The development of the human body begins from the very first day of fertilization of the egg by the sperm. The stages of embryogenesis are counted from the moment the cell begins to develop, which subsequently forms an embryo, and a full-fledged embryo appears from it.
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 carried out in the mother's body.
First stage 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 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 characteristic of embryogenesis is 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 interstitial fluid is also distributedunequally. 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 stage
When cell division ends, a blastula is formed from them. It is a single-layer embryo the size of an egg. Blastula already carries all the necessary DNA information and contains unequal cell sizes. This happens already on the 7th day after fertilization.
After that, a single-layer embryo passes through the stage of gastrulation, which is the movement of existing cells into several germ layers - layers. First they form 2, and then a third appears between them. During this period, a new cavity is formed in the blastula, called the primary mouth. The previously existing cavity completely disappears. Gastrulation enables the future embryo to clearly distribute cells for the further formation of all organs and systems.
From the first formed outer layer, all skin integuments, connective tissues and the nervous system are formed in the future. The lower, formed second, layer becomes the basis for the formation of the respiratory organs, the excretory system. The last, middle cell layer is the basis for the skeleton, circulatory system, muscles and other internal organs.
The layers in the scientific environment are named accordingly:
- ectoderm;
- endoderm;
- mesoderm.
Third stage
After all the above stepsembryogenesis are completed, the embryo begins to grow in size. In a short time, it begins to be a cylindrical organism with a clear distribution of the head and tail ends. The growth of the finished embryo continues until the 20th day after fertilization. At this time, the plate formed earlier from the cells, the precursor 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, cells are distributed to further division between muscle tissues, skin and internal organs, which are formed from all cell layers.
Extra-embryonic development
All the initial stages of embryogenesis take place in parallel with the development of extra-embryonic parts, which will further provide the embryo and fetus with nutrition and support vital activity.
When the embryo is fully formed and out of the tubes, the embryo is attached to the uterus. This process is very important, since the vital activity of the fetus in the future depends on the correct development of the placenta. It is at this stage that IVF embryo transfer takes place.
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, is responsible for the efficiency of attachment of the embryo to the walls of the uterus. With its help, the embryo penetrates the mucous membranes of the femalebody, 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 is carried out in parallel with its separation from the litter. The process is ensured by the presence of a trunk fold, which, as it were, repels the walls of the extraembryonic organ from the body of the embryo. At this stage of embryo development, the only connection with the placenta is the umbilical stalk, which later forms the cord and provides nutrition for the baby for the rest of the intrauterine period of his life.
Interestingly, the early stages of embryogenesis in the region of the umbilical stalk also have a yolk 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, this organ, although it is formed, has no effect on the further embryonic development of the organism, 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 fetus 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 a water shell. The baby swims in it until he is born. This bag is called amnion, and its filling is called amniotic fluid. All theseorgans are enclosed in another shell - the chorion. It has a villous surface and provides the embryo with breathing and protection.
Step by step review
To understand human embryogenesis in more detail in a language understandable to most, you need to start with its definition.
So, what is embryogenesis? This phenomenon represents the intrauterine development of the fetus from the day of its fertilization until birth. This process begins only after 1 week has passed 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 be attached in any, except for the lower, part of the uterus. It is important to understand that this whole process is carried out for at least 40 hours, since only gradual actions can ensure complete safety and comfort for both organisms. The place of attachment of the embryo after attachment is gradually filled with blood and overgrown, after which the most important period of development of the future person begins - embryonic.
First organs
A fetus attached to the uterus already has organs that are somewhat reminiscent of a head and a tail. The very first after a successful attachmentThe embryo develops a protective organ - the chorion. To more accurately imagine what it is, we can draw an analogy with a 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 for the crumbs. Already after the second week of pregnancy, the appearance of allantois, or the umbilical cord, can be observed.
Third week
Embryo transfer to the fetal stage is carried out only upon completion of its formation, but already in the third week you 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 own heart of the unborn baby begins to beat.
Meal change
This period of development is also marked by another important stage. Starting from the third week of life, the embryo ceases to receive 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 already from the mother's blood. At this point, to ensure the effectiveness of the whole process, 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 for the rest of the intrauterine time.
Fourth week
At this time, it is already possible to clearly define future limbs and even placeseye sockets. Outwardly, the embryo changes slightly, since the main emphasis of development is given to the formation of internal organs.
Sixth week of pregnancy
At this time, the expectant mother should pay special attention to her own he alth, because during this period the thymus gland of her unborn baby is being formed. It is this organ that will be responsible for the performance of the immune system in the future. It is very important to understand that the ability of her child to withstand external stimuli throughout her independent life will depend on the he alth of the mother. You should not only pay attention to the prevention of infections, but also warn yourself against nervous situations, monitor your emotional state and the environment.
Eighth Seven Days
Only starting from this threshold of time, the expectant mother can find out the sex of her child. Exclusively at week 8, the sexual characteristics of the fetus and the production of hormones begin to be laid. Of course, you can find out the gender if the child himself wants it and turns the right side on the ultrasound.
Final stage
Starting from the 9th week of pregnancy, the embryonic period ends and the fetal period begins. By this time, a he althy baby should already have all the organs formed - they just have to grow. At this time, the child’s body weight is actively gaining, his muscle tone increases, hematopoietic organs are actively developing; the fetus begins to move randomly. Interestingly, the cerebellum is usually not yet formed at this point, so the coordination of fetal movements occursover time.
Dangers during development
Different stages of embryogenesis have their weaknesses. To understand this, you need to consider them in more detail. So, in some periods, human embryogenesis is sensitive to infectious diseases of the mother, and in others - to chemical or radiation waves from the external environment. If problems occur during such a critical period, the risk of birth defects in the fetus will increase.
To avoid this phenomenon, you should know all the stages of embryo development and the dangers of each of them. So, the blastula period is a special sensitivity to all external and internal stimuli. At this time, most of the fertilized cells die, but since this stage passes in the first 2 weeks after conception, most women do not even know about it. The total number of embryos dying at this time is 40%. Embryo transfer during IVF is currently 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 the greatest vulnerability of the embryo. At this time, the risk of rejection is not so great, but from the 20th to the 70th days of pregnancy, all vital organs are laid, with any negative effects on the mother's body at this time, the likelihood of the future baby developing congenital he alth abnormalities increases.
Usually, by the end of the 70th day, all organs are already formed, but there are cases of delayed development. Suchsituations with the onset of the fetal period, there is a danger to 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 take care of your he alth both before and after the moment of conception. Lead the right lifestyle. And then there should be no problems.
