1. The Skeletal System

• The Skeletal System is made up of:
  • Bones
  • Joints
  • Cartilage
  • Ligaments
• Axial Skeleton consists of bones along the centre of the body:
  • Skull
  • Vertebral
  • Column
  • Rib Cage
• Appendicular Skeleton consists of:
  • Pelvic
  • Pectoral Girdle
  • Limbs
• Adult body contains 206 bones
• Bone is a connective tissue

Functions

• Support soft tissue and provide sites for attachment for Skeletal Muscles
• Enable movements and postural control together with Skeletal Muscles
• Protection of internal tissue and vital organs
• Storage of minerals: especially $Ca^{2+}$ and $P$
• Red Bone Marrow, in Skull, Pelvis and Ribs, produces Blood Cells
• Yellow Bone Marrow serves as chemical energy storage, Triglycerides

2. Composition and Types of Bone

• Bone contains 60% minerals and 40% collagen
  • Contains Hydroxyapatite, $CaCO_3$ and some $Mg$, $Na$, $K$, $F$, $S$
  • Allows it to be hard/stable instead of being brittle

• Sandwich Design | | Long Bone | Arms, Legs | - Structural support
• Hollow | | Short Bone | Wrist, Ankle | - Large range of movement | | Irregular Bone | Vertebra | - Irregularly shaped projections
• Attachment sites for muscles, tendons and ligaments | | Sesamoid Bone | embedded in Tendon | - Allow connection of Tendon far away to allow better muscle leverage |

Sandwich Design (Flat Bone), is impact-resistant so can deal with mechanical stress and serve as protective function

• Trabecular “Spongy” Bone

Long Bone Structure

• Lone Bone is structured so ends (epiphysis, 骺) consist of Spongy Bone and the shaft (diaphysis, 骨干) consist of Compact Bone

Compact Bone

• Hard, dense layer of bone
• Osteocytes reside in Lacunae which contain protein fibres and mineral deposits
• Haversian System: system of Canals allows blood vessels to pass through bone ensuring circulation

Spongy Bone

• Strong but light reducing overall bone weight
• Contains Osteocytes
• Many Trabeculae, plate like structures that follow stress lines in bone
  • Act like braces and provide support to the bone

Marrow

• Along the Diaphysis, compact bone surrounds a hollow centre called Medullary Cavity
• Medullary Cavity contains Bone Marrow
  • in the Red Bone Marrow, a process called Haematopoiesis forms Blood Cells
  • Yellow Bone Marrow functions as fat storage

Layer

• Endosteum
  • Thin layer of connective tissue that lines surface of bony tissue that forms the Medullary Cavity
  • Reabsorbed when malnourished
• Periosteum
  • Lines outer surface of the bone
  • Continuous with ligaments/tendons connecting to muscles
  • Blood vessels pass from Periosteum into the bone, providing circulation

3. Bone Growth and Remodelling

• Osteoblasts and Osteoclasts found in both compact and spongy bones
• Bone Remodelling and Growth can happen in Diaphysis and Epiphysis
• Osteoblasts become Osteocytes as they build up the bone surrounding them and get immobilised/tapped within the matrix
• Osteocytes send cytoplasmic protrusions with other Osteocytes

Osteogenic Cells (Osteoprogenitor Cells)

• Mitotic
• Present in periosteum and endosteum

Osteoblasts

• Non-mitotic, bone pre-cursor cell
• Build bone
• Collagen $\rightarrow$ Ossification

Osteocytes

• Non-mitotic, bone cell
• Maintain bone

Osteoclasts

• Fused Monocytes
• Multinuclear
• Degrade and resorb bone tissue

Bone Growth

Bone Remodelling

• All bones in the body are replaced every 3 years