Vertebrate Anatomy: Skeletal System and Bone Formation

Bone Formation and Growth

• Bone formation occurs in two ways: Endochondral (Cartilage) bone formation and Intramembranous (Intraosseous) bone formation
• Endochondral bone formation:
  • Majority of bone formation
  • Most bones start as cartilage in the fetus
  • Long bone formation begins in the primary growth centre in diaphysis and radiates outwards
  • Cartilage is removed gradually as bone is created
  • Additional secondary growth centres develop in the epiphysis of bone (the ends)
• At birth, most cartilage has been replaced by bone
• Only growth plates/epiphyseal plates remain as cartilage
• These allow long bone growth
• Full skeletal maturity: epiphyseal plates ossify

Intramembranous Bone Formation

• Bone forms from fibrous membranes covering the brain in the developing fetus
• Only occurs in certain skull bones, mandible, and clavicle (not in dogs and cats)
• This creates bones of the cranium which surround the brain
• Why might we want these bones to ossify earlier than the rest of the skeleton?

Bone Healing

• Bones are one of the best healing tissues in the body
• Fractures (breaks) can occur in any bones but are most common in long bones
• Three factors necessary for optimal healing:
  • Alignment
  • Immobilisation
  • Time

Bone Healing - Alignment

• Alignment is also known as “reduction” of a fracture (or “fx”)
• Fractured ends must be brought together in reasonable alignment and kept from moving apart during the healing process
• Internal and external devices (such as plates, pins, and screws implanted during surgery) are used to reduce a fx

Bone Healing - Immobilisation

• Also called “fixation” of a fracture
• Achieved through internal fixtures and external fixtures – pins, plates, screws, and splints, casts
• Length of time external fixators are in place depends on species, age, activity level – all affect healing time

Fracture Healing

• Bones have a large blood supply – this results in a large amount of bleeding (haemorrhage) when a fracture occurs
• A clot is formed – known as the fracture haematoma
• Osteoblasts form healing tissue known as a callus
• First a fibrocartilage/soft callus followed by a bony/hard callus
• Callus bridges the fracture gap – the size of the callus indicates how much movement has occurred during healing
• Remodeling occurs after callus has formed and fracture has stabilised

Joints

• Joints are the junctions between bones, which can be immovable, slightly movable, or freely movable.
• Joint terminology:
  • Arthro- and Articular- refer to joints.
  • Arthrology is the study of joints.
  • Articular surfaces are smooth, bony surfaces that come together to form freely movable joints.

Types of Joints

• Fibrous Joints:
  • Immoveable joints with no joint cavity.
  • Held together by fibrous connective tissue (ligaments, tendons).
  • Examples: skull bones (sutures), mandible bone to tooth (gomphosis joint).
• Cartilaginous Joints:
  • Joints held together with cartilage.
  • Allow more movement between bones than fibrous joints but less than synovial joints.
  • Examples: vertebral discs, pubic symphysis, mandibular symphysis.
• Synovial Joints:
  • Freely movable joints.
  • Articular cartilage covers the ends of bones.
  • Have a fibrous joint capsule containing synovial fluid.
  • Ligaments reinforce and stabilize the joint.
  • Examples: hinge, gliding, pivot, and ball and socket joints.

Cartilage

• Found in: larynx, trachea, bronchi, flexible portions of nose, external ear, and connects ribs to sternum.
• Functions:
  • Provides tough yet flexible support.
  • Reduces friction and acts as a cushion between joints.
  • Helps support weight during movement.
  • Important in embryonic development (most bones are first formed as cartilage and later replaced as bone).
• When damaged, cartilage has limited healing ability due to lack of direct blood supply, leading to issues like osteoarthritis.

Synovial Fluid

• Found in synovial joints.
• Helps to minimize friction in the joint and allow for free movement.
• Has a thick, viscous consistency similar to oil.
• Can be sampled through a "joint tap" for examination and analysis.

Joints

• Joints are the junctions between bones, which can be immovable, slightly movable, or freely movable.
• Joint terminology:
  • Arthro- and Articular- refer to joints.
  • Arthrology is the study of joints.
  • Articular surfaces are smooth, bony surfaces that come together to form freely movable joints.

Types of Joints

• Fibrous Joints:
  • Immoveable joints with no joint cavity.
  • Held together by fibrous connective tissue (ligaments, tendons).
  • Examples: skull bones (sutures), mandible bone to tooth (gomphosis joint).
• Cartilaginous Joints:
  • Joints held together with cartilage.
  • Allow more movement between bones than fibrous joints but less than synovial joints.
  • Examples: vertebral discs, pubic symphysis, mandibular symphysis.
• Synovial Joints:
  • Freely movable joints.
  • Articular cartilage covers the ends of bones.
  • Have a fibrous joint capsule containing synovial fluid.
  • Ligaments reinforce and stabilize the joint.
  • Examples: hinge, gliding, pivot, and ball and socket joints.

Cartilage

• Found in: larynx, trachea, bronchi, flexible portions of nose, external ear, and connects ribs to sternum.
• Functions:
  • Provides tough yet flexible support.
  • Reduces friction and acts as a cushion between joints.
  • Helps support weight during movement.
  • Important in embryonic development (most bones are first formed as cartilage and later replaced as bone).
• When damaged, cartilage has limited healing ability due to lack of direct blood supply, leading to issues like osteoarthritis.

Synovial Fluid

• Found in synovial joints.
• Helps to minimize friction in the joint and allow for free movement.
• Has a thick, viscous consistency similar to oil.
• Can be sampled through a "joint tap" for examination and analysis.