Topic 54 - Formation of germ layers and axial organs
Covers gastrulation, the formation of the three germ layers and which organs each one gives rise to. Walks through the numbered steps from germ disc formation through primitive streak, notochord and neural tube development to mesoderm subdivision. Then explains how the anteroposterior and left–right body axes are set up, naming the key genes and signalling molecules involved.
Topic 54 of 59 in the MU Pleven syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 4 sections, about 472 words (3 min of reading), with 15 flashcards.
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Topic 54 - Formation of germ layers and axial organs
Gastrulation
Steps of Gastrulation
Establishment of the Body Axes
Gastrulation
Gastrulation is the process in which the three germ layers of the embryo are formed.
The three germ layers are:
Ectoderm (external): forms the integumentary system, brain, spinal cord, and pituitary gland.
Mesoderm (middle): forms the musculoskeletal system (bones, cartilage), cardiovascular system, excretory system, and reproductive system.
Endoderm (internal): forms the epithelial layer of lungs, digestive system, liver, pancreas, and bladder, as well as the thyroid and parathyroid glands.
Steps of Gastrulation
1. Formation of the germ disc (embryonic disc): occurs after implantation and before embryonic folding, around 14–21 days after fertilization.
2. Following implantation, trophoblast cells differentiate into two types, and inner cell mass cells differentiate into hypoblast and epiblast, forming the flat germ disc.
a. Hypoblast contributes to the umbilical vesicle.
b. Epiblast forms the amniotic cavity.
3. Gastrulation begins with formation of the primitive streak on the surface of the epiblast. Epiblast cells migrate toward the streak and create an invagination.
4. Some migrating cells displace the hypoblast to form the endoderm. Other cells migrate between the epiblast and endoderm to form the mesoderm.
5. Remaining epiblast cells become the ectoderm.
6. Mesodermal cells produce the notochord, a cylindrical rod running along the length of the embryo.
7. The notochord induces thickening of ectodermal cells to form the neural plate.
8. Central cells of the neural plate move outward, forming the neural groove and neural folds. Neural folds eventually fuse to form the neural tube, establishing the anterior–posterior body axis.
9. The notochord extends beneath the neural tube from head to tail.
10. Mesoderm cells proliferate along the midline to form the paraxial mesoderm.
11. On each side, mesoderm remains thin as the lateral plate mesoderm.
12. Intermediate mesoderm lies between paraxial and lateral plate mesoderm.
13. Between days 13–15, proliferation of extraembryonic mesoderm, primitive streak, and embryonic mesoderm occurs.
14. The notochord process develops between days 15–17.
15. Between days 17–19, the notochord canal and axial canal form, and the first three somites appear.
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