DSF Dev of MSK

Question 1

What 3 sections does the mesoderm differentiate into? What structures do these sections make up?

Answer 1

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

Question 2

Somite Dev and Differentiation

Answer 2

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

Question 3

Bone Dev - Axial Skeleton

Answer 3

Sclerotomes from somites: vertebral column, ribs, base of skull
Mesenchyme of neural crest cells: calvarium, facial bones

Question 4

Bone Dev - Face and Skull

Answer 4

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

Question 5

Bone Dev - Base of Skull (Chondrocranium)

Answer 5

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

Question 6

Lateral Somitic Frontier

Answer 6

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

Question 7

Bone Dev - Axial Skeleton

Answer 7

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

Question 8

Bone Dev - Intramembranous and Endochondral ossification

Answer 8

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

Question 9

Endochondral Ossification

Answer 9

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

Question 10

Joint Dev

Answer 10

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

Question 11

Sutures and Fontanelles

Answer 11

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

Question 12

Lateral Plate Mesoderm

Answer 12

Parietal layer: forms CT of limbs - vasculature, skeletal components
Tightly condensed mesenchyme
Induced to form small elevations - limb buds

Question 13

Limb buds

Answer 13

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”

Question 14

Apical Ectodermal Ridge

Answer 14

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

Question 15

Hand and Feet Dev

Answer 15

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

Question 16

Limb extension

Answer 16

Ventral surface - medial
Dorsal surface - lateral
Limbs bend anteriorly: Elbow and knee - lateral, Palms and sole - face trunk

Question 17

Limb Rotation

Answer 17

Upper extremity: 90 degree rotation laterally
Ventral - flexor compartment - Anterior

Lower extremity: 90 degree rotation medially
Ventral - flexor compartment - Posterior

Question 18

Muscle Formation:
Mytomes
LSF
Primaxial Domain
Abaxial Domain

Answer 18

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

Question 19

Muscle Formation - Primaxial Domain

Answer 19

Only paraxial mesoderm (somites)

Form muscles of: back (innervated by dorsi rami), intercostals, rhomboids, levator scapulae, latissimus dorsi

Question 20

Muscle Formation - Abaxial Domain

Answer 20

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)

Question 21

Upper Limb Muscle Formation

Answer 21

Anterior condensation: Flexor and Pronator musculature

Posterior condensation: Extensor and Supinator musculature

Question 22

Lower Limb Muscle Formation

Answer 22

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

Question 23

Motor innervation of the upper limb

Answer 23

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

Question 24

Anterior condensation of Upper limbs

Answer 24

Medial and Lateral cords branches:

Musculocutaneous n., Ulnar n., Median n.

Question 25

Posterior condensation of Lower limbs

Answer 25

Posterior cord branches:

Axillary n., Radial n.

Question 26

Motor innervation of Lower limbs

Answer 26

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

Question 27

Dermatomes

Answer 27

Map showing cutaneous innervation
Clear, orderly pattern
C4 at base of neck 
C7 at middle finger
C8/T1 at armpit
T4 level of nipple