Development of the embryo

Basically, the term embryo is defined as a living being that is in an early stage of its development. Thus, this definition applies not only to humans, but to all living beings. An embryo is created through the development of a fertilized egg cell and is normally called an embryo as long as it is in the womb.

In humans, however, an exception is made here by definition. In humans, the embryo is referred to as the foetus from the ninth week of pregnancy. This is where the embryonic period passes into fetal development.

In humans, however, an exception is made here by definition. In humans, the embryo is referred to as the fetus or foetus from the ninth week of pregnancy. In this case, the embryonic period passes into fetal development.

What happens to the embryo in the 1st trimester?

The first 12 weeks of pregnancy are summarized as the 1st trimester or 1st trimester. The first day is defined as the first day of the last menstruation. From this day on, the embryo needs exactly 40 weeks to develop fully.

Often the pregnancy of the future mother is not yet visible, even if pregnancy symptoms are already noticeable. The first trimester is also the critical phase with regard to external toxins. The consumption of alcohol, cigarettes or other drugs can lead to massive disturbances in the development of the child.

Furthermore, the risk of miscarriage is highest in the first trimester. This topic might also be of interest to you: How dangerous is smoking during pregnancy? The development of the individual organs and the disproportionate growth of the head are the main focus of the 1st trimester.

Only in the second and third trimesters do massive growth in size and weight gain take place. At the beginning of the first week of pregnancy there is initially a single fertilized egg in the fallopian tube. In the first days of pregnancy, this migrates towards the uterus to become a nidation.

During this phase, the first divisions of the ovum already take place. This is followed by the 2-cell stage. On the third and fourth day further cell divisions take place, so that a slight cell specialization (differentiation) can already be observed.

The resulting cell group is called blastocyst and already contains several cells. On the fifth to sixth day the actual implantation into the uterus takes place. During the normal cycle, the uterus has prepared itself optimally for this situation.

It is well supplied with blood, contains many nutrients and has a spongy surface so that the cell group (blastocyst) can settle down well. This is where the placenta develops. In the first week of pregnancy it can also happen that two independent organisms begin to form during cell division.

The result is identical twins. The second week of pregnancy is characterized by the complete implantation of the blastocyst. The placenta develops and the so-called germinal discs and cavities such as the amniotic cavity (amninonic cavity) and the yolk sac are formed.

The high rate of cell division at this stage not only increases the number of cells, but also enables further differentiation and specification for specific tasks. Different layers of cells are formed, forming a cave. This is where the embryonic stem cells are protected.

This inner cell group is called embryoblast. The formation of the germinal discs results in many different processes. This is how the main axis of the developing embryo is determined, i.e. where above and below is.

In addition, there is a large shift in the individual cavities and, among other things, the amniotic cavity is formed, which now slowly fills with amniotic fluid and later fills the entire uterus. The third week of pregnancy is dominated by the formation of the three cotyledons. These cotyledons are the ectoderm, the mesoderm and the endoderm.

The different cotyledons continue to differentiate in the course of development and ultimately form all organs of the human body. However, a clear division can already be seen here. The ectoderm forms the nervous system, the skin, teeth, hair and many other structures.

In contrast, the mesoderm develops into the muscles, most of the internal organs, bones and connective tissue. From the endoderm the glands, such as the thyroid gland and the thymus, develop. All this is the result of very complex biochemical and anatomical processes that take place during this developmental phase.

For example, it is at this point that the so-called neurulation begins.This term describes the emergence of a neural tube from parts of the ectoderm and the subsequent differentiation of embryonic stem cells into precursors of nerve cells. In this very complex phase of development, the embryo is very sensitive to external pollutants such as alcohol. You can find out what effect alcohol consumption has during pregnancy at Alcohol during pregnancyDuring the fourth week of pregnancy, there are significant changes in the body shape of the embryo.

Due to the increasing development of the systems for different organs and the growth of the brain, the embryo slowly takes on the familiar C-shaped appearance. In addition, the first signs of the upper and lower extremities, the so-called limb buds, develop. The neural tube that developed in the 3rd week now closes at the upper and lower opening.

Folic acid is particularly important for proper closure. Otherwise, serious disorders such as spina bifida can occur. In addition, the heart system migrates to the front of the body and now begins to develop further in the upper chest area.

The first systems for the ears and eyes can also already be guessed at this stage. Overall, the body shape of the embryo changes only slightly in the fifth week of pregnancy. The embryo is about 2 mm in size at this stage.

The only exception to this is the head, which now appears very large compared to the other body structures due to the disproportionate growth of the brain. The upper and lower limb buds also continue to grow. Between these two structures, the development of so-called urination occurs.

However, in humans these recede after a certain time. However, some structures remain, such as the testicles or the ovary (ovary) and the spermatic ducts, which have developed from the cells of the urinary tubules. At the same time, the first facial structures are formed.

Often it is also the fifth week when the heart starts beating for the first time. The heart rate is approximately twice as high as that of the mother. The extremities make enormous progress during this week.

In the arms you can now see individual structures such as the crook of the arm and the palms of the hands. These show the first facilities for the development of the fingers in the form of so-called finger beams. The lower extremity follows a similar development, but only a few days later.

By fusing certain bulges in the head area, the precursors of the external auditory canal and the auricle are now formed. Due to the formation of certain color pigments in the retina, the eyes can now also be easily recognized. In addition, some literature reports that embryos at this stage can already react to external stimuli with reflex twitches, which is evidence of the progressive development of the nervous system.

The great leaps in development of the extremities extend into the seventh week of pregnancy. The finger systems are now well developed and the resulting indentations between the fingers already give an idea of the future shape. Due to the enormous growth of the internal organs, especially the intestines, a physiological umbilical hernia occurs, which was planned by nature in this way.

Since the abdominal cavity does not grow as fast as the intestine, an acute lack of space occurs. The solution is to move the intestine outwards for a short time, which is achieved by the umbilical hernia. During this week the large ossification processes in the extremities also begin, starting with the humerus.

The embryo is now about 15mm in size. The rapid development of the upper extremity is now followed by the lower extremity. The individual toes are now also distinguishable and the first signs of ossification in the area of the thigh can be seen.

Furthermore, all four extremities move further to the front. The facial contours also become more refined. Eyelids are now visible and the auricles are almost completely formed.

At this stage the development of the external sexual characteristics also begins. However, due to their size, these cannot be distinguished at this stage in an ultrasound examination. The embryo now shows many human characteristics and the first organs are already working.

For example, the heart pumps blood at a high frequency through the small vessels of the embryo, the kidneys produce the first urine, which is excreted into the amniotic fluid and the stomach begins to produce gastric acid. By the ninth week of pregnancy, all organs are now sufficiently developed to be able to take up their functions. Now the maturation and size growth of the individual organs begins.At this time, the embryo is already about 2 cm in size.

The brain is now protected to the outside by larger cartilage plates. However, these can only grow together after birth, as the brain is still growing rapidly and therefore requires a lot of space. Furthermore, a specialization of the nerve cells takes place.

Some take over the control of movements, while others are responsible for the perception and processing of sensory impressions. At this time, the nerve cells in particular are very sensitive to external toxins, such as alcohol. The consumption of alcohol can lead to severe disorders in the development of the brain. By the end of this week, the embryo is so far developed that it is now called a fetus by definition.