spine and limbs

Question 1

Paraxial mesoderm transformation occurs when the PM condenses into segmental tissues called__________.

Answer 1

Somites

•First pair of somites form on day 20

•For the next 2 weeks, new pair forms every 8 hrs caudal to previous pair

Question 2

What happens to the cranial most somites?

Answer 2

They are not as distinct balls of paraxial mesoderm, thus called somitomeres, contribute to neurocranium (skull bones)

Question 3

What happens to the rest of the somites? (non cranial)

Answer 3

Each somite differentiates into three segments

Question 4

Resegmentation of sclerotomes

Answer 4

• Each somite receives a spinal nerve
• Spinal nerve splits each sclerotome
• Fusion of cranial & caudal portions of two sclerotomes forms a vertebra = resegmentation

Question 5

Vertebral Column Resegmentation

Before resegmentation:
–8 cervical sclerotomes
–8 cervical spinal nerves (sn)

Answer 5

•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

Question 6

What happens to the myotomes?

Answer 6

-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

Question 7

What happens to the Epimeres?

Answer 7

• Give rise to the deep muscles of the back
• Maintain segmental innervation via dorsal rami

Question 8

What happens to the Hypomeres?

Answer 8

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

Question 9

Development of the limbs

•Begins as small outgrowth, limb bud, of the ____________ (somatic lateral plate mesoderm + overlying ectoderm)

Answer 9

Somatopleure

Question 10

What is the anatomy of the limb bud?

-_________ covers the outside.

-_________ “fills” the inside

Apical ectodermal ridge (AER) at distal edge (ectodermal thickening).

Answer 10

1- Ectoderm

2-Somatic lateral plate mesoderm

Question 11

How does the limb bud grow?

Answer 11

•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

Question 12

How does the limb bud grow?

Answer 12

• Site of mesodermal cell proliferation = undifferentiated/progress zone
• Proximal mesodermal cells start to differentiate as progress zone becomes distant

Question 13

How does the limb bud grow?

Answer 13

•Limbs grow in recognizable stages

-Segmental apoptosis bY AER is important to shape the limbs

Question 14

What happens when reciprocal induction is altered?

Answer 14

•Congenital deformity of the limbs
–Phocomelia
–Amelia
–Syndactyly
–Polydactyly

Question 15

Developmental components of the limb:

Answer 15

• 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).

Question 16

Innervation of limb muscles

Answer 16

•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

Question 17

Sensory Innervation

Answer 17

• Sensory nerves segmentally innervate each dermatome
• Subsequent morphogenesis distorts the simple segmented pattern

Question 18

Skeletal system overview

Answer 18