Bone Tissue: Composition & Function

• Bone is a living tissue comprised of cells and a calcified extracellular matrix
• It changes based on body use, responds to external forces, and remodels throughout life
• Bone cells respond to trauma to unite broken parts, enabling self-repair

Bone Tissue Composition

• Bone is connective tissue, providing support and maintaining form
• It has two extracellular components: organic and inorganic

Extracellular Components - Organic

• Organic components make up 33% of bone ECM
• Primarily made of Collagen (protein) and ground substance (proteoglycans)
• Organic components Function to resist tension
• Without collagen, bone becomes brittle and breaks easily

Extracellular Components - Inorganic

• Inorganic components make up 67% of bone ECM
• Hydroxyapatite and other Ca minerals are key inorganic components
• Mineral component makes bone hard and resistant to compression
• Bone lacking inorganic components becomes too flexible

Cellular Components of Bone

• Cellular components comprise only 2% of bone by weight
• Four cell types: osteogenic cells, osteoblasts, osteocytes, and osteoclasts
• These cells work together to maintain bone homeostasis
• Amount of bone stays the same through a balance of bone destruction and formation

Bone Cell Functions

• Osteogenic cells: Stem cells which produce osteoblasts
• Osteoblasts: Produce new bone matrix
• Osteocytes: Recycle protein and minerals from matrix
• Osteoclasts: Remove bone matrix

Gross Structure of Bones

• Two types of bone tissue: compact and cancellous
• Made of the same things but structured differently
• Compact bone features an osteon structure, while cancellous bone has a trabecular structure

Compact Bone

• Macroscopically, outer surfaces appear dense and impenetrable (periosteum) with foramina/holes for blood supply
• Microscopically, composed of circumferential lamellae and osteons

Osteons

• Osteons are lengthwise units within compact bone
• Facilitate nutrient pathways to cells in the ECM
• Central canals contain blood vessels and nerves
• Lamellae are cylinder series of ECM surrounding the central canal, shaping the osteon
• Collagen fibers within lamellae resist forces
• Lacunae house osteocytes
• Canaliculi are nutrient channels to osteocytes through the ECM

Cancellous Bone

• Trabeculae are struts of lamella bone
• Marrow fills cavities between trabeculae
• Osteocytes are housed in lacunae between lamellae or on the surface

Trabecular Bone

• Trabecular bone resists force from multiple directions
• Directs force from body weight in a single direction down the shaft
• Spreads force distally
• Trabeculae channel weight around the ilia into femora

Bone Remodelling

• Bones grow through appositional growth
• Osteoblasts add bone matrix in lamellae to the bone surface
• Osteoclasts remove bone from the medullary cavity

Bone Homeostasis

• Balance of osteoblast (OB) and osteoclast (OC) activity
• Bone is constantly formed/destroyed, allowing the body to mobilize calcium, phosphate, and other minerals from the bone matrix
• Process called REMODELLING, allows bone to respond plastically
• Shape change is possible through life to resist strain

Imbalance in Bone Homeostasis

• Body requires adequate calcium in diet and moderate exercise to maintain homeostasis
• Imbalances can disrupt osteoblastic/osteoclastic activity

Imbalance in OB/OC activity

• Osteoporosis and osteopenia occur when OC activity exceeds OB activity
• Osteoporosis is the clinically significant version

Osteoporosis

• Characterized by a Loss of cortical bone
• Cancellous bone trabeculae become thinner
• Can leads to Compression fractures of vertebrae

Factors Affecting Osteoporosis Risk

• Biological females are at higher risk due to loss of estrogen post-menopause
• Lifestyle factors like lack of exercise or nutritional factors contribute
• Risk depends on starting point; low peak bone mass in 20s increases susceptibility