Skeletal System and Bones

functions of skeletal system

• structural framework and attachment for muscles

• protection

• movement

• mineral homeostasis

• blood cell protection by red bone marrow

• triglyceride storage by yellow bone marrow

2 skeletal sysems

axial and appendicular skeleton

axial skeleton

skull, thoracic cage and spine

appendicular skeleton

limbs including pectoral and pelvic girdle

pectoral

scapula and clavicle = attachment points for arms to axial

pelvic

hips - attach legs to axial

are bones living tissue

yes - they’re vascular and innervated

• vascular - blood supply for nutrients incl venous drainage for waste and deoxygenated blood

• innervated - neuronal supply mostly in periosteum

composition of bones

• 45% is organic components

• 55% chrystalised mineral salts

organic components of bones

30% collagen and 15% water

• collagen resists tension = absorbs shock

chrystalised mineral salts

calcium phosphate and calcium carbonate - why bones are opaque and able to use x-rays

• resists compression

type of bones

compact and spongy/trabecular

• compact surrounds spongy

compact bones

dense - forms outer shell of all bones

• hollow pillars of bone matrix containing nerves and blood vessels

spongy bones

trabeculae arranged around lines of compressive and tensile stresses = strength for weight bearing while staying light. contains red bone marrow

compressive and tensile

• compressive - physical force pushing downward

• tensile - pulling/stretching forces

trabeculae

small thin plates of bone arranged like honeycomb

bone shapes

flat, short, long anf irregular

flat bones

longer than they’re wide = bear lot of weight e.g. femur and hummerus

short bones

similar width and length = cube shape. for stability and support with little movement e.g. carpal bones of wrist

flat bones

thin, flattened and usually bit curved = protects vital organs and provides surfaces for large muscle attachments e.g. skull bones and sternum

irregular bones

doesn’t fit other shape categories bc complicated shapes e.g vertebrae

sections of long bone

1. diaphysis

2. epiphysis

3. metaphysis

diaphysis

long axis middle section of bone

epiphysis

proximal and distal ends where bone attaches to other bones via articulations

metaphysis

proximal and distal ends - transition space between diaphysis and epiphysis

• location of epiphyseal growth plate/epiphyseal line

epiphyseal growth plate

hyaline cartilage where bones grow from

• becomes epiphyseal line when done growing

articular cartilage

hyaline cartilage that covers the surface of bones within synovial/moveable joints = smooth movement

periosteum

outer fibrous covering all of bone except joints

medullary cavity

hollow space within diaphysis and contains yellow bone marrow and blood vessels

endosteum

thin membrane that lines medullary cavity, trabeculae and canals of compact bones

periosteal arteries

supplies periosteum and outer compact bone - enters thru many small canales in compact to access spongy

nutrient artery

large artery - main supplier to bone

• each bone has 1

• enters compact thru nutrient foramen at centre of diaphysis and enters cavity and then splits up and goes toward each epiphysis

epiphyseal and metaphyseal arteries

supplies ends of long bones

which type of nerves do bones have

only sensory

types of cells in bone tissue

osteoblasts and osteoclasts

osteoblasts

bone building cells - synthesise and secrete collagen fibres and other organi components to build extracellular matrix and initiates calcification (hardening)

osteoclasts

breaks down extracellular matrix and involved in bone resoprtion - releases lysosomal enzymes and acids to digest protein and mineral components of ECM

ossification

bone formation - 2 processes - both start with mesenchyme

1. intramembranous

2. endochondral

intramembranous ossification

directly from mesenchyme during embryonic period - forms flat bones of skull, facial bones, mandible, parts of clavicale and hardening fontanelles

endochondral ossification

from cartilage derived from mesenchyme - forms all other bones

intramembranous ossification steps

1. development of ossification centre and osteoblasts secrete organic extracellular matrix

2. calcium and other mineral salts are deposited and ECM hardens

3. ECM develops into trabeculae that fuse to form spongy bone

4. development of periosteum from mesenchyme at periphery of bone

endochrondal ossification steps

1. mesenchymal cells develop into chondroblasts forming cartilage model

2. model grows via cell division

3. arrival of nutrient artery triggers development of primary ossification centre = bone replaces cartilage in diaphysis

4. medullary caivty develops via oseoclasts

5. secondary ossification centre at epiphysis

6. formation of articular cartilage and epiphyseal plate

bone growth

in 2 directions until 25 yrs - length and diameter/appositional

• only diameter after 25

longitudinal bone growth

2 major events

• interstitial growth of cartilage on epiphyseal side of epiphyseal plate

• replacement of cartilage on diaphyseal side with bone via endochondral ossification

when does epiphyseal plate close

• 18 in females

• 21 in males

appositional bone growth

thickening - new bone deposited on outer surface by osteoblasts and old bone lining medullary cavity destroyed by osteoclasts

• cavity enlarges as bone thickness increases

osteoporosis

bones become thin weak and porous = break easily

bone remodelling

combo of bone deposit and resorption

• can change shape in response to mechanical stress

frequency of bone remodelling

• spongy replaced 3-4 yrs

• compact every 10

• 
  

effects of aging on bones

demineralisation begins earlier in people producing oesrogen as dominant sex hormone

factors that allow fractured bone to heal completely

• adequate immobilisation in early stage

• reasonably good alignment - bone not shattered into many pieces and no chronic infection

• extensive remodelling of bone over months

callus

lump that forms around healing fracture

• initially soft fibrous tissue that becomes ossified

osteomalacia

failure to properly mineralise bone earlier in life due to lack of vitamin D, calcium or phosphate