mother to fetus, resulting in miscarriages and stillbirth. Detected syphilis must be immediately treated with antibiotics, which is important for the health of the mother and her unborn child.
Fetal erythroblastosis can cause stillbirth or severe anemia in the newborn with the development of mental retardation. The disease occurs in cases of Rh incompatibility between the blood of the mother and the fetus (usually with a repeat pregnancy with an Rh-positive fetus). See also BLOOD.
Another hereditary disease is cystic fibrosis, the cause of which is a genetically determined metabolic disorder, affecting primarily the function of all exocrine glands (mucous, sweat, salivary, pancreas and others): they begin to produce extremely viscous mucus, which can clog both the ducts themselves glands, preventing them from secreting secretions, and small bronchi; the latter leads to severe damage to the bronchopulmonary system with the eventual development of respiratory failure. In some patients, the activity of the digestive system is primarily disrupted. The disease is detected soon after birth and sometimes causes intestinal obstruction in the newborn on the first day of life. Some manifestations of this disease are amenable to drug therapy. Galactosemia is also a hereditary disease, caused by the lack of an enzyme necessary for the metabolism of galactose (a product of the digestion of milk sugar) and leading to the formation of cataracts and damage to the brain and liver. Until recently, galactosemia was a common cause of infant mortality, but methods for early diagnosis and treatment through a special diet have now been developed. Down syndrome (see DOWN SYNDROME), as a rule, is caused by the presence of an extra chromosome in cells. A person with this condition is usually short in stature, with slightly slanted eyes and reduced mental abilities. The likelihood of a child having Down syndrome increases with maternal age. Phenylketonuria is a disease caused by the lack of an enzyme necessary to metabolize a certain amino acid. It can also be a cause of mental retardation (see PHENYLKETONURIA).
Some birth defects can be partially or completely corrected through surgery. These include birthmarks, club feet, heart defects, extra or fused fingers and toes, anomalies in the structure of the external genitalia and genitourinary system, spina bifida, cleft lip and cleft palate. Defects also include pyloric stenosis, i.e. narrowing of the transition from the stomach to the small intestine, absence of the anus and hydrocephalus - a condition in which excess fluid accumulates in the skull, leading to an increase in the size and deformation of the head and mental retardation (see also CONGENITAL VICES).
Medicines and drugs. Evidence has accumulated, much of it from tragic experience, that some drugs may cause abnormalities in fetal development. The best known of these is the sedative thalidomide, which has caused underdeveloped limbs in many children whose mothers took the drug during pregnancy. Currently, most doctors recognize that drug treatment in pregnant women should be kept to a minimum, especially in the first three months when organ formation occurs. The use of any medications by a pregnant woman in the form of tablets and capsules, as well as hormones and even inhalation aerosols, is permissible only under the strict supervision of a gynecologist.
Drinking large amounts of alcohol by a pregnant woman increases the risk of the baby developing many conditions, collectively called fetal alcohol syndrome, which include growth retardation, mental retardation, cardiovascular abnormalities, a small head (microcephaly), and poor muscle tone.
Observations have shown that cocaine use by pregnant women leads to serious problems in the fetus. Other drugs such as marijuana, hashish and mescaline are also potentially dangerous. A link has been found between pregnant women's use of the hallucinogenic drug LSD and the incidence of spontaneous miscarriages. Experimental evidence suggests that LSD can cause abnormalities in chromosome structure, indicating the possibility of genetic damage in an unborn child (see LSD).
Smoking by expectant mothers also has an adverse effect on the fetus. Studies have shown that, in proportion to the number of cigarettes smoked, cases of premature birth and fetal underdevelopment increase. Smoking may also increase the frequency of miscarriages, stillbirths, and infant mortality immediately after childbirth.
Radiation. Doctors and scientists are increasingly pointing out the danger associated with the continuous increase in the number of sources of radiation, which can cause damage to the genetic apparatus of cells. During the early stages of pregnancy, women should not be unnecessarily exposed to X-rays and other forms of radiation. More broadly, strict control of medical, industrial and military sources of radiation is vital to preserving the genetic health of future generations. See also REPRODUCTION; HUMAN REPRODUCTION; EMBRYOLOGISTS
Http://www.krugosvet.ru/enc/medicina/EMBRIOLOGIYA_CHELOVEKA.html
SITUATIONAL TASKS IN EMBRYOLOGY
During the lesson, students discuss the importance of extra-embryonic organs in human embryonic development and, in particular, the yolk sac. It is known that in evolution the yolk sac performed a trophic function. In humans it contains a very small amount of yolk. Why? discuss the situation while answering the following questions
What extraembryonic organs are formed during embryogenesis in humans?
Function of the yolk sac in fish and birds
What structures (cells) form the human yolk sac?
Why does the yolk sac in humans lose its trophic function?
What functions does the yolk sac perform in humans?
During a presentation at a conference on the topic “Stages of Human Embryonic Development,” students discuss the following questions. answer them too
Fertilization (3 phases)
Type of cleavage and blastocyst formation
Types of gastrulation in humans and the formation of germ layers and axial organs
Histogenesis and organogenesis
Systemogenesis
In an experiment on an animal, the source of development of the adrenal medulla was damaged. During the discussion, questions arose. answer them too
What germ layers arise as a result of gastrulation?
During what process does this source arise?
Which germ layer gives rise to the development of this source
From what part of the ectoderm does this source develop?
What is the name of the part of the ectoderm that goes into the development of the source?
Premature placental abruption occurred, resulting in the death of the fetus. what is the reason? As you discuss the situation, answer the questions.
What extraembryonic organs develop in humans during embryogenesis?
Which organ ensures the connection between the embryo and the mother's body?
What type of placenta does a person have?
What two parts of the chorion are formed in humans?
What is the reason for premature placental abruption?
5. When discussing the topic “Sex Cells,” questions arose about the classification of eggs. answer them too
What is the name of the period in which egg formation occurs?
What is characteristic of eggs?
How are eggs classified by the amount of yolk?
How are eggs classified according to the location of the yolk?
What conditions determine the amount of yolk in the cytoplasm of the egg?
6. During the inflammatory process, the blastocyst was in the fallopian tube on the 7th day of embryogenesis. discuss the outcome of the pregnancy as you answer the following questions
Structure of the blastocyst (5-6 days)
Stages of implantation
What changes occur in the blastocyst on the 7th day?
What can happen to the blastocyst in the fallopian tube after the seventh day
What is the outcome of pregnancy
7. Students discuss in class the period of embryonic development, when cells begin to separate and specialize due to the activity of certain genes. The following questions arose. answer them too
What is this period called?
What are the 4 stages here?
At what stage do unequal blastomeres appear?
When do germ layers appear?
At what stage do the rudiments of different tissues appear?
8. At what stage of embryogenesis and how (methods) the embryo becomes multilayered. discuss this as you answer the following questions
What is this stage called and what is formed as a result of it?
What methods do lancelets and amphibians have?
How does gastrulation occur in humans?
What organs are formed in the human embryo between the two stages of gastrulation?
Time frame for the formation of all 3 germ layers in the human embryo
9. During an autopsy of a woman who died during a car accident, a vesicle-shaped embryo was found in the uterus. what stage of development and what is the gestational age? discuss the situation while answering the following questions
Stages of embryonic development and their essence
Human egg type
Site of reproductive tract of fertilization
Type of crushing in humans and localization of the process
Human blastocyst (structure, place and time of formation after fertilization)
10. The fetus was born in a “shirt”. What does it mean? what is this “shirt” made of? discuss the situation while answering questions
Gastrulation in humans
Human extraembryonic organs. Formation between gastrulation phases
Provisional organs formed after gastrulation in humans
Amnion structure
The meaning of the amnion (actions of the obstetrician if the fetus is born in a “shirt”)
ANSWERS TO SITUATIONAL PROBLEMS IN EMBRYOLOGY
In humans, during emryogenesis, extraembryonic organs are formed: amnion, yolk sac, chorion, allantois, placenta.
In fish and birds, the yolk sac performs trophic and hematopoietic functions.
In humans, the yolk sac is formed by extraembryonic endoderm and extraembryonic mesoderm (mesenchyme).
The yolk sac appears in the second week of development, and from the third week hemotrophic nutrition begins, so the yolk sac loses its trophic function.
The yolk sac is an organ that mainly performs hematopoietic function and the formation of primary germ cells in humans.
There are three phases in fertilization.
Remote interaction. Chemicals are used to prevent many sperm from entering the egg.
Contact interaction - the cytoplasm of contacting gametes combines.
Penetration of the sperm into the cytoplasm of the egg, compaction of the peripheral part of the cytoplasm of the egg and formation of the fertilization membrane.
The crushing is complete, uneven, asynchronous. Some blastomeres are large, dark, and fragment slowly. This is an embryoblast. It forms the body of the embryo and extra-embryonic organs, except for the trophoblast. The second type of blastomeres - small, light, rapidly dividing - is the trophoblast. After 50-60 hours, the crushed embryo takes the form of a dense ball - a morula. On the third day, the blastocyst begins to form, which is a hollow vesicle formed from the outside by the trophoblast and filled with liquid, with the embryoblast in the form of a knot of cells, which is attached from the inside to the trophoblast at one pole of the blastocyst.
In humans, gastrulation occurs in two phases. As a result of delamination, two layers are formed: the outer one is the epiblast (primary ectoderm) and the inner one is the hypoblast (primary endoderm). At the second stage, as a result of the formation of the primitive streak and the immigration of cell masses, the mesoderm and notochord are formed. By day 17, the human embryo has formed 3 germ layers. On days 20–21, the notochord, the neural tube (from the ectoderm), is finally formed, closing by day 25. The intestinal tube is formed.
The formation of tissue primordia occurs due to determination and commitment, differentiation, proliferation and cell death. Determination is a genetically programmed path of development of cells and tissues. At the gastrulation stage, cells are not sufficiently determined, so they are the source of the development of several tissues. Commitment is a restriction of the possible paths of cell development. Differentiation is changes in the structure of cells that are associated with their functional specialization, caused by the activity of certain genes. During the process of differentiation, specialization of tissue primordia and the formation of various types of tissues occur.
The rudiments of organs and systems are formed from tissues.
In humans, gastrulation occurs in two phases. As a result of delamination, two casts are formed: the outer one is the epiblast (primary ectoderm) and the inner one is the hypoblast (primary endoderm). At the second stage, as a result of the formation of the primitive streak and the immigration of cell masses, the mesoderm and notochord are formed.
The source arises in the process of differentiation.
Primary ectoderm.
Germinal ectoderm.
Rudiments of the sympathetic ganglia.
4.1. In human embryogenesis, extraembryonic organs are formed: amnion, yolk sac, chorion, allantois, placenta.
4.2. The connection between the embryo and the mother's body is ensured by the placenta.
4.3. The human placenta is hemochorial, discoidal, villous.
4.4. A branched chorion is distinguished, the villi of which greatly grow and branch. Smooth chorion - no villi. It is located in the region of the parietal and bursa parts of the falling shell.
4.5. At the border of the smooth and branched chorion, part of the main sheath along the edge of the placental disc grows tightly to the chorion and does not collapse, forming an endplate. If the formation of this plate is disrupted, blood flows out of the lacunae with maternal blood and the placenta exfoliates.
The process of development and maturation of eggs is called progenesis.
Eggs are characterized by the presence of yolk, which is a protein-lipid complex. This inclusion is located in the cytoplasm and is used to nourish the embryo.
Based on the amount of yolk, yolkless (alecithal) are distinguished. Small yolk (oligolecithal). Among them, primary (in the lancelet) and secondary (in placental mammals and humans) are distinguished. Polylecithal (multiyolk) (in birds).
In low-yolk eggs, the yolk granules are evenly distributed - called isolecithal. In polylecithal eggs, the yolk is often located at one pole. Such eggs are called telolecithal. Among them, a distinction is made between moderately telolecithal (mesolecithal in amphibians) and strongly telolecithal (in birds). If the yolk is in the center of the cell, then the cells are called centrolecithal.
The amount of yolk in the egg depends on the development conditions (in the external or internal environment) and the duration of development. In humans, the small amount of yolk is explained by the fact that the development of the embryo occurs in the mother’s body and nutrition is provided by the mother through the placenta.
On the fifth day, the blastocyst enters the uterus and is freely located in it. At first, the blastocyst has the appearance of a hollow vesicle, which is formed from the outside by the trophoblast and the embryoblast in the form of a nodule attached from the inside to the trophoblast. The bubble is filled with liquid. From the fifth day, the number of lysosomes in the trophoblast increases, and trophoblast outgrowths appear. The embryonic nodule turns into the embryonic shield. Preparations are underway for the first phase of gastrulation.
There are two stages in implantation: adhesion (sticking) and invasion (penetration). On the resulting trophoblast villi, two layers are formed: the inner cytotrophoblast and the outer symplastotrophoblast, which produces proteolytic enzymes that melt the uterine mucosa. An implantation pit appears in it, into which the blastocyst penetrates.
On the 7th day, process cells - extra-embryonic mesoderm - are evicted from the embryonic shield. It participates in the formation of the amnion (together with the extra-embryonic ectoderm), the yolk sac (together with the extra-embryonic endoderm) and the chorion (together with the trophoblast).
The blastocyst, ready for implantation, can immerse itself in the mucous membrane of the fallopian tube.
Pipe rupture with intra-abdominal bleeding; surgery (tubectomy)
The stage is called germ layer differentiation.
There are 4 stages of differentiation:
Ootypic differentiation - at the zygote stage is represented by presumptive rudiments - sections of the fertilized egg.
Blastomer differentiation at the blastula stage.
Rudimentary differentiation at the early gastrula stage.
Histogenetic differentiation at the late gastrula stage.
At the stage of blastomere differentiation, unequal blastomeres appear (roof, bottom blastomeres...).
Germ layers arise during the early gastrulation stage.
The rudiments of various tissues appear at the stage of histogenetic differentiation. The rudiments of organs and systems begin to form from tissues.
8.1. The embryo becomes multilayered at the gastrulation stage.
8.2. In the lancelet, gastrulation proceeds by invagination (invagination), and in amphibians it occurs mainly by epiboly (fouling) plus partial invagination.
8.3. In humans, gastrulation occurs in two ways: delamination (splitting - two layers are formed: the outer layer - the epiblast (primary ectoderm) and the inner hypoblast (primary endoderm)). The second method is immigration (eviction, movement of cellular material).
8.4. Between the two stages of gastrulation, extraembryonic organs are formed: the amniotic and vitelline vesicles, the chorion - organs that provide conditions for the development of the embryo.
8.5. By day 17, the embryo has formed all 3 germ layers - ectoderm, endoderm and mesoderm.
Fertilization is the formation of a single-celled organism - a zygote. As a result of fragmentation, the embryo becomes multicellular. Gastrulation is the formation of a multilayer embryo. Histogenesis, organogenesis and systemogenesis - tissues appear in the embryo, from the combination of which organs and systems are formed. The human body is formed.
Secondary isolecithal egg
In the fallopian tubes
Complete, uneven, asynchronous. It begins in the fallopian tube and ends in the uterus.
Blastocyst is a germinal vesicle. At the early blastocyst stage (3–4 days), formed in the fallopian tube, the embryo enters the uterine cavity. There, the free blastocyst stage (days 5–6) is followed by implantation (days 6–7), lasting 40 hours. In a blastocyst there is a trophoblast (outer cell mass) on the outside, and a cavity with liquid on the inside. At one pole of the vesicle there is an embryoblast (inner cell mass) attached to the trophoblast. Gestation period is the end of the first week from fertilization (about 6 days).
Gastrulation in humans occurs in two phases: phase 1 – delamination on the 7th day with the formation of epi- and hypoblast; Phase 2 on day 14 (immigration). In the embryo, three germ layers are formed and the main organs (axial) are formed.
The amnion, yolk sac, and chorion appear between the two phases of gastrulation
After the second stage, the allantois is formed and the placenta begins to form
Composition of the amnion: extraembryonic ectoderm and mesoderm (mesenchyme)
The amnion is a water membrane - a hollow organ filled with fluid produced by its epithelium. The fetus develops in it and makes movements. The amnion protects the fetus from drying out and has the effects necessary for the formation of the digestive and respiratory organs. The fetus swallows amniotic fluid and secretes urine into it. The obstetrician opens the amnion for normal labor (if it has not opened itself).
