Bone Tissue, Homeostasis, and Pathology

• The lecture covers bone tissue and its microscopic structure, bone homeostasis, and bone pathology related to lack of bone homeostasis (osteoporosis).
• By the end of the lecture, one should be able to describe the microscopic structure of bones, how bones maintain homeostasis, and osteoporosis.

Bone: A Dynamic Tissue

• Bone is a living tissue comprised of cells and a calcified extracellular matrix.
• Bone changes in response to how the body is used and can repair itself when broken.
• Bone cells respond to external forces, causing it to remodel and change shape throughout life based on how the skeleton is used.
• Bone cells respond to trauma to unite bone fracture.

Bone Tissue Composition

• Bone is a connective tissue.
• The functions of connective tissue are to support other tissues/organs and to maintain form.
• Bone has organic and inorganic extracellular components.

Organic Extracellular Matrix

• 33% of bone ECM is organic.
• Organic components include collagen (protein) and ground substance (proteoglycans).
• Function: resists tension.
• Without collagen, bone becomes brittle and breaks easily.

Inorganic Extracellular Matrix

• 67% of bone ECM is inorganic.
• The inorganic component is primarily hydroxyapatite and other calcium minerals (mineral salts).
• Creates bone hardness and resistance to compression.
• Without inorganic compounds, bone becomes too flexible.

Cellular Component of Bone

• Cells make up only 2% of bone by weight.
• Four types of bone cells include: osteogenic cells, osteoblasts, osteocytes, and osteoclasts.
• Bone homeostasis is maintained when these cells work together.
• Bone must maintain a balance between bone destruction and formation so the amount of bone stays the same.

Bone Cell Functions

• Osteogenic cells: stem cells that produce osteoblasts.
• Osteoblasts: "MAKERS" that produce new bone matrix.
• Osteocytes: "MAINTAINERS" that recycle protein and minerals from the matrix.
• Osteoclasts: "DESTROYERS" that remove bone matrix.

Gross Structure of Bones

• Two types of bone tissue: compact and cancellous (spongy bone).
• Compact and cancellous bone is made of the same things but has different structures.
• Compact bone consists of osteon structures.
• Cancellous bone consists of trabecular structures.

Compact Bone

• Macroscopically, an impenetrable (periosteum) tissue with holes for blood vessels (foramina)
• Microscopically: made of circumferential lamellae and units called osteons.

Osteons

• Osteons are the longitudinal (lengthwise) unit within compact bone.
• Osteons contain a central canal, which contains blood vessels and nerves and provides a pathway for nutrients to get to cells in the ECM.
• Lamellae are a series of cylinders formed of ECM around the central canal; collagen fibres within the lamellae resist forces.
• Lacunae are 'lakes' for osteocytes.
• Canaliculi are channels for nutrients to travel to osteocytes through the ECM.

Cancellous Bone

• Trabeculae are struts of lamella bone.
• Cavities between trabeculae are filled with marrow.
• Osteocytes are housed in lacunae between lamellae/on the surface.
• Organisation of trabeculae resists force from multiple directions.
• It directs force from body weight in a single direction down the shaft.
• Spreads force distally.
• Trabeculae channel weight around the ilia into the femora.

Bone Remodelling

• Bone remodelling is the process that allows bones to grow (appositional growth).
• Osteoblasts add bone matrix in lamellae to the bone surface.
• Osteoclasts remove bone from the medullary cavity.

Bone Homeostasis

• Bone homeostasis depends on the balance of osteoblast (OB) and osteoclast (OC) activity.
• Bone is constantly being formed/destroyed, which allows the body to mobilise calcium, phosphate, and other minerals from the bone matrix.
• The process is called remodelling.
• Able to respond plastically.
• Shape change is possible through life to resist strain, etc.

Imbalance in Bone Homeostasis

• Adequate calcium in the diet or moderate exercise are required to maintain bone homeostasis.
• Without these, there can be an imbalance in osteoblastic/osteoclastic activity.
• Osteoporosis and osteopenia are both related to OC > OB activity.
• Osteoporosis is a clinically significant version of osteopenia.

Osteoporosis

• Osteoporosis results in a loss of cortical bone, which causes compression fractures in the vertebrae.
• In cancellous bone trabeculae become thinner.
• Biological females at risk of osteoporosis due to loss of estrogen post-menopause as they enter adulthood.
• Lifestyle factors such as lack of exercise and nutritional factors can lead to risk of osteoporosis.
• Risk of osteoporosis depends on one's starting point (peak bone mass in their 20s); if it is already low, they are more likely to get osteoporotic.