1. Osteogenic Cells

• Stem cells that differentiate into osteoblasts.

2. Osteoblasts (Bone-Building Cells)

• Form new bone matrix through ossification.

• Responsible for bone formation and located on bone surfaces.

• Secrete lamellae (rings of compact bone).

3. Osteocytes (Mature Bone Cells)

• Maintain the bone matrix and reside in lacunae (small spaces in the bone).

• Extend cellular processes through canaliculi to communicate and transfer nutrients.

4. Osteoclasts (Bone-Destroying Cells)

• Break down bone matrix to remodel bone and release calcium into the blood.

Bone Tissue Structure

• Lamellae: Concentric rings of compact bone created by osteoblasts.

• Central Canal: Surrounded by lamellae; contains blood vessels to nourish bone tissue.

• Osteon (Haversian System): The functional unit of compact bone, consisting of the central canal, lamellae, and osteocytes within lacunae.

Functional Classification of Joints

1. Synarthroses

• Immovable joints, providing stability (e.g., skull sutures).

2. Amphiarthroses

• Slightly movable joints, allowing limited movement (e.g., pubic symphysis).

3. Diarthroses

• Freely movable joints, primarily synovial joints (e.g., knee, elbow).

Structure of a Synovial Joint

• Articular (Hyaline) Cartilage: Covers bone ends, reducing friction.

• Synovial Membrane: Secretes synovial fluid for lubrication.

• Joint Cavity: Contains synovial fluid.

• Fibrous Articular Capsule: Encloses the joint.

• Ligament: Connects bone to bone, stabilizing the joint.

• Tendon: Connects muscle to bone, aiding movement.

• Bursa: Fluid-filled sac that reduces friction.

Types of Synovial Joints Based on Movement

1. Nonaxial: Limited gliding movement (e.g., carpals in the wrist).

2. Uniaxial: Movement in one plane.

• Hinge Joint: Elbow and knee (flexion and extension).

• Pivot Joint: Rotation, e.g., radius and ulna (supination and pronation).

3. Biaxial: Movement in two planes.

• Condyloid Joint: Wrist joint allowing flexion, extension, and circumduction.

4. Multiaxial: Movement in multiple planes.

• Ball and Socket Joint: Shoulder and hip (most mobile but least stable).

Hormonal Mechanism of Calcium (Ca²⁺) Regulation

• Calcitonin

• Released by the thyroid gland in response to high blood calcium levels.

• Stimulates osteoblasts to deposit calcium in the bone, lowering blood calcium.

• Parathyroid Hormone (PTH)

• Released by the parathyroid glands when blood calcium levels are low.

• Activates osteoclasts to break down bone matrix, releasing calcium into the blood.