Development of the embryo


Basically, the term embryo is defined as a living being that is in an early stage of its development. This definition therefore applies not only to humans, but to all living things. An embryo is created by the development of a fertilized egg and is usually called an embryo for as long as it is in the womb.

More about this topic can be found: Fertilization of the egg

In humans, however, an exception is made here by definition. In humans, from the ninth week of pregnancy onwards, the embryo is called a fetus or fetus. So here the embryonic period passes over into fetal development.

What happens to the embryo in the 1st trimester?

The first 12 weeks of pregnancy are called 1st trimester of pregnancy or as 1st trimester summarized. The first day is defined as the first day of the last menstruation. From this point on, the embryo needs exactly 40 weeks to full development. Often the pregnancy of the future mother cannot yet be seen, even if pregnancy symptoms are already noticeable.

The first trimester also represents the critical phase in terms of external Poisons The consumption of alcohol, cigarettes or other drugs can lead to massive disturbances in the development of the child. Also, the risk is for one Miscarriage highest in the 1st trimester.
You may also be interested in this topic: How dangerous is smoking during pregnancy?

The focus of the 1st trimester is primarily on the Development of the individual organs and the disproportionate growth of the head. Only in the second and third trimester do massive growth in height and weight take place.

Development in the 1st week of pregnancy

At the beginning of first week of pregnancy there is initially a single one fertilized egg in the fallopian tube. This migrates towards in the first few days of pregnancy uterus (Uterus) to get there Nesting (Nidation). During this phase, the first divisions of the egg take place.

It follows that 2-cell stage. On the third and fourth day, further cell divisions take place, so that a slight cell specialization (differentiation) can already be determined. The resulting Cell association is called a blastocyst and already comprises several cells.

The actual day now takes place on the fifth to sixth day Implantation in the uterus. In the course of the normal cycle, this person has optimally prepared for this situation. she is strong blood supply, contains a lot of nutrients and has a spongy surface so that the cell structure (blastocyst) can nestle well. This is where the later one is created placenta.

In the first week of pregnancy it can also happen that two independent organisms begin to form during cell division. The consequences are identical Twins.

Development in the 2nd week of pregnancy

The second week of pregnancy is characterized by the full implantation (Implantation) of the blastocyst. The placenta develops and the so-called germinal discs and cavities, like that, arise Amniotic fluid cavity (Amninon cavity) and the Yolk sac.

The high rate of cell division at this stage not only increases the number of cells, but also allows for further differentiation and specification for specific tasks. Different cell layers form, which form a cavity. The embryonic ones lie there protected Stem cells. This inner group of cells is called an embryoblast.

The formation of the germinal disks results in many different processes. This is how the main axis of the developing embryo is determined, i.e. where it is above and below. In addition, there is a great shift in the individual caves and, among other things, the amniotic fluid cave is created, which is now slowly moving with it Amniotic fluid fills and later fills the entire uterus.

You might also be interested in: Amniotic fluid examination

Development in the 3rd week of pregnancy

That is ruled third week of pregnancy by training the three Cotyledons. These cotyledons are that Ectoderm, the Mesoderm and the Endoderm. The various cotyledons further differentiate in the course of development and ultimately form all organs of the human body. But you can already see a clear division here.

So from the Ectoderm the Nervous system, the Skin, teeth, hair and many more structures. In contrast to this arise from the Mesoderm the muscles, most inner Organs, bones and connective tissue. From the Endoderm arise the Glands, like the thyroid and thymus. All of this is the result of very complex biochemical and anatomical processes that occur during this phase of development.

At this point, for example, so-called neurulation begins. This term describes the emergence of a Neural tube from parts de Ectoderms and the resulting differentiation of embryonic stem cells into precursors of the Neurons.
The embryo is in this very complex phase of development very sensitive on external pollutants such as alcohol.

You can find out about the effects of alcohol consumption during pregnancy at: Alcohol during pregnancy

Development in the 4th week of pregnancy

During the fourth week of pregnancy there are significant Changes in body shape of the embryo. Due to the increasing development of the systems for different organs and that Growth in size of the brain the embryo slowly has the familiar C-shaped appearance. In addition, the first signs of upper and lower extremities, the so-called extremity buds.

The one created in the 3rd week Neural tube occludes now at the upper and lower opening. So that it comes to a proper closure is above all Folic acid significant. Otherwise it can too serious disorderslike one Spina bifida come. Read on here: Spina bifida

Furthermore, the Cardiac system on the front of the body and now begins to develop in the upper chest area. The first systems for the Ears and eyes can already be guessed at at this stage.

Development in the 5th week of pregnancy

Overall, the body shape of the embryo changes only slightly in the fifth week of pregnancy. Of the embryo is at this stage approx. 2 mm in size. The only exception to this is the head, which is now disproportionate Brain growth appears very large in comparison with the other body structures.

The upper and lower limb buds also continue to grow. So-called urns develop between these two systems. In humans, however, these regress after a certain time. However, some structures remain, such as the testes or the ovary (ovary) and the spermatic ducts, which have arisen from the cells of the urinary tubules.

In addition, the first are formed at the same time Facial structures out. Often it is also the fifth week in which this Heart starts beating for the first time. The heart rate is about twice as high as that of the mother.

Development in the 6th week of pregnancy

The extremities are making tremendous strides this week. Both Poor you can now create individual structures like the Elbow and the Palms detect. These show the first facilities for the development of the fingers in the form of so-called finger jets.
The lower extremity follows a similar development, but only a few days later.

The merging of certain protrusions in the head area now creates the precursor of the outer Ear canal and the auricle. Due to the formation of certain color pigments in the retina, the eyes now easy to see.

In addition, some literature reports that embryos at this stage already respond to external stimuli Reflex twitches can react, which is evidence of the progressive development of the nervous system.

Development in the 7th week of pregnancy

The big ones Leap in development of the extremities extend into the seventh week of pregnancy. The Finger systems are now well defined and the notches that arise between the fingers already give an idea of ​​the future shape.

By the enormous growth of internal organs, especially of the intestine, it comes to physiological, so planned by nature, Umbilical hernia. Since the abdominal cavity does not grow as fast as the intestine, it comes to one acute lack of space. The solution is to move the bowel outwards for a short period of time, which is achieved through the umbilical hernia.

The big ones start this week too Ossification processes in the extremities, starting with the upper arm bone (humerus).

Development in the 8th week of pregnancy

Of the embryo is now about 15mm tall. The rapid development of the upper extremity is now followed by the lower. It is now also the individual Toes distinguishable and the first signs of one ossification in the area of Thigh can be recognized. Furthermore, all four extremities continue to migrate to the front.

The Facial contours also refine. So are meanwhile Eyelids to recognize and the Auricles are almost completely developed.

At this stage the Development of the external sexual characteristics. However, due to their size, these cannot yet be distinguished in an ultrasound examination at this point in time. The embryo now shows many human characteristics on and the first organs work already. The heart pumps blood through the small vessels of the embryo at a high frequency, the kidneys produce the first urine that is excreted into the amniotic fluid and the stomach begins to produce stomach acid.

Development in the 9th week of pregnancy

In the ninth week of pregnancy All organs are now trained to the point that they can take up their function. Now they begin maturation and the Size growth of the individual organs. At this point the embryo is already about 2 cm in size.

The brain is now getting bigger Cartilage plates protected from the outside. However, these can only grow together after birth, as the brain another rapid growth and therefore requires a lot of space.
There is also a Specialization of the nerve cells instead of. Some take control of movements, while others are responsible for perceiving and processing sensory impressions.

At this point, especially those are Neurons very sensitive to external poisonslike alcohol. So can its consumption severe disorders in the development of the brain have as a consequence. By the end of this week, the embryo has developed to the point that it is now, by definition, a fetus referred to as.

The transition from the embryo to the fetus

The transition of an embryo into one fetus is not a biological process, but a pure matter of definition. This does not happen abruptly, but over the past few weeks. It is now all organs trained and can be partially yours Perform functions. In addition, it is now recognizable that it is a developing human being.

The following weeks of development are shaped by the Specialization of the cells and the Increase in size of the individual organs. This is the only way to create such a complex organism as humans.

A big difference to embryonic development is the extent of the Head growth If this is strongly disproportionate in the first few weeks, the speed now slows down and other areas of the body are the focus of growth. So the body now grows in length. Only from the 5th month does the weight follow this rapid progression.

The viability of the fetus outside the uterus, as a result of a Premature birth, is currently (2017) the 23rd week of pregnancy and a birth weight of at least 500g.