Embryogenesis

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Under embryogenesis ( AltGr. Ἔμβρυον ( Embryo ) ' unborn fetus ' and γένεσις ( genesis ) ' Development, 'Origin' ) or embryonic development is understood that phase of germ development, from fertilization of the ovum through segmentation, Blastulation and gastrulation to form the organ systems ( organogenesis ) and leads in which there is a substantial change in the external appearance of the embryo blasts and embryos. This period is also called the embryonic period.

The placental development proceeds in the bud stage of the fertilized egg ( zygote ) to the blastocyst implants itself on the 5th to 6th day of development in the womb. With the formation of the chorionic villi and the uptake of the compound to the maternal circulation, the embryonic stage begins.

In humans, the development of embryogenesis is after 8 weeks ( after conception ) ended. Approximately half the proportion of now reached, crown-rump length (CRL ) from an average of 28-30 mm now occupies the head. P.c. With the beginning of the third month is the further development, which is characterized mainly by rapid body growth, as fetal period, means the further development as Fetogenese.

The human intrauterine development can basically be divided into three main sections. It consists

The study of embryogenesis is the responsibility of embryology.

• 2.1 Early embryogenesis 2.1.1 of the primitive streak
• 2.1.2 gastrulation
• 2.1.3 Development of the notochord
• 2.1.4 Folding of the neural tube ( neurulation )

• 2.2.1 Thus emergence
• 2.2.2 curvature movements
• 2.2.3 pharyngeal arches

Blastogenesis

Cell development

The zygote begins immediately after fertilization, to divide. Consequently, the zygote is after the first partition of two blastomeres. The cells divide from now until they form the morula ( mulberry ) on the 4th day with 8 to 32 blastomeres. The development of the first division until the morula stage is called segmentation.

Although the cells multiply, the total volume of the morula towards the zygote is unchanged. At this stage, for example, on the fourth day, there is a differentiation of the cells into an outer and an inner layer of cells. From the outer cell layer of the trophoblast and later the placenta and fetal membranes will develop first, the inner cell layer is to embryoblast, the predecessor of the actual embryo. Water penetrates into the spaces between the cells, and finally collected at a position at which therefore forms the blastocyst after this stage is referred to as a blastocyst.

Nidation

As nidation or implantation is the process in which the embryo descends into the womb, usually 5 - 6th Day after fertilization.

In the second week of the chorion with the placenta grows together.

Embryogenesis

Under embryogenesis that phase of germ development is understood that ( organogenesis ) leads from gastrulation to form the organ systems and requires a significant change in the external appearance of the embryo blasts and embryos. This period is also called the embryonic period. It takes humans from the third to the eighth week of development (pc).

Early embryogenesis

The Early embryogenesis - the people in the third week of development - the period in which the embryo developed the fastest. It comes to the determination of its axes by the formation of the primitive streak. At gastrulation the three germ layers, which show any tissue of the embryo arise.

In the next step it comes to folding of the neural tube ( the beginning of neurulation ) and to provide a rough system of each organ system, which then continues into the next period of embryogenesis.

Formation of the primitive streak

On the 15th day of development can be in the middle of a thickening of the epiblast cells recognize - the primitive streak. This ribbon-like structure allows for the first time a spatial Achsendetermination: The longitudinal axis is defined. It begins at the caudal adhesive stalk near end of the primitive streak. In the latter direction of the primitive streak also continues its growth in length.

The sagittal axis is defined by the dorsal epiblast and ventral hypoblast. After defining these two axes, it falls an easy to set the last axis. If you look at the median plane before, which is spanned by the axes above, and passes exactly through the middle of the primitive streak, one can easily find the transverse axes. They are orthogonal to the median plane ( vertical ) axes. One can speak of right and left in the embryo for the first time.

At the cranial end of the primitive streak of the primitive node is located. Its cells provide for the growth of the head extension in cranial direction. This growth is stopped by the prechordal plate.

After the fourth week of development of the primitive streak disappears almost completely.

Gastrulation

Under gastrulation ( from Lat Gastrum " clay pot bellied " ) is understood to Dreiblättrigkeit the transition from the two-bladed embryoblasts. The Epiblastenzellen the median plane from the ventral fold and then migrate between hypoblast and epiblast laterally. This is how the embryonic mesoblast. Meanwhile, cells invade the hypoblast and displace these laterally. After this immigration differentiating ectoderm (former epiblast ), mesoderm ( mesoblast former ) and endoderm (instead of the hypoblast; former hypoblast ).

The ectoderm forms an epithelial layer of cylindrical cells and the endoderm, a layer of small, polygonal cells. In two places up the embryo only from the ecto- and endoderm, so here is missing the mesoderm. That's the prechordal plate, which will differentiate for later pharyngeal membrane and the cloacal membrane.

Development of the notochord

The formation of the notochord is of great importance, as it serves as a lead to the formation of the spine and induces the folding of the neural tube.

Situated at the primitive node primitive pit forms the Chordakanal by cranially into stretches in the head extension. The median cell cord from the head extension fuses with the endoderm and forms the notochord. When fusing the meantime created openings that connect the yolk sac with the amniotic cavity ( neurenteric canal ).

Folding of the neural tube ( neurulation )

The ectoderm differentiates medially located to the neural plate, while it forms the lateral surface ectoderm. Induced by the notochord, the neural plate folds to the median plane and forms the neural groove. Approximately in the middle of the neural groove to seal them again through the merging of Neuralplattenzellen that are to Neuralrinnenzellen so. This creates the neural tube. The remaining Neuralplattenzellen, the neural crest cells between surface ectoderm and Neuralrinnenzellen migrate from bilateral and form the neural tube located next to the dorsal root ganglia. The surface ectoderm is now closed dorsally over the neural tube and the dorsal root ganglia.

Further development

In the course of embryogenesis - the people in the fourth to eighth week of development - there will be a very wide variety of differentiation of tissue that here first only the main building blocks of organogenesis are presented.

Thus emergence

The somites arise from the paraxial mesoderm. This begins to organize into somites at the end of the third week of development. The main period of the somites is located between the 20th and 30th day of development. In the time arises about every 90 minutes a new Thus pair. The somite formation is induced by the notochord.

These somites are not the same as the vertebrae of the adult human. The latter arise exactly between two somites. There are 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and about 8 kokzygeale somite pairs.

During the development of the somites into two segments, the sclerotome and the Dermatomyotom differentiate. From the Dermatomyotom, among other skeletal muscles developed.

Curvature movements

In the main phase of organogenesis many organs caused by curvature motion. A craniocaudal curvature of the embryo is caused by the rapid growth of the neural tube. It allows, for example, the emergence of the pericardial cavity (see heart # development). Thus, at the origin, there is a bilateral bending movement. From this movement came to the conclusion of the neural tube, the notochord, the intestines and the abdominal cavity.

Pharyngeal arches

In all vertebrates gill arches arise during embryonic development. However, these have undergone a change in their function and therefore should be better called pharyngeal arches.

Usually, a pharyngeal arch apparatus the pharyngeal arches, pharyngeal clefts, pharyngeal pouches and pharyngeal arch membranes. The pharyngeal arch itself consists of a branchial arch artery, a tracheal cartilage, muscle element and a branchial arch nerves. Your core is of mesodermal origin. Outside they are covered by ectodermal tissue inside of endodermal ( pharyngeal pouches ).

From the pharyngeal arches arise, among others, the lower jaw and the chewing muscles.

Fetogenese

From the 9th week ( with the beginning of the third month of pregnancy pc) begins the Fetogenese as the stage of organ development ( morphogenesis ) and the differentiation of tissue ( histogenesis ). It is then already a clear human form to recognize the organs begin gradually with intrinsically active function in the sense of their later stop working.