Topic 12 - Bones Flashcards
Learning Outcomes
- Describe basic structure and function of bone
- Explain how and why bone remodelling occurs and how it effects bone throughout life
- Describe the basic structure of the skeleton
Roles of the skeleton
Structure:
- Enables movement
- Support
- Protect organs
Homeostasis:
- Store minerals (85% of phosphorus, 99% of Calcium)
- Houses bone marrow (Haematopoiesis)
- Site of energy metabolism
- Endocrine functions
Bone structure
Cortical (Dense) Bone:
- Outside section of bone
- 80% of total bone mass
- 10% porus
- Gives strength to structure
- Mostly made up of parallel, densly packed Collagen Type 1 fibrils for strength
Cancellous (Trabecular) Bone:
- Makes up 20% bone mass
- Highly porus, 50-90%
- Thin network of porus trabeculae in directions of stress for shock absorption
- Has a Large SA:V for mineral exchange
- Bones are a storehouse for Ca2+, very important for various functions of the body (nerves, muscle contraction)
Collagen and minerals
- Collagen and Hydroxyapatite form the bone matrix
- Ca2+ is used in the structure of Hydroxyapatite crystals
-
Collagen Type 1 binds with collagen cross links tightly, hydroxyapatite minerals then fill the spaces between the alignment. Forming the bone matrix
Bones must be hard, strong and flexible - Too much Hydroxyapatite leads to brittle bones, too much collagen leads to bendy bones (Ratio must be correct)
Key functions of Bone Remodelling
- Repair damage to skeleton (fracture healing)
- Prevent accumulation og microdamage/ aged tissue
- Supply calcium and phosphorus for mineral homeostasis
Cells involved in bone remodelling
- Osteocytes
- Osteoclast (reabsorb bone)
- Osteoblast (form new bone)
Stages of Bone Remodelling
.1. Quiescent
* A signal tells a site in the skeleton it needs to remodel
.2. Activation
* Activates the site
* Stimulates the recruitment og osteoclast precursers, forming osteoclasts
.3. Reabsorption
* Osteoclasts break down the bone matrix with demineralisation, secreting acids and create a proease as they break down the collagen
.4. Formation
* Osteoblasts are then stimulated and recruited to form new bone and fill in the hole
.5. Mineralisation
* The bone is returned into its quiescent stage
Osteoblasts
- Form bone through producing the collagen matrix (osteoid) and then mineralise it
- Derives from Mesenchymal linage
- Once the osteoblasts have done their job, they become a resting cell, OR imbed themselves in the bone becoming an Osteocyte (They cannot then return as an osterblast)
Osteoclasts
- Reabsorption and degrades bone matrix
- Derives from hematopoietic precursor cells
- Multinucleated
- Attaches to bone, ejecting acid, shifting the pH demineralising it, then breaks down the collagen with protease
- The Ca2+ is then released into the blood
Osteocytes
Master regulators of bone remodelling
- Inside the bone matrix
- Connected through a network of ‘arms’ similar to nerves
- Secrete factors that control Osteoblasts and Osteoclasts
- Mechano sensing cell
Bone Growth of flat bones
Protect Organs
- Form through Intramembranous ossification through mesenchymal condensation
- Mesenchymal cells condense during development and have signals that turn them into osteoblasts and make bone
- The Osteoblasts form spicules which fuse to form cancellous woven bone
Regions of Long bones
- Epiphysis
- Metaphysis
- Diaphysis
- Endosteum
- Periosteum
- Marrow cavity
- Major blood supply
Diagram in notes
Formation of Long Bones
Occurs through Endochondral Ossification
- Cartilage in shape of bone is formed first as a foundation
- Cartilage is then penetrated and release factors are injected that lead to blood vessel formation inside the cartilage
- The blood vessels bring cells inside to make new bone, slowly converting cartilage into bone
- The epiphysis of long bones ossify after birth
See diagram in notes
Longitudinal growth
- Proliferation
- Hypertrophy and calcification
- Degradation and invasion
- Bone Formation
See Diagram
Osteoperosis
- Disease characterised by compromised bone structure and strength (Increases risk of fracture)
- Is an imbalance in bone remodelling (Osteoclasts work too hard/ Osteoblasts not enough)
- Obviously seen in cancellous bone (Loss of structure and ‘rods’)
- Often in vertebrae as the bone is mostly cancellous
- 1/5 men and 1/3 women over 50 will have it.
- Women more common as they lose estrogen from menopause
