Bone Physiology & Ossification

Question 1

Types of bones

Answer 1

• long bones; largely tubular with the majority being dense cortical bone
• irregular bones; larger amounts of spongy cancellous bone. Thinner cortical shell.
• flat bones; thin cortical plates with little cancellous bone between

Question 2

Osteocytes

Answer 2

• found in lacunae
• small canals called canaliculi connect adjacent lacunae
• they surround the Haversian canal
• communicate through gap junctions

Question 3

Osteoblasts

Answer 3

• synthesise and secrete collagen and other components of the bone matrix
• Calcium phosphate crystals are formed by osteoblasts

Question 4

Osteoclasts

Answer 4

• destroys bone matrix
• derived from haematopoietic lineage
• multinucleate
• acidify their subcellular space to dissolve CaPO4
  and enzymatically digest collagen and other matrix
  proteins
• As osteoclasts resorb bone, they leave behind mitogens (growth factors) to
  encourage osteoblasts to mature and lay down matrix forming a cement or
  reversal line that forms the border of the newly remodelled bone unit.
• Osteoclasts remove damaged matrix to avoid catastrophic failure

Question 5

Too much osteoblast/osteoclast activity

Answer 5

• If too much osteoblast activity; osteopetrosis (rare) with very dense bones
• If too much osteoclast activity; osteoporosis e.g. in hormone deficiency

Question 6

Blood supply & nerves

Answer 6

The nutrient arteries enters via the nutrient foramen
- supplies inner 2/3 of bone from the endosteum
- periosteal vessels supply outer 1/3 via attachments of tendons and muscles

Question 7

Endochondral ossification

Answer 7

1) bones develop from structures of hyaline cartilage; lined with perichondrium
2) growth of blood vessels into perichondrium forms periosteum
3) inner layer of periosteum, osteoprogenitor cells. Nourished by vascularisation, differentiate into osteoblasts
4) Osteoblasts form osteoid, forms bone collar
5) Bone cavity formed; hypertrophic chondrocytes increases phosphate conc. causes calcification
6) blood vessels enter the calcified matrix; osteoclasts can migrate along these. Matrix replaced by bone.
7) Osteoclasts break down tissue in middle of bone to form marrow cavity.

Question 8

Types of bone growth

Answer 8

• Longitudinal; occurs via growth plates
• Appositional; allows bone to become thicker

Question 9

Intramembranous Ossification

Answer 9

• No cartilaginous precursor structure
• Bone forms between two periosteal membranes
• Bilayer of cortical bone with a thin sandwich of cancellous bone between