Lecture Wednesday August 14: Musculoskeletal Development

Skull development - NEURAL CREST
Neurocranium: skull - intramembraneous ossification (base of skull endochondral)
Viscerocranium: face - intramembraneous ossification

Sagital sutures allow for growth of skull/brain in width
Coronal sutures allow for growth of skull/brain in length

Limb Development
Mesodermal derivatives of the limbs:
Paraxial mesoderm - skeletal muscle
Somites
1. Sclerotome - bone and connective tissue of vertebral column
2. Dermatome- dermis of the skin
3. Myotome- skeletal muscles of limbs and body wall

Lateral plate (somatic mesoderm) - bone, connective tissue


Apical Ectodermal Ridge (AER)
- Proximal to distal growth of the limb
- Produces FGF 4 and 8 to cause cell division in the mesoderm below the AER (progress zone) therefore growth in length of the limb
- Loss of AER causes limb truncation. Normal loss of AER necessary for formation of digits


Zone of Polarizing Activity (ZPA)
- cranial to caudal growth and pattern formation (thumb on one side pinky on the other) through the production of HOX (homeobox) genes in response to sonic hedgehog (Shh).

polydactyly = extra digits

Bone Formation
1. Intramembraneous ossification: mesenchyme to osteoblasts
2. Endochondral ossification: mesenchyme to chondroblasts to cartilage model. Blood vessels bring osteoprogenitor cells that remodel the cartilage to bone.


Muscle Development
Myogenic progenitor cell } myoblast } myotubes } myofiber
Don't memorize the transcription factors!


LIMB ANOMALIES
Reduction defects = AER
Duplication defects = ZPA
Dysplasia = persistent AER


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