Chapter 6 – Bones and Bone Tissue

Functions of the Skeletal System

The skeletal system provides the framework for the body and serves several essential functions.

• Support: Bones support the body and cradles soft organs.

• Protection: Bones protect vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Bones act as levers for muscles to produce movement.

• Mineral Storage: Bones store minerals, especially calcium and phosphorus.

• Blood Cell Formation: Hematopoiesis occurs in the red marrow of certain bones.

• Triglyceride Storage: Yellow marrow stores fat as an energy reserve.

Bone Classification and Structure

Bones are classified by shape and internal structure.

• Shape Classification:

  • Long bones: Longer than wide (e.g., femur, humerus).

  • Short bones: Cube-shaped (e.g., carpals, tarsals).

  • Flat bones: Thin, flattened, and usually curved (e.g., sternum, skull).

  • Irregular bones: Complicated shapes (e.g., vertebrae, hip bones).

  • Sesamoid bones: Embedded in tendons (e.g., patella).

• Compact vs. Spongy Bone:

  • Compact bone: Dense, forms the outer layer of bones, provides strength.

  • Spongy bone (cancellous): Porous, found at the ends of long bones and inside flat bones, contains red marrow.

Gross Anatomy of a Long Bone

Long bones have distinct regions and structures.

• Diaphysis: Shaft of the bone, composed of compact bone.

• Epiphysis: Ends of the bone, mostly spongy bone covered by compact bone.

• Epiphyseal line/plate: Growth plate; site of bone lengthening during development.

• Articular cartilage: Hyaline cartilage covering joint surfaces.

• Periosteum: Dense connective tissue covering the bone's outer surface.

• Endosteum: Membrane lining the internal bone surfaces.

• Medullary cavity: Central cavity containing yellow marrow in adults.

Microscopic Structure of Bone Tissue

Bone tissue consists of cells and matrix organized into structural units.

• Osteon (Haversian system): Structural unit of compact bone.

• Central canal: Contains blood vessels and nerves.

• Lamellae: Concentric rings of bone matrix.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae for nutrient exchange.

• Perforating (Volkmann's) canals: Connect central canals transversely.

Bone Cells

Bone tissue contains several cell types, each with specific functions.

• Osteogenic cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells; synthesize bone matrix.

• Osteocytes: Mature bone cells; maintain bone tissue.

• Osteoclasts: Bone-resorbing cells; break down bone matrix.

Bone Development and Growth

Bones form and grow through two main processes.

• Intramembranous ossification: Bone develops from a fibrous membrane (e.g., flat bones of the skull).

• Endochondral ossification: Bone forms by replacing hyaline cartilage (e.g., long bones).

• Longitudinal growth: Occurs at the epiphyseal plate, increasing bone length.

• Appositional growth: Increases bone thickness.

Bone Remodeling and Repair

Bone is continuously remodeled and repaired throughout life.

• Bone remodeling: Balance between bone resorption (osteoclasts) and formation (osteoblasts).

• Hormonal regulation:

  • Calcitonin: Lowers blood calcium levels.

  • Parathyroid hormone (PTH): Raises blood calcium levels.

  • Growth hormone: Stimulates bone growth.

  • Thyroid hormone: Modulates growth and development.

  • Sex hormones: Promote bone growth and closure of epiphyseal plates.

• Mechanical stress: Weight-bearing activities stimulate bone remodeling.

Red and Yellow Bone Marrow

Bone marrow exists in two forms with distinct functions.

• Red marrow: Site of hematopoiesis; found in spongy bone of flat bones and epiphyses of long bones.

• Yellow marrow: Fat storage; found in medullary cavities of long bones in adults.

• Age differences: Children have more red marrow; adults have more yellow marrow.

Table: Comparison of Compact and Spongy Bone

Chapter 7 – The Skeleton: Bones and Bone Markings

Overview of the Skeleton

The skeleton is divided into axial and appendicular components, each with specific bones and functions.

• Axial skeleton: Skull, vertebral column, rib cage.

• Appendicular skeleton: Limbs and girdles (shoulder and pelvic).

Bone Markings and Features

Bones have markings that serve as attachment points, passageways, and articulations.

• Foramen: Opening for nerves and blood vessels.

• Process: Projection for muscle or ligament attachment.

• Fossa: Depression for articulation.

• Crest, tubercle, condyle: Various raised or rounded areas for attachment or articulation.

Major Bones and Regions

• Skull: Protects the brain; includes facial and cranial bones.

• Vertebral column: Supports the body and protects the spinal cord.

• Thoracic cage: Ribs and sternum protect thoracic organs.

• Upper and lower limbs: Enable movement and manipulation.

Table: Comparison of Axial and Appendicular Skeleton

Chapter 8 – The Skeletal System: Articulations

Joints and Articulations

Joints (articulations) are sites where two or more bones meet, allowing for movement and flexibility.

• Classification by structure:

  • Fibrous joints: Bones joined by dense connective tissue; little or no movement (e.g., sutures).

  • Cartilaginous joints: Bones joined by cartilage; limited movement (e.g., intervertebral discs).

  • Synovial joints: Bones separated by a fluid-filled cavity; freely movable (e.g., shoulder, knee).

• Classification by function:

  • Synarthroses: Immovable joints.

  • Amphiarthroses: Slightly movable joints.

  • Diarthroses: Freely movable joints.

Structure of Synovial Joints

Synovial joints have a complex structure that allows for a wide range of movements.

• Articular cartilage: Covers bone surfaces at the joint.

• Joint (synovial) cavity: Space filled with synovial fluid.

• Articular capsule: Encloses the joint; consists of fibrous and synovial layers.

• Synovial fluid: Lubricates and nourishes the joint.

• Ligaments: Strengthen and stabilize the joint.

• Bursae and tendon sheaths: Reduce friction.

Movements at Synovial Joints

Synovial joints allow various types of movement.

• Flexion/Extension: Decreasing/increasing the angle between bones.

• Abduction/Adduction: Moving limb away/toward the midline.

• Rotation: Turning bone around its axis.

• Supination/Pronation: Rotating forearm so palm faces up/down.

• Elevation/Depression: Lifting/lowering a body part.

• Protraction/Retraction: Moving a body part forward/backward.

• Opposition: Thumb movement to touch other fingers.

Table: Types of Synovial Joints and Movements

Accessory Structures of Synovial Joints

• Bursae: Fluid-filled sacs that reduce friction.

• Tendon sheaths: Elongated bursae around tendons.

• Fat pads: Cushion joints.

Major Joints and Their Features

• Shoulder (glenohumeral) joint: Ball-and-socket; most mobile, prone to dislocation.

• Elbow joint: Hinge; allows flexion and extension.

• Hip joint: Ball-and-socket; stable due to deep socket and strong ligaments.

• Knee joint: Largest, most complex; hinge with additional rotational capability.

Table: Ligaments of the Knee Joint

Hormonal Regulation of Bone Remodeling

• Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts.

• Calcitonin: Decreases blood calcium by inhibiting osteoclasts.

• Growth hormone: Stimulates bone growth during childhood.

• Thyroid hormone: Regulates bone growth and development.

Key Equations

• Calcium Homeostasis:

Summary

• Bones and joints are essential for support, movement, protection, and mineral homeostasis.

• Bone tissue is dynamic, constantly remodeled by cellular and hormonal mechanisms.

• Understanding bone structure, markings, and joint types is fundamental for studying human anatomy and physiology.