Title: Embyrogenesis Timeline

Embryo-genesis

focusing on Skeleton Development

1 to 2: Day 1&2

First cleavage and second cleavage. Cell becomes two then four.

7 to 13: Second Week

2 layered germ: ectoderm and endoderm

14 to 20: Third Week

- formation of mesoderm: cells of primitive streak invade between ectoderm and endoderm
- formation of notochord
- differentiation of neural crest cells
- formation of somites starts

19: Day 19

Formation of neural plate

20: Day 20

Folding of neural plate -> neural folds (day 20) and neural groove

21 to 27: Fourth Week

- differentiation of somites to sclerotome, myotome and dermatome
- cells of neural crest move into mesoderm -> epithelium-mesenchym-transformation
- formation of somites

22: Day 22

Closure of neural groove to neural tube

23: Day 23

Closure of neural folds (neuroporus anterior)

27: End of Fourth Week

Secretion of FGF-10 from lateral plate mesoderm

- formation of apical ectodermal ridge (AER) -> secretion of FGF-8
- proliferation of mesoderm -> progress zone
- zone of polarizing activity (ZPA)

28 to 34: Fifth Week

Limb sprouts of parietal mesoderm, covered with ectoderm -> limb bud

35 to 41: Sixth Week

Circular constriction -> hand/foot plate, later elbow, knee
Apoptosis -> formation of five fingers/toes
Hyaline cartilage anlange

42 to 48: Seventh Week

Rotation of Limb

77 to 83: Twelfth Week

Primary ossification centers in all long bones

273 to 279: After Birth

Secondary ossification centers appear

500: After Puberty

Closure of growth plates