The development of the human body begins from the very first day of fertilization of the egg with a sperm. The stages of embryogenesis are counted from the moment the cell begins to develop, which subsequently forms the embryo, and a full-fledged embryo emerges 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 being carried out in the mother's body.
The first stage of the 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 again becomes double. The cell formed at the same time is called a zygote.
The characteristic of embryogenesis is such that the development of the zygote is also divided into several stages. Initially, a newly formed cell begins to divide into new cells of different sizes, called morules. Intercellular fluid is also not distributed equally. A feature of this stage of embryogenesis is that the morula formed as a result of division does not grow in size, but only increase in quantity.
Second phase
When cell division ends, a blastula is formed from them. It is a single-layer egg-sized embryo. The blastula already carries all the necessary DNA information and contains an uneven cell size. This happens already on the 7th day after fertilization.
After this, a single-layer embryo passes through the stage of gastrulation, which is the movement of existing cells into several germinal sheets - layers. At first 2 of them are formed, and then a third appears between them. During this period, a new cavity called the primary mouth forms in the blastula. 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 in the future, all skin integuments, connective tissues and the nervous system are formed. The lower, formed by the second layer becomes the basis for the formation of respiratory organs, 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 called, respectively:
- ectoderm;
- endoderm;
- mesoderm.
Third stage
After all of the above stages of embryogenesis are completed, the embryo begins to grow in size. In a short time, it begins to represent a cylindrical organism with a clear distribution at the head and tail ends. The growth of the finished embryo continues until 20 days after fertilization. At this time, the plate previously formed from the cells, the precursor of the nervous system, is transformed into a tube, further representing the spinal cord. Other nerve endings that fill the entire embryo gradually grow from it. Initially, the processes are divided into the dorsal and abdominal. Also at this time, the cells are distributed and further divided between muscle tissue, skin and internal organs, which are formed from all cell layers.
Extra embryonic development
All the initial stages of embryogenesis go parallel to the development of extra-embryonic parts, which in the future will provide the embryo and fetus with nutrition and support vital functions.
When the embryo has fully formed and emerged from the tubes, the embryo is attached to the uterus. This process is very important, since the fetal life activity in the future depends on the proper development of the placenta. It is at this stage that embryos are transferred during IVF.
The process begins with the formation of a nodule around the embryo, which is a double layer of cells:
The latter is the outer shell, therefore, is responsible for the effectiveness of the 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 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 extra-germinal organ from the body of the embryo. At this stage of embryo development, the umbilical stalk becomes the only connection with the placenta, which later forms the cord and provides nutrition to the baby for the entire remaining intrauterine period of his life.
Interestingly, the early stages of embryogenesis in the umbilical stem also have a vitelline duct and a vitelline sac. In non-placental animals, birds and reptiles, this bag is an egg yolk 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 body, and over time it simply reduces.
The umbilical cord has blood vessels in it 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 bond is formed from allantois or part of the urinary sac.
The embryo developing inside the placenta is protected by two shells. In the internal cavity is a protein fluid, which is an aqueous membrane. In it, the baby swims before his birth. This bag is called amnion, and its filling is called amniotic fluid. All these organs are enclosed in yet another shell - the chorion. It has a villous surface and provides the embryo with breathing and protection.
Phased review
To analyze human embryogenesis in more detail in a language that is understandable to most, it is necessary to begin with its definition.
So what is embryogenesis? This phenomenon represents the intrauterine development of the fetus from the date of its fertilization until birth. This process begins only after passing 1 week after fertilization, when the cells have already finished dividing and the finished embryo moves into the uterine cavity. It was at this time that the first critical period began, 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 fixed in any, except 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 attachment point of the embryo after joining is gradually filled with blood and overgrows, after which the most important period of the development of the future person begins - the embryonic one.
First organs
The embryo attached to the uterus already has organs that resemble the head and tail in some way. The very first after successful attachment of the embryo is developing 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 to the crumbs. After the second week of pregnancy, the appearance of allantois, or umbilical cord, can be observed.
Third week
The transfer of embryos to the stage of the fetus is carried out only at the end 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 is secured, the trunk fold becomes noticeable, the head stands out and, most importantly, the own heart of the unborn baby begins to beat.
Food change
This period of development is also marked by another important stage. Starting from the third week of life, the embryo ceases to receive food according to the old system. The fact is that the egg stocks are depleted by this moment, and for further development the embryo needs to get the substances necessary for further formation already from the mother’s blood. At this point, to ensure the effectiveness of the entire process, allantois begins to transform into the umbilical cord and placenta. It is these organs that will provide the fetus with nutrition and liberate from the waste products all the remaining intrauterine time.
Fourth week
At this time, it is already possible to clearly identify future limbs and even the places of the eye sockets. Outwardly, the embryo changes insignificantly, since the main emphasis is on the formation of internal organs.
Sixth week of pregnancy
At this time, the future mother should pay special attention to her own health, since at this time the thymus gland of her unborn baby is formed. It is this body that in the future will be responsible for the functioning of the immune system all its life. It is very important to understand that the ability of her child to resist external stimuli throughout her independent life will depend on her mother’s health. You should not only pay attention to the prevention of infections, but also warn yourself from nervous situations, monitor the emotional state and the environment.
Eighth day
Only starting from this threshold of time, the future mother can find out the gender of her child. Exclusively at 8 weeks, 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 this and turns to the right side on the ultrasound.
The final stage
Starting from the 9th week of pregnancy, the embryonic period ends and the fetal period begins. At this point, a healthy baby should have already formed all the organs - they can only grow. At this time, the child’s body weight is actively gaining, his muscle tone is increasing, hematopoietic organs are actively developing; the fetus begins to move erratically. It is interesting that the cerebellum is usually not yet formed at this point, therefore, the coordination of fetal movements occurs over time.
Developmental dangers
Different stages of embryogenesis have their weak points. 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 environment. If problems arise during such a critical period, then the risk of developing birth defects in the fetus will increase.
To avoid this phenomenon, one should know all stages of the development of the embryo 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, as there is a risk of maternal rejection of the embryo. Therefore, during this period you need to take maximum care of yourself.
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 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 a future baby developing birth defects increases.
Usually, by the end of the 70th day, all organs are already formed, but there are also cases of late development. In such situations, with the onset of the fetal period, there is a danger to these organs. For the rest, 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 way of life. And then no problems should arise.