Week 7

Question 1

What is a neural crest cell?

Answer 1

• NCCs are a group of cells that originate from the dorsal part of the vertebral neural tube (ectodermal in origin)
• They can give rise to a wide range of cell types (have the broadest developmental potential)
• Neural tube NCCs are multipotent stem cells

Question 2

Describe the process of neural crest induction:

Answer 2

• NCCs arise at the juxtaposition between neural plate ectoderm and non-neural plate ectoderm
• The important signalling molecules are FoxD3 and Slug

Question 3

Describe the different migration pathways of trunk NCCs:

Answer 3

1. Following induction NCCs undergo epithelial to mesenchymal transition (EMT) as they delaminate from the neural folds (N-CAM levels decrease and FN and LN levels increase)
2. The migration pathway they go down is determined by intrinsic properties of the NCCs e.g. adhesion molecules as well as the extracellular environment
3. Trunk NCCs that travel ventrally through the anterior sclerotome of somties become sympathetic ganglion neurons, DRGs, schwann cells and adrenomedullarly cells
4. Trunk cells that move through the dorsolateral route settle between the epidermis and the dermis and form melanocytes

Question 4

What are the functions of bone?

Answer 4

1. Support
2. Protection
3. Movement
4. Storage of minerals (calcium and phosphorous)
5. Blood cell production (in red marrow)

Question 5

Describe the composition of bone?

Answer 5

• Bone is a regular, dense connective tissue
• This means it contains specialised cell and ECM: ground substance and fibres
• Adult bone is mainly comprised of ECM which contains type I collagen fibres and hydroxyapatite crystals comprised of calcium phosphate
• The specialised cells are osteoprogenitors, osteoblasts, osteocytes and oesteoclasts

Question 6

Describe the specialised cell types present in bone:

Answer 6

1. Osteoprogenitor cells:
   - Arise from mesenchymal stem cells
   - Proliferate and form osteoblasts
   - Persist for life in low abundance
   - Present in the periosteum and endosteum (outer and inner layer of bone)
2. Osteoblasts:

Question 7

Describe the specialised cell types present in bone:

Answer 7

1. Osteoprogenitor cells:
   - Arise from mesenchymal stem cells
   - Proliferate and form osteoblasts
   - Persist for life in low abundance
   - Present in the periosteum and endosteum (outer and inner layer of bone)
2. Osteoblasts:
   - Derived from osteoprogenitor cells
   - Responsible for forming new bone
   - Secrete the organic components of bone matrix (osteoid)
   - Controls mineralisation of osteoid by releasing Ca2+
   - Differentiate into osteocytes when surrounded by bone matrix
3. Osteocytes:
   - Differentiate from osteoblasts
   - Surrounded by mineralised bone matrix
   - Responsible for the maintenance of bone
   - Connect to neighbouring osteocytes via cytoplasmic processes
4. Osteoclasts:
   - Dervied from haemopoietic stem cells (a different lineage to the other bone cells)
   - They are large, multinucleated phagocytic cells
   - Responsible for the breakdown and reabsoption of bone
   - Secrete acids and enzymes that digest bone matrix

Question 8

What is the anatomy of a long bone?

Answer 8

• There is an epiphysis at either end
• The diaphysis (bone shaft) is in the middle
• There is a medullary cavity in the centre of the diaphysis which is the ite of bone marrow (red turns into white as we age)
• Compact bone is on the outside
• Trabecular bone is on the inside

Question 9

What are lamellae?

Answer 9

• Thin sheets of calcium bone matrix that are deposited in layers
• Osteocytes are embedded in between lamella
• Collagen fibres are in a parallel arrangement within lamella

Question 10

What is the structure of compact bone vs trabecular bone?

Answer 10

Compact bone:

• Lamellae form a series of rings surrounding a central canal
• The central canal contains arteries, nerves and veins
• These lamellae surrounding the central canal form osteons
• Osteons run longitudinally along the bone
• Osteocytes within compact bone are trapped in small spaces called lacunae and undertake nutrient and waste change via cytoplasmic processes that connect them to other osteocytes and the central canal

Trabecular Bone:

• Bone is arranged into thin parallel struts called trabeculae
• Does not contain osteons
• The trabeculae are formed from lamellae
• There are no blood vessels present as the open network allows the cell to maintain contact with the haemotopoietic tissue within the bone marrow

Question 11

What are the developmental origins of bones?

Answer 11

1. Axial skeleton:
   - Derived from paraxial mesoderm (sclerotome)
2. Appendicular skeleton:
   - Derived from lateral plate mesoderm
3. Craniofacial bones:
   - Derived from neural crest cells

Question 12

What are the two processes of ossification?

Answer 12

1. Intramembranous ossification:
   - Mesenchymal tissue is replaced by bone
   - Gives rise to craniofacial bone and part of the clavicle
2. Endochondral ossificatin:
   - A hyaline cartilage model is replaced by bone
   - Gives rise to most of the bones in the body