The skeleton

Question 1

What are the types of mesoderm that it differentiate into?

Answer 1

The mesoderm differentiates into lateral, which forms the truck, and paraxial mesoderm, which forms medial-cranial and generates somites.

Question 2

What are the tissues formed from somite differentiation? Under which factors are they regulated?

Answer 2

Somitogenesis forms:
1. dermatome
2. myotome
3. sclerotome
Somites differentiate into these tissues since the mesoderm is epithelial (form the lumen); the ventral and medial part of the mesoderm become mesenchymal and migrates to the neural tube and notochord, forming the sclerotome; then the  dermatomyome forms, which is regulated by BMP4 and Wnt.

Factors:

• Shh and FGF8 upregulate nogging and chordin, causing the formation of the sclerotome from the ventral part of the somites.
• PAX3 and NT3 regulate dermatome, causing the formation of the dermis
• MYF5 and Wnt specify the cells in the dermamyotome, causing muscle formation
• MYOD specify cells in the dermamyotome, causing the formation of muscle cells precursors.

Question 3

How do vertebrae form?

Answer 3

Somites migrate around the neural tube and notochord, fusing with each other: this process, resegmentation, cause the formation of 8 cervical sclerotomes, which will develop into 7 cervical vertebrae by splitting and recombination. The upper cervical sclerotome form the skull base, or occipital bone. Each somite also form a spinal nerve, which is the ventral root of the motor neurons that innervate the myotome.

The vertebral column is form by the vertebrae, which surround the notochord (nucleus pulposus) and the spinal cord (CNS), and are divided by the intervertebral disc/annulus fibrosus.

The spinal cord is both the ventral root of the motor neurons that innervate the muscles, and the dorsal root ganglia of the PNS, which arise from the neural crest.

Question 4

What is scoliosis?

Answer 4

Scoliosis occur when the vertebral column is not straight, when seen frontally.

Types:

• congenital
• idiopathic
• neuromuscular

Question 5

What is flat back syndrom?

Answer 5

Flat back syndrome occurs when the vertebral column don’t have the natural curves that it should have.

Question 6

Which germ lines does the skull derive from?

Answer 6

The skul can be divided in two part:

1. neurocranium:
   
   • flat bones
   • neural crest + paraxial mesoderm, in which mesenchymal undergo ossification withouth forming cartilage
     i. cartilaginous neurocranium
     ii. chondrocranium
     
     • fusion between intermediate cartilages:
       i. prechondral chondrocranium
       ii. hypophyseal chondrocranium
       iii. chordal chondrocranium
   • it grows as much as the brain needs
     
     • bones lie separately, but are linked by connective tissue (suture/fontanelle)
2. viscerocranium
   
   • ectodermal origin
   • neural crest
     i. two pharyngeal arches
     ii. mandibula
     iii. maxillary
     iv. nasal prominence

Question 7

What is an important factor for craniosynostis?

Answer 7

Craniosynostis occurs when the skull bones do not develop properly, due to a premature closure of sutures. It is caused by a mutation in the FGF ligands and receptors, which cause an unbalance between proliferation and differentiation of sutures.