Eighth week (stages 21 to 23; ca. 51-56 days)
The eighth week represents the last phase of the embryonic period. The fingers and toes are still connected with webbing. Through the apoptosis (physiologic, programmed death of cells) that occurs in the ectoderm as well as in the mesoderm that lies below it, this gradually disappears (interdigital necrosis zones [INZ]). They become separated from one another in this way and can lengthen.
(stage 21)
The head has risen up and is connected via the neck with the rest of the trunk. The length of the head is still as large as half the entire embryo. The face is well developed. One can already recognize lips and nose, giving the embryo its human appearance.
(stage 22)
Eyes and ears have almost developed to their definitive shapes.
A part of the intestines is still found in the proximal section of the umbilical cord (physiologic umbilical hernia).
The external sexual organs are not yet differentiated to the point that one can determine the baby's gender.
(stage 23)
At around the 56th day one can well distinguish the various features of both the upper and the lower extremities (elbows, fingers, toes). The first movements of the extremities occur at this time.
The tail anlage has completely atrophied at the end of the eighth week.
The generation of the extremities
In the course of the fourth week the upper limb buds develop. They become visible in the deep cervical region on around the 31rst day. The lower limb buds develop in the deep, lumbar region and appear in the course of the 32nd day.
For their growth, the limb buds require numerous interactions between cell layers and secreted products. Their internal axis is formed by the mesoblast of the somatopleura - see picture. The ectoderm ("apical extodermal ridge") forms the outer layer.
The genesis of this ridge is induced by FGF10. Following a successful induction, it secretes FGF8, which affects the mesoderm that lies under the ectoderm, and there stimulates cellular division, leading to the growth of the extremities.These two proteins belong to a family of « fibroblast growth factors » and are coded by genes (FGF 10) that can vary the strength of their expression. . In the course of the development, the expressions of these genes slowly move into the region of the future appendages. In experimental embryology the turning off of the FGF 10 gene's expression leads to the lack of differentiation of this ridge.
Retinol (a derivative of vitamin A) is synthesized by several embryonic cells. To these also belong the primitive node cells during the mesoblast immigration and the activation zone that polarizes (zone of polarizing activation [ZPA]) the arm and leg buds. Results of experimental embryology show that the antero-posterior organization of the extremities (which also determine the appearance sequence of the fingers) depends on a mesenchymal region that is called the ZPA. In this region large amounts of retinol is produced that diffuses and activates further genes in the neighboring cells. Although retinol is absolutely necessary for the development of the embryo, in large amounts it can also have teratogenic effects.
In summary it can be said that at the end of the embryonic period the organogenesis is almost completely finished. The embryo, which originally arose from a single, 0.14 mm sized cell, now consists of millions of cells and measures 30 mm. The outer sexual organs are not yet differentiated enough that one can determine the baby's gender.
During the fetal period growth stands especially in the foreground and no longer the differentiation of the organs, which in some tissues nevertheless continues even after delivery (central nervous system). In the course of the fetal period out of a 30 mm sized embryo a 500 mm sized fetus will come.