Skeletal System Development

Question 1

What is derived from mesodermal sclerotomes/somites?

Answer 1

• vertebral column
• ribs
• sternum

Question 2

What is derived from later plate mesoderm?

Answer 2

• limb bones

- girdles

Question 3

What is derived from the head mesoderm?

Answer 3

-calvaria and base of skull

Question 4

What is derived from neural crest cells?

Answer 4

-facial bones

Question 5

What interacts in the limb skeleton?

Answer 5

-continuous interaction between apical ectoderm all ridge and limb bud mesoderm

Question 6

What interacts in the vertebral column?

Answer 6

-inductive interaction between sclerotome and notochord or neural tube

Question 7

What interacts in the head bones?

Answer 7

• interaction between preskeletal neural crest cells and information along migratory paths
• interactions between areas of the brain and overlying mesenchyme

Question 8

What is the common pathway of bone/cartilage differentiation?

Answer 8

• mesenchyme is induced to enter the common pathway
• production of N-cadherins which promote mesenchymal cell condensation
• TGF-beta stimulates synthesis of fibronectin and N-CAM
• aggregated state of mesenchymal cells is stabilized

Question 9

Describe the membranous bone pathway.

Answer 9

• requires transcription factors Runx-2 and Osx

- mesenchymal cells differentiate into osteoblasts

Question 10

Describe the permanent cartilage pathway.

Answer 10

• mesenchymal condensation forms chondroblasts
• Sox-9 causes chondroblasts to secrete collagen II and cartilage matrix
• Sox-9 is continually expressed in permanent cartilage

Question 11

Describe the endochondral bone pathway.

Answer 11

• Runx-2, ihh, and BMP-6 induce cartilage to undergo hypertrophy
• hypertrophic cartilage cells secrete bone proteins and vascular endothelial growth factor
• invading blood vessels erode the hypertrophic cartilage and bring in osteoblasts to replace cartilage with bone

Question 12

What is the centrum derived from?

Answer 12

-derived from ventral and medial parts of paired sclerotomes

Question 13

Skeletal tissue is _______________ in origin.

Answer 13

Mesenchymal

Question 14

What do the neural arches arise from?

Answer 14

-arise from dorsal regions of sclerotomes

Question 15

What are the costal processes/ribs derived from?

Answer 15

• proximal development depends on expression of myotomic myogenic factors, Myf-5 and Myf-6
• distal development depends on BMP signals from somatopleural mesoderm

Question 16

What Hox genes create the boundaries in the ribs and vertebra?

Answer 16

Hox 3: occipital-cervical boundary

Hox 6: cervical-thoracic boundary

Hox 9: attached floating ribs boundary

Question 17

Describe the development of the sternum.

Answer 17

• derived from lateral plate mesoderm
• arises as a pair of cartilaginous bands
• fused cartilaginous bands secondarily subdivide into craniocaudal elements

Question 18

Describe the development of the clavicle.

Answer 18

• arises from neural crest
• follows intramembranous pathway
• one of first bones to become ossified

Question 19

What are the three structures the developing skull is composed of?

Answer 19

• neurocranium
• viscerocranium
• occipital sclerotomes

Question 20

What is the neurocranium?

Answer 20

• part of the skull that surrounds the brain
• has a cartilaginous portion and a membranous portion

-base is formed from the chondrocranium (cartilage)
+origin of occipital, sphenoid, ethmoid, and parts of temporals

-membranous part:
+origin of part of occipital, parietals, frontal so, and part of temporals

Question 21

What is the viscerocranium?

Answer 21

• surrounds oral cavity and pharynx
• has a cartilaginous portion and a membranous portion

-cartilaginous portion forms:
+pharyngeal arch I (Meckel’s cartilage, malleus, incus)
+pharyngeal arch II (Reichert’s cartilage, stapes, styloid)

-membranous portion:
+part of temporal, zygomatic, maxillary, nasal, lacrimal, alatine, vomer, ptygoid plates, mandible, tympanic ring

Question 22

Describe the pharyngeal arches in the viscerocranium.

Answer 22

• mesenchymal that forms the viscerocranium is mostly from neural crest
• 6 pairs of pharyngeal arches

Question 23

What are the 4 components that each pharyngeal arch consists of?

Answer 23

• skeletal element
• muscles
• a branch of a specific cranial nerve
• artery

Question 24

Basioccipital portion of chondrocranium is derived from _________ cartilages and _____________ sclerotomes.

Answer 24

• parachordal cartilages
• occipital sclerotomes
• other components of the original cartilages fuse to form final chondrocranium

Question 25

How does the chondrocranium/neurocranium form?

Answer 25

• ossification centers form within the cartilage allow continued growth and are separated from each other by synchondroses -> growth is due to Shh
• membranous components may be incorporated into the endochondral components
• bones of the neurocranium arise as a result of inductive interactions if surrounding mesenchyme -> characterized by transient appearance of type II collagen and cartilage specific proteoglycan
• intersection of more than two bones form fontanelles
• fusion of sutures

Question 26

What transcription factors are involved in the closure of the sutures?

Answer 26

FGF-2 -blocked-> Noggin -blocked -> BMP -> closure of sutures

-Noggin is expressed in all sutures

Question 27

What’s the difference between a suture and a fontanelle?

Answer 27

• a suture is when two structures fuse together

- a fontanelle is when more than two structures fuse together