spine and limbs Flashcards
(18 cards)
Paraxial mesoderm transformation occurs when the PM condenses into segmental tissues called__________.
Somites
•First pair of somites form on day 20
•For the next 2 weeks, new pair forms every 8 hrs caudal to previous pair
What happens to the cranial most somites?
They are not as distinct balls of paraxial mesoderm, thus called somitomeres, contribute to neurocranium (skull bones)
What happens to the rest of the somites? (non cranial)
Each somite differentiates into three segments
Resegmentation of sclerotomes
- Each somite receives a spinal nerve
- Spinal nerve splits each sclerotome
- Fusion of cranial & caudal portions of two sclerotomes forms a vertebra = resegmentation
Vertebral Column Resegmentation
Before resegmentation:
–8 cervical sclerotomes
–8 cervical spinal nerves (sn)
•After resegmentation:
–7 cervical vertebrae
–8 cervical sn retained
–Thus, C1-7 sn emerge above corresponding cervical vertebrae and C8 sn emerges below C7 vertebra
–T1 and the rest of sn emerge below corresponding vertebrae
What happens to the myotomes?
-Form segmentally innervated skeletal muscles
-Each myotome divides into dorsal (epimere) and ventral (hypomere) groups
–Segmental sn’s split into 2 branches to accommodate the two groups; dorsal & ventral rami
What happens to the Epimeres?
- Give rise to the deep muscles of the back
- Maintain segmental innervation via dorsal rami
What happens to the Hypomeres?
Give rise to the muscles of the body wall and limbs
–Muscles of the tongue
–Strap muscles of the neck
–3 layers of intercostals
–3 layers of the abdominal muscles
–Rectus abdominis
–Quadratus lumborum
–Limb musculature
•Generally maintain segmental innvervation via ventral rami except for the limb muscles
Development of the limbs
•Begins as small outgrowth, limb bud, of the ____________ (somatic lateral plate mesoderm + overlying ectoderm)
Somatopleure
What is the anatomy of the limb bud?
-_________ covers the outside.
-_________ “fills” the inside
Apical ectodermal ridge (AER) at distal edge (ectodermal thickening).
1- Ectoderm
2-Somatic lateral plate mesoderm
How does the limb bud grow?
•Reciprocal induction b/w AER and adjacent mesoderm (epithelial-mesenchymal interaction)
–AER induces mesodermal cells to proliferate
–Mesodermal cells induce AER to persist and continue to signal
How does the limb bud grow?
- Site of mesodermal cell proliferation = undifferentiated/progress zone
- Proximal mesodermal cells start to differentiate as progress zone becomes distant
How does the limb bud grow?
•Limbs grow in recognizable stages
-Segmental apoptosis bY AER is important to shape the limbs
What happens when reciprocal induction is altered?
•Congenital deformity of the limbs
–Phocomelia
–Amelia
–Syndactyly
–Polydactyly
Developmental components of the limb:
- Myotomes (hypomere) -> limb musculature
- Axons from neural tube -> motor innervation
•Axons from spinal ganglia -> sensory innervation
•
Neural crest -> melanocytes (skin coloration)
•Somatic lateral plate mesoderm -> connective tissues (bone, blood).
Innervation of limb muscles
•Nerves are already associated with myotomes as they enter the limb
- Forming bone divides the limb into dorsal & ventral compartments
- Dorsal muscles -> extensors, supinators & abductors
- Ventral muscles -> flexors, pronators and adductors
Sensory Innervation
- Sensory nerves segmentally innervate each dermatome
- Subsequent morphogenesis distorts the simple segmented pattern
Skeletal system overview
