3B Scientific Embryonic Development Model, 12 stages - Handbuch (Seite 3)

®
3
English
1. The human ovum with a diameter of 120 to 150 μm is the greatest cell of the body. In its cell nucleus
of about 30 μm there is a big nuclear corpuscle. The body consists, for its greater part, of germinal
plasma (idioplasma) and contains only little yolk as nutritive plasma (deutoplasm) . The human
embryo, therefore, must soon take its foodstuffs from the mother.
A membrane (in the model, of transparent plastics), the area pellucida, envelops the ovum. It pre-
vents an untimely nidation of the embryo into the mucous membrane of the uterine tube. Fine
pores serve to absorbe foodstuffs. Male generative cells, spermatozoa, secrete a biochemical catalyzer
(hyaluronidase), by the help of which they break through the membrane. Only one male generative
cell gets into an ovum to be fecundated. The intrusion of the spermatozoon (head and parts of the
tail) is called impregnation. Then the ovum accomplishes its last maturation division. Henceforth it is
called a „spermovium“ (fertilized egg). Now follows the unification of the nuclei of the ovum and the
male generative cell (conjugation).
2. In the next genetic phase the fertilized macrogamete divides into 2 cells (blastomeres): The area
pellucida still exists.
3. After 24 hours other cell divisions have taken place. The daughter cells do not reach the size of the
mother cells, so that the whole embryo is not greater than the „spermovium“. This process is called
segmentation.
4. During the further divisions an odd number of cells may also occur, as the segmentation processes are
not always synchronous.
5. There finally arises an association of smaller cells resembling a mulberry, and therefore called moru-
la. The embryo gets to the uterine cavity within 7 days. The area pellucida envelops it only uncom-
pletely.
6. On the 7th day liquid develops within the morula, and the cells limit a cavity (blastocele). This stage
is called blastocyst, two types of cells are distinguished. The parietal cells of the blastocyst are the
nutrient cells of the embryo (trophoblast). Out of the polar cell complex situated in the interior of the
blastocyst (yellow) develops the embryo (embryoblast). The area pell ucida has been dissolved.
7. The blastocyst grows by propagation of the cells and increase of the cavity. The embryoblast subdi-
vides into extoderm (yellow) and entoderm (red). In the further evolution its cells arrange on the sur-
face and are called ectoderm and entoderm.
8. In this stage the blastocyst migrates through the uterine mucous membrane (implantation or nida-
tion). The trophoblast cells propagate at the nidation side. The embryo is provided with foodstuffs
from the maternal tissues by the release of enzymes which attack the mucous membrane (histiotroph
phase). Besides blood lacunae arise in the trophoblast. Later on the embryo gets foodstuffs directly
from the maternal blood over the tmphoblast cells (hemotroph phase). The other wall of the blasto-
cyst at first remains membrane which protrudes into the uterine cavity. The embryoblast consists of
high cylindrical cells (ectoderm) and flat cubic cells (entoderm), which form together the embryonic
plate. Above the ectoderm arises the amniotic cavity like a dome.
9. The trophoblast grows with villi (chorionic villi) forward into the mucous membrane of the uterus.
Between the trophoblast and the embryoblast lies a net-work of starlikebranched cells, the extro-
embryonic mesenchyme. The embryoblast consists of 2 vesicles, the extoderm vesicle, or amniotic
cavity (yellow), and the entoderm vesicle (red), or vitelline sac. Out of the ectoderm will arise the ner-
vous system, the sense organs and the skin, out of the entoderm the digestive system and the lungs.
The evolution of the human embryo from the
fertilized macrogamate to the 2nd month