Function of bone
- support of body shape
- system of levers for muscle action
- internal organ protection
- blood cell formation site
- mineral storage pool
Bone mechanical properties
- Cable-like flexibility and resistance to tension because of collagen framework (plus other proteins)
- Pillar-like stiffness and resistance to compression conferred by impregnation of collagen with crystalline mineral (hydroxyapatite=complex calcium hydroxyphosphate)
Two main types of bone tissue
WOVEN BONE -immature -only found in repairing fractures in adults or in bone disease (Paget's) LAMELLAR BONE -mature -structure= concentric rings of cells
Lamellar bone arrangment
- outer hard layer of compact lamellar bone (cortical bone)
- inner layer of interlacing struts (cancellous/spongy/trabecular bone)
Trabecular bone
- trabeculae are irregularly arranged and made of lamellar bone
- otherwise known as cancellous bone
Epiphysis (long bone)
-end of a long bone
Metaphysis (long bone)
-portion between the epiphysis and the diaphysis
Diaphysis (long bone)
-shaft of a long bone
Bone blood supply
- rich blood supply
- fracturing bones can cause lots of bleeding
- nutrient foramina (tunnel through cortex of long bone) allows nutrient artery to pass into the bone
Periosteum
- outer surface of bone
- consists of a fibrous and cellular layer
- key roles in bone growth and repair, vascular effects and has a good sensory nerve supply
Bone development
-skeleton starts to form at 6 weeks of fetal life and growth continues in some bones until individual is 25 years old
Intramembranous ossification
- in existing vascular connective tissue
- bone matrix (ostein) deposited around collagen
- mineralises to form woven bone
- remodels to lamellar bone
Endochondral ossification
- within existing fetal cartilage models
- cartilage calcifies and chondrocytes die
- periosteal osteoclasts cut channels for sprouting vessels
- osteoblasts enter with vessels to build bone round them
Endochondral ossification and bone length growth
-most long bones must support large forces whilst growing which would disrupt terminal appositional growth
TO STOP THIS:
- the shaft ossifies first, followed by the epiphyses
- growth continues by ossification at growing cartilage plate between them
- growth cessation when cartilage growth ceases and plate is over-run by ossification
Age-related changes in the appearance of normal bones
- In a child’s wrist, epiphyses ossify in the 2nd year
- Epiphyseal plates remain cartilaginous until growth ceases after puberty
Bone adaptability
- can grow without compromising its support function
- increases or decreases bulk and density in reponse to pattern of use
- can alter its external and internal shape in response to pattern of use (remodelling)
- can repair when fractured
The key to bone growth and remodelling
- bone has large blood supply (cells never far from nutrients and oxygen)
- osteocytes maintain matrix but can activate osteoblasts for new bone building
- osteoclasts are giant cells specialised for bone matrix destruction
Bone diameter growth
- apposition=addition to exterior at periosteum
1) osteoblasts and osteoclasts create ridges and grooves on bone surface
2) blood vessels align in grooves
3) osteoblasts build new osteons around vessels
4) osteoclasts remove bone from endosteal surface
Calcium control
BLOOD CALCIUM LEVELS HIGH
-calcitonin released by parafollicular thyroid cells
-inhibited bone matrix breakdown by osteoclasts
-calcium uptake into bone matrix promoted
BLOOD CALCIUM LEVELS LOW
-PTH released by chief cells of parathyroid gland
-osteoclast bone resorption activity promoted
-increased calcium re-absorption by the kidneys
Fractures
TYPES:
-tranverse, oblique, spiral, comminuted, segmental, avulsed, impacted, torus and greenstick
HEALING:
-bleeding is important=haematoma forms and becomes infiltrated by fibrous matrix and invaded by cartilage/bone progenitors
REPAIR:
-via woven bone formation
Ossification
OSTEOGENESIS
-laying down new bone material by osteoblasts
