DSF Dev of MSK Flashcards
What 3 sections does the mesoderm differentiate into? What structures do these sections make up?
Paraxial mesoderm - somite = axial skeleton, trunk and limb muscles, dermis
Intermediate mesoderm - UG systems
Lateral plate mesoderm - Parietal layer = parietal pleura and peritoneum, skeleton of limbs, Visceral layer = visceral pleura, peritoneum, mesenteries
Somite Dev and Differentiation
From Paraxial Mesoderm
Form along neural tube, in ~35 pairs
Crainocaudal - first appear in occipital region
Differentiate into 3 tissue types:
Dermatome - Dermis
Sclerotome - Axial skeleton, ribs, skull base
Myotome - Trunk and limb musculature
Bone Dev - Axial Skeleton
Sclerotomes from somites: vertebral column, ribs, base of skull
Mesenchyme of neural crest cells: calvarium, facial bones
Bone Dev - Face and Skull
Paraxial mesoderm and neural crest cells: flat bones of skull, neurocranium - frontal, nasal, lacrimal, vomer, parietal and occipital
Neural crest cells: Facial bones, viscerocranium - maxilla, zygomatic, portion of parietal bone, mandible, bones of inner ear
Bone Dev - Base of Skull (Chondrocranium)
Neural crest cells form base of skull anterior to pituitary fossa
Paraxial mesoderm forms base of skull posterior to pituitary fossa
Pituitary fossa is cranial most extension of notochord
Lateral Somitic Frontier
A border separates mesodermal domains
Sclerotomes migrate across the LSF to develop into bones in different regions:
Primaxial region - remain in axial region
Abaxial region - migrate across LSF
Bone Dev - Axial Skeleton
Vertebra and bony part of ribs - Derived from scleotomes which remain in primaxial domain
Cartilaginous portion of ribs - Sclerotomes in abaxial domain (crossed LSF)
Sternum - Develops independently in parietal layer of lateral plate mesoderm
Bone Dev - Intramembranous and Endochondral ossification
Intramembranous ossification: Direct ossification w/o a cartilaginous model, derived from neural crest cells
Endochondral ossification: Bones which form from a cartilaginous model, derived from paraxial and lateral plate mesoderm
Endochondral Ossification
Condensation of mesenchyme - Cartilaginous model
Primary ossification: occurs in diaphysis (shaft) of long bones, becomes vascularized, ossifies toward epiphysis (ends), completed by birth
Secondary ossification: occurs in epiphysis, begins after birth
Joint Dev
Synovial joints form at same time as cartilaginous models of bone
Interzone: zone b/w 2 chondrifying bone primordia, differentiates into dense fibrous tissue
Fibrous tissue forms: articular cartilage, synovial membranes, menisci and ligaments w/in joint capsule
Fibrous joints - sutures in skull - remain dense fibrous tissue
Sutures and Fontanelles
Sutures: CT separating flat bones
Fontanelles: pts where more than 2 bones meet
Anterior Fontanelle - Frontal and Parietal bones, not fully ossified until after 2nd year - “soft spot”
Posterior Fontanelle - Occipital and Parietal bones, closes w/in first months of birth
Sutures and Fontanelles allow expansion for brain growth, and overlap/molding during birth
Lateral Plate Mesoderm
Parietal layer: forms CT of limbs - vasculature, skeletal components
Tightly condensed mesenchyme
Induced to form small elevations - limb buds
Limb buds
Upper limb bud forms ~24 days
Lower limb bud forms ~26-28 days
Formed by parietal layer of lateral plate mesoderm
Each bud consists of Mesodermal core and Ectodermal “cap”
Apical Ectodermal Ridge
Thickening of ectoderm along apex of limb bud
Necessary for outgrowth of distal limb structures
Mesenchyme Proximal: condenses - blood vessels, cartilage
Mesenchyme Adj: loose organization - undifferentiated, flatten into hand and foot plates
Hand and Feet Dev
Distal ends of buds flatten - hand and foot plates
Condensing of hand plate mesenchyme - digital rays
End of 6th wk: Finger buds
7th wk: Toe buds
Mesenchymal apoptosis results in separation of digits
Limb extension
Ventral surface - medial
Dorsal surface - lateral
Limbs bend anteriorly: Elbow and knee - lateral, Palms and sole - face trunk
Limb Rotation
Upper extremity: 90 degree rotation laterally
Ventral - flexor compartment - Anterior
Lower extremity: 90 degree rotation medially
Ventral - flexor compartment - Posterior
Muscle Formation: Mytomes LSF Primaxial Domain Abaxial Domain
Mytomes: Muscle forming areas of the somites
LSF (Lateral Somitic Frontier): well-defined border b/w somites (paraxial mesoderm) and parietal layer of lateral plate mesoderm
Primaxial Domain: region around neural tube which contains only paraxial mesoderm (somite cells)
Abaxial Domain: consists of parietal layer lateral plate mesoderm and paraxial mesoderm which migrated across the LSF
Muscle Formation - Primaxial Domain
Only paraxial mesoderm (somites)
Form muscles of: back (innervated by dorsi rami), intercostals, rhomboids, levator scapulae, latissimus dorsi
Muscle Formation - Abaxial Domain
Consists of paraxial mesoderm (mytomes from somites) and lateral plate mesoderm
Cells migrate ventrolaterally to form: trunk muscles and limb buds (Posterior condensation - dorsal mass, Anterior condensation - ventral mass)
Upper Limb Muscle Formation
Anterior condensation: Flexor and Pronator musculature
Posterior condensation: Extensor and Supinator musculature
Lower Limb Muscle Formation
Posterior condensation: Extensor and ABductor musculature
Anterior condensation: Flexors and ADductor musculature
Due to limb rotation:
Anterior condensation - posterior and medial thigh, posterior leg
Posterior condensation - gluteal region, anterior and lateral thigh, anterior leg
Motor innervation of the upper limb
Ventral rami from C5-8 and T1 arrive at base of limb bud and join in specific pattern resulting in:
Posterior divisions - Middle and upper trunks join to form the posterior cord and grow into the posterior condensation
Anterior divisions form - Medial cord (lower trunk), Lateral cord (middle and upper trunk), and grow into anterior condensation
Anterior condensation of Upper limbs
Medial and Lateral cords branches:
Musculocutaneous n., Ulnar n., Median n.
Posterior condensation of Lower limbs
Posterior cord branches:
Axillary n., Radial n.
Motor innervation of Lower limbs
Ventral rami from L2-L5 and S1-S3 arrive at base of limb bud and divide:
Posterior divisions - Grow into posterior condensation
Anterior divisions - Grow into anterior condensations
Dermatomes
Map showing cutaneous innervation Clear, orderly pattern C4 at base of neck C7 at middle finger C8/T1 at armpit T4 level of nipple
