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What are the embryological origins of bone?
- Mesoderm
- -paraxial - parts of skull, axial skeleton (from sclerotomes)
- -lateral plate (somatic layer) - appendicular skeleton
- Ectoderm
- -Neural crest cells (parts of skull)
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How does intramembranous ossification work?
- -Occurs in some bones of the skull
- -Cells from paraxial and neural crest cells differentiate directly into osteoblasts
- -These osteoblasts secrete the bony matrix
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Describe endochondral ossification
- -Occurs in all other bones
- -Cells form cartilage models of bone, which later ossify into bones
- 1) cells differentiate into chondrocytes
- 2) secrete cartilage matrix model of bone
- 3) Vascularization and invasion by osteoblasts and osteoclasts
- 4) Calcification of cartilage
- 5) resorption of cartilage
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Which are the bones of the axial skeleton?
Skull, vertebral column, ribs, sternum
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Describe the purpose of the Apical Ectodermal Ridge
- -promotes limb outgrowth and differentiation.
- -lays down cartilage model from proximal to distal
- -causes proximal cells to differentiate appropriately
- -AER undergoes apoptosis to separate digits
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Describe the development of the skull
- Top of skull (intramembranous ossification)
- -posterior (paraxial mesoderm)
- -anterior (neural crest)
- Bottom of skull (endochondral ossification)
- -posterior (paraxial mesoderm)
- -anterior (neural crest)
- Face (both)
- -neural crest
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Describe the development of the vertebral column.
- 1) sclerotome cells migrate around neural tube and notochord
- 2) caudal half of upper sclerotome fuses with the cranial half of the lower sclerotome to form the future vertebral body
- -imagine that the spinal nerves are bisecting these sclerotomes
- 3) endochondral ossification
- Discs:
- -midpoint of original sclerotome becomes annulur fibrosis
- -the notocord becomes the nucleus pulposis
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Describe the develoment of the appendicular skeleton
- 1) AER moved from proximal to distal, inducing differentiation of cells just proximal and leaving behind a cartilage model of the skeleton
- 2) Ossification begins in the diaphysis, completed by birth
- 3) Epiphysis ossifies after birth
- 4) Epiphysial plate continues to be important in long bone development.
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Embryological origins of achondroplasia, congenital scoliosis, congenital kyphosis
- -achondroplasia: mutation of FGR gene, chondrocyte proliferation disrupted in growth plates, stops long bone growth.
- -congenital scoliosis: failure of either left or right scelerotome to ossify, causes a kink from side to side
- -congenital kyphosis: failure of both ossification sites to form, causes an excessive front to back curve (hunchback)
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Define the following: amelia, meromelia, phocomelia
- amelia: absence of 1 or more limb
- meromelia: absence of part of limb
- phocomelia: absence of proximal part of limb
- (all caused by thalidomide)
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What are the two types of craniosynostosis?
- Scaphocephaly: premature fusion of saggital suture (long alien head)
- Brachycephaly: premature coronal suture fusion (tall head)
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What are the embryological origins of the three muscle types?
- skeletal: paraxial mesoderm
- Cardiac: lateral plate mesoderm (splanchnic layer)
- Smooth:
- -viscera and blood vessels: LPM (splanchnic layer)
- -blood vessels of body wall: LPM (somatic layer)
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Define epimere and hypomere.
- Mytome is in two parts:
- epimere: adjacent to neural tube, forms the intrinsic extensor muscles of the back, innervated by dorsal rami
- hypomere: lateral part, all muscles of body walls and limbs (not head and neck), innervated by ventral rami
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