2.1. Bone anatomy and physiology

What are the four types of bone cell?

Osteogenic cells, osteoblasts, osteocytes, and osteoclasts.

What are osteogenic cells?

Stem cells found in the periosteum, endosteum and blood vessel canals. They undergo cell division and their daughter cells become osteoblasts.

What do osteoblasts do?

Synthesise and secrete collagen/matrix, initialise calcification, and become trapped in the matrix — 10% become osteocytes.

What are osteocytes?

Mature bone cells making up 90% of bone cells. They maintain daily bone metabolism, act as mechanosensors/mechanotransmitters, and exchange nutrients and waste.

What are osteoclasts?

Large multinucleated bone-resorbing cells formed from monocyte fusion. Concentrated in the endosteum with a ruffled border facing the bone surface.

How do osteoclasts resorb bone?

They release powerful lysosomal acids and enzymes that digest the protein and mineral components of bone as part of a balanced maintenance and repair system.

What is the sequence of bone cell development?

1) Osteogenic cell → 2) Osteoblast (forms matrix) → 3) Osteocyte (maintains tissue) → 4) Osteoclast (resorbs matrix).

What is an osteon (Haversian system)?

The structural unit of compact bone. Made up of concentric lamellae arranged like tree rings, with collagen fibres and bone salt crystals. Withstands torsion.

What are Volkmann's (perforating) canals?

Transverse canals connecting the periosteum, Haversian canals, and medullary cavity.

What do Haversian (central) canals contain?

Blood vessels and nerve fibres running longitudinally through osteons.

What are lacunae?

Small spaces within bone matrix containing osteocytes, connected by canaliculi.

What are canaliculi?

Tiny channels connecting lacunae, allowing osteocytes to communicate and exchange nutrients.

What are interstitial lamellae?

Remnants of old osteons that fill gaps between osteons and circumferential lamellae.

How does cancellous bone differ histologically from compact bone?

Cancellous bone has trabeculae with no osteons or canals. Osteocytes are superficial and receive nutrients directly from blood in the medullary cavity.

What does the diaphysis contain?

The shaft of a long bone — compact bone surrounding a medullary cavity containing yellow bone marrow.

What does the epiphysis contain?

The ends of a long bone — spongy bone within compact bone, articular cartilage, and red bone marrow.

What is the metaphysis?

The region where the diaphysis and epiphysis meet. Contains the epiphyseal plate (cartilage, during growth) or epiphyseal line (bone, after growth).

What is the periosteum?

The outer sheath of bone. Outer layer = dense irregular connective tissue. Inner layer = osteogenic layer containing primitive stem cells.

What is the endosteum?

Thin membrane lining the medullary cavity and covering internal spongy bone and canals within compact bone.

What is compact bone tissue?

Dense bone with few spaces forming the external layer of all bones and bulk of the diaphysis. Made of aligned osteons. Surrounds yellow bone marrow.

What is cancellous bone tissue?

Bone with many spaces in a latticework of trabeculae. Makes up most short, flat and irregular bones and epiphyses of long bones. Lighter than compact bone, supports red bone marrow.

What are long bones?

Tubular bones with a long axis and two articulating ends — e.g. humerus.

What are flat bones?

Thin, flat, possibly curved bones that protect internal organs — e.g. cranial bones.

What are short bones?

Cuboidal-shaped bones — e.g. carpal bones.

What are sesamoid bones?

Bones lying within tendons — e.g. patella.

What are irregular bones?

Bones with complex shapes — e.g. vertebrae.

What is the composition of bone matrix?

50% crystallised mineral (hydroxyapatite, magnesium hydroxide, fluoride, sulphate), 25% collagen fibres, 25% water.

What is calcification?

The process of crystallisation of minerals within a collagen framework. Minerals provide hardness; collagen provides flexibility.

What happens if you dissolve the minerals from bone?

The bone becomes rubbery and flexible.

What happens if you remove collagen from bone?

The bone becomes brittle.

What is the difference between axial and appendicular bones?

Axial = head and torso. Appendicular = appendages/limbs.

Where is red bone marrow found?

In the epiphyses of long bones and within cancellous bone.

Where is yellow bone marrow found?

In the medullary cavity of the diaphysis.