Bone Growth & Joint Tissues

• Lecture 5 delves into bone growth and joint tissues/structures, including the principles of bone growth and the microanatomy of joint tissues.
• Fibrous and cartilaginous joints are examined.

The Story So Far

• Basic tissues, bone tissue, and the skeleton are familiar concepts.
• The focus is on how bones grow (shape and size), how they fit together (joints), and how they're held together (soft tissues).

Bone Growth

• Bone growth starts with a cartilage model about 6 weeks after fertilization.
• Endochondral ossification converts cartilage into bone.

Primary Ossification Centres

• The diaphysis (shaft) is the primary ossification centre.
• The epiphyses remain as cartilage.

Secondary Ossification Centres

• The epiphyses undergo the same ossification process as primary centres.
• The epiphyseal plate/growth plate separates the epiphysis from the diaphysis.

Bone Growth in Length

• Epiphyseal plates, also known as 'growth plates,' enable bone growth in length.
• These plates are made of cartilage.
• Starting from puberty, epiphyses begin to fuse with diaphyses.

Bone Growth in Width

• Appositional growth is responsible for the increasing width of bones.
• Osteoblast activity results in circumferential lamellae.
• Osteoclasts mould the bone's shape and create the medullary cavity.

How Skeletons Move

• Bones articulate at joints.
• A joint holds bones together, involves bone ends and soft tissues, and allows controlled movement.

Key Soft Tissues

• Key soft tissues include various types of cartilage and dense fibrous connective tissue (DFCT).
• Cartilage types discussed are hyaline (articular) and fibrocartilage.
• DFCT components include ligaments, tendons, and joint capsules.

Cartilage

• Chondrocytes are within lacunae.
• Collagen fibres are embedded in a ground substance.
• Cartilage is avascular; it lacks blood vessels, and receives nutrients through the matrix via joint loading.
• Hyaline (articular) cartilage and fibrocartilage are two key types found in joints.

Hyaline (Articular) Cartilage

• This cartilage's primary function is to resist compression.
• The matrix has a high water content.
• Collagen fibres are sparse.
• It moulds to the surfaces of bones where they articulate, creating a smooth surface that allows frictionless movement.
• This cartilage is found on articulating bone surfaces and degrades with age.

Fibrocartilage

• Fibrocartilage resists both compression and tension.
• Many collagen fibres are found in bundles.
• The orientation of the fibres aligns with stresses.
• It's useful at joints that experience both compression and tension.
• Fibrocartilage acts as a buffer/shock absorber by distributing force over a wider area, and it deepens articular surfaces.

DFCT

• Fibroblasts are present.
• It contains collagen fibres and sometimes elastin fibres.
• It is tightly packed
• The primary function is to resist tension.
• DFCT has little vascularity, resulting in slow healing.

Ligaments

• Ligaments connect bone to bone.
• Collagen and elastin are present to resist tension.
• Allows a little stretch and recoil.
• Movement is restricted.

Tendons

• Tendons connect muscle to bone.
• They facilitate and control movement.
• Tendons have less elastin than ligaments
• Muscle contraction is transmitted to bone by tendons

Bony Congruence

• Bony congruence is the sum of bone surfaces that form an articulation.
• Less congruence means a greater need for soft tissue support.

Joint Classifications

• Fibrous joints have the least amount of movement.
• Cartilaginous joints have some movement.
• Synovial joints have the most amount of movement.

Terminology

• Tissue consists of cells grouped together in a highly organized manner for a specific structure and function (e.g., DFCT).
• Structure is something formed of a tissue (e.g., ligament).

Fibrous Joints

• The tissue is DFCT.
• The structure is a ligament.
• The function is to limit movement and provide stability as seen in cranial sutures, which provide stability for the skull and protect the brain.
• Another example is the distal tibiofibular joint, which provides stability for the ankle and prevents rotational movements.

Cartilaginous Joints

• The tissue is fibrocartilage.
• The structure varies.
• The primary function is to allow some movement.
• Bones are connected entirely by fibrocartilage.
• Examples of cartilaginous joints are the intervertebral disc and the pubic symphysis.

Synovial Joints

• Synovial joints consist of many tissues and structures and allow lots of movement.
• Most joints in the appendicular skeleton are synovial joints.

Lecture Summary

• Bone starts as cartilage, then ossifies, involving primary and secondary ossification centres.
• Bone growth requires growth/epiphyseal plates and appositional growth.
• Joints need connective tissues like cartilage and DFCT to provide structure and function.
• The amount of movement defines the three classes of joints.