The Skeletal System: Structure, Function, and Clinical Relevance

The Skeletal System

Functions of the Skeletal System

The skeletal system provides the essential framework for the human body, supporting and protecting organs, enabling movement, and serving as a reservoir for minerals and blood cell production.

• Support: The skeleton forms the internal framework that supports the body and maintains its shape.

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

• Movement: Bones act as levers and points of attachment for muscles, facilitating movement through muscle contraction and relaxation.

• Mineral Storage: Bones store calcium and phosphorus, essential for various cellular activities.

• Hematopoiesis: Blood cell formation occurs in the red bone marrow, a process known as hematopoiesis.

Classification and Structure of Bones

Types of Bones

Bones are classified according to their shapes, which relate to their functions.

• Long Bones: Longer than they are wide; e.g., humerus.

• Short Bones: Approximately equal in length and width; e.g., carpals (wrist bones).

• Flat Bones: Thin and often curved; e.g., frontal bone (skull).

• Irregular Bones: Complex shapes; e.g., vertebrae.

• Sesamoid (Round) Bones: Develop in tendons; e.g., patella (kneecap).

Structure of Long Bones

Long bones have a specialized structure that supports their function in movement and weight-bearing.

• Diaphysis: The shaft, composed of compact bone, providing strength.

• Medullary Cavity: Central cavity containing yellow marrow (fat storage).

• Epiphyses: The ends of the bone, consisting of spongy bone filled with red marrow.

• Articular Cartilage: Covers joint surfaces, reducing friction and absorbing shock.

• Periosteum: Tough, fibrous membrane covering the bone except at joint surfaces.

• Endosteum: Thin membrane lining the medullary cavity.

Structure of Flat Bones

Flat bones consist of a layer of spongy bone (diploë) sandwiched between two layers of compact bone, providing protection and broad surfaces for muscle attachment.

Microscopic Structure of Bone

Bone Tissue Types

• Spongy (Cancellous) Bone: Composed of trabeculae with spaces containing red or yellow marrow; found mainly in epiphyses.

• Compact Bone: Dense and organized into osteons (Haversian systems), providing strength and support.

Osteon (Haversian System)

The osteon is the fundamental unit of compact bone, consisting of concentric lamellae (rings of calcified matrix), lacunae (spaces containing osteocytes), canaliculi (tiny canals connecting lacunae), and a central canal (containing blood vessels and nerves).

Cartilage Tissue Structure

• Chondrocytes: Cartilage cells located in lacunae.

• Matrix: Flexible, gel-like, and avascular (lacks blood vessels), making cartilage slower to heal than bone.

Bone Formation and Growth

Bone Cells

• Osteoblasts: Bone-forming cells responsible for synthesizing new bone matrix.

• Osteoclasts: Bone-resorbing cells that break down bone tissue.

• Osteocytes: Mature bone cells derived from osteoblasts, maintaining bone tissue.

Ossification Processes

• Endochondral Ossification: Most bones develop from cartilage models. The primary ossification center appears in the diaphysis, followed by secondary centers in the epiphyses. The epiphyseal plate (growth plate) allows for lengthwise growth until it is replaced by bone (epiphyseal line) at maturity.

• Intramembranous Ossification: Some flat bones (e.g., skull) form directly from connective tissue membranes.

Early Bone Development

Before birth, the skeleton is primarily cartilage and fibrous structures, which are gradually replaced by bone through ossification. Osteoblasts build new bone, while osteoclasts resorb bone, allowing for growth and remodeling.

Divisions of the Skeleton

Axial and Appendicular Skeleton

• Axial Skeleton (80 bones): Includes the skull, vertebral column, and thorax.

• Appendicular Skeleton (126 bones): Includes the upper and lower extremities, pectoral (shoulder) girdle, and pelvic (hip) girdle.

Skull

• Cranium: 8 bones protect the brain.

• Face: 14 bones form the structure of the face.

• Middle Ear: 6 tiny bones transmit sound vibrations.

Sinuses

Sinuses are air-filled cavities within certain cranial bones (frontal, maxilla, sphenoid, ethmoid) that lighten the skull and enhance voice resonance.

Mastoiditis

Mastoiditis is the inflammation of the air spaces within the mastoid portion of the temporal bone, which can lead to serious complications if untreated.

Hyoid Bone

The hyoid bone is unique as it does not articulate with any other bone. It anchors tongue muscles and supports the larynx.

Vertebral Column

The vertebral column consists of 33 vertebrae in childhood, which fuse to 26 in adulthood. It provides support, protects the spinal cord, and allows flexibility.

• Cervical: 7 vertebrae (neck)

• Thoracic: 12 vertebrae (chest)

• Lumbar: 5 vertebrae (lower back)

• Sacrum: 1 bone (5 fused vertebrae)

• Coccyx: 1 bone (3–5 fused vertebrae)

Name	Number	Description
Cervical	7	Upper 7 vertebrae; first is atlas, second is axis
Thoracic	12	Next 12 vertebrae; ribs attach here
Lumbar	5	Lower back
Sacrum	1	5 fused vertebrae
Coccyx	1	3–5 fused vertebrae

Thorax

The thorax consists of the sternum, ribs, and thoracic vertebrae, forming the rib cage that protects the heart and lungs.

• True Ribs: First 7 pairs, directly attached to the sternum.

• False Ribs: Pairs 8–10, attached to the sternum via cartilage of the seventh rib.

• Floating Ribs: Pairs 11–12, not attached to the sternum.

Name	Number	Description
True ribs	14	Upper seven pairs; attached to sternum by costal cartilages
False ribs	10	Lower five pairs; lowest two pairs do not attach to sternum (floating ribs); next three pairs attached to sternum by costal cartilage of seventh ribs
Sternum	1	Breastbone; shaped like a dagger

Appendicular Skeleton

The appendicular skeleton includes the bones of the limbs and girdles that attach them to the axial skeleton.

• Pectoral Girdle: Scapula and clavicle connect upper limbs to the axial skeleton.

• Pelvic Girdle: Coxal bones, sacrum, and coccyx connect lower limbs to the axial skeleton.

Upper Extremity

• Arm: Humerus

• Forearm: Radius and ulna

• Wrist: 8 carpal bones

• Hand: 5 metacarpals, 14 phalanges

Name	Number	Description
Clavicle	2	Collar bones; only joints between shoulder girdle and axial skeleton are those between each clavicle and sternum
Scapula	2	Shoulder blades; forms shoulder girdle
Humerus	2	Arm bone
Radius	2	Bone on thumb side of forearm
Ulna	2	Bone on little finger side of forearm
Carpal bones	16	Wrist bones
Metacarpals	10	Palm bones
Phalanges	28	Finger bones

Lower Extremity

• Thigh: Femur

• Kneecap: Patella

• Leg: Tibia and fibula

• Ankle: 7 tarsal bones

• Foot: 5 metatarsals, 14 phalanges

Name	Number	Description
Coxal bone	2	Hipbones; ilium, ischium, pubis fused in adult
Femur	2	Thigh bones
Patella	2	Kneecap
Tibia	2	Shinbone
Fibula	2	Long slender bone of lateral side of leg
Tarsal bones	14	Ankle bones
Metatarsals	10	Foot bones
Phalanges	28	Toe bones

Sexual and Age Differences in the Skeleton

• Male Skeleton: Generally larger, pelvis deep and narrow, smaller pelvic inlet, narrower pubic angle.

• Female Skeleton: Pelvis broad and shallow, wider pelvic inlet, wider pubic angle to facilitate childbirth.

• Age Changes: Skeleton matures around age 25; bone density decreases after age 50.

Joints (Articulations)

Classification of Joints

• Synarthrosis: Immovable joints (e.g., sutures of the skull).

• Amphiarthrosis: Slightly movable joints (e.g., symphysis pubis).

• Diarthrosis (Synovial Joints): Freely movable joints, most common in the body.

Types of Diarthrotic Joints

• Ball-and-Socket: Shoulder, hip

• Hinge: Elbow, knee

• Pivot: Atlas and axis (neck)

• Saddle: Thumb

• Gliding: Intercarpal joints

• Condyloid: Wrist

Structure of a Synovial Joint

• Joint Capsule and Ligaments: Hold bones together while permitting movement.

• Articular Cartilage: Covers bone ends, absorbs shock.

• Synovial Membrane: Lines the capsule, secretes lubricating synovial fluid.

• Joint Cavity: Space between articulating bones.

Clinical Conditions of the Skeletal System

Bone and Cartilage Tumors

• Osteosarcoma: Most common malignant bone tumor, often in distal femur, proximal tibia, or humerus.

• Chondrosarcoma: Malignant cartilage tumor, often in humerus, femur, ribs, or pelvis.

Metabolic Bone Diseases

• Osteoporosis: Loss of bone matrix and trabeculae, leading to fragile bones. Common in wrists, hips, vertebrae. Treated with drugs, exercise, calcium, and vitamin D.

• Rickets: Loss of bone minerals in children before skeletal maturity, causing deformities. Treated with vitamin D.

• Osteomalacia: Loss of minerals in mature bones, increasing fracture risk. Treated with vitamin D.

• Paget Disease: Faulty bone remodeling, resulting in deformed, easily fractured bones. May be genetic or viral in origin.

• Osteogenesis Imperfecta: Brittle bone disease due to lack of organic matrix; treated with splinting and drugs.

Bone Infection

• Osteomyelitis: Bacterial infection of bone, often requiring surgery and prolonged antibiotics.

Bone Fractures

• Open (Compound) Fracture: Bone pierces the skin.

• Closed (Simple) Fracture: Bone does not pierce the skin.

• Other Types: Complete, incomplete, linear, transverse, oblique.

Joint Disorders

• Osteoarthritis (Degenerative Joint Disease): Most common noninflammatory joint disorder; "wear and tear" arthritis.

• Dislocation/Subluxation: Bones in a joint lose proper contact.

• Sprain: Ligament injury around a joint.

• Strain: Injury to muscle or tendon.

• Rheumatoid Arthritis: Systemic autoimmune disease causing chronic inflammation of synovial membranes and other tissues.

• Gouty Arthritis: Inflammation due to urate crystal deposition in joints.

• Infectious Arthritis: Joint inflammation caused by infection (e.g., Lyme disease).

Sample Review Questions

1. What substance is stored in bones? Calcium

2. Red bone marrow is found in the epiphyses of long bones.

3. Osteocytes and chondrocytes are found in lacunae.

4. Bone-forming cells are osteoblasts; bone-resorbing cells are osteoclasts.

5. Which bone is part of the axial skeleton? Ribs

6. The normal curves of the spine provide balance necessary for upright posture.

7. An example of an amphiarthrotic joint is the symphysis pubis.

8. Rickets is a disorder involving demineralization of developing bones in children.