The Skeletal System: Structure, Function, and Clinical Relevance

Skeletal System Overview

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: Forms the internal framework that supports the body.

• Protection: Shields internal organs and aids in immune defense by producing white blood cells.

• Movement: Facilitates movement by serving as levers for muscle action.

• Mineral Storage: Stores calcium and phosphorus, vital for physiological processes.

• Hematopoiesis: Produces blood cells in red bone marrow.

Classification and Structure of Bones

Types of Bones

Bones are classified by shape, which relates to their function and location in the body.

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

• Short Bones: Cube-shaped (e.g., carpals of the wrist).

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

• Irregular Bones: Complex shapes (e.g., vertebrae).

• Sesamoid Bones: Develop within tendons (e.g., patella).

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.

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

• Epiphyses: Ends of the bone, made of spongy bone containing red marrow.

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

• Periosteum: Tough membrane covering the bone except at joints; contains nerves and blood vessels.

• 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 marrow; found in epiphyses.

• Compact Bone: Dense and organized into osteons (Haversian systems) for strength.

Osteon (Haversian System)

The osteon is the fundamental unit of compact bone, consisting of concentric lamellae, lacunae (housing osteocytes), canaliculi (nutrient channels), and a central canal with blood vessels.

Cartilage Tissue Structure

• Chondrocytes: Cartilage cells located in lacunae.

• Matrix: Flexible, gel-like, and avascular (lacks blood vessels).

Bone Development and Growth

Bone Cells

• Osteoblasts: Bone-forming cells.

• Osteoclasts: Bone-resorbing cells.

• Osteocytes: Mature bone cells maintaining bone tissue.

Ossification Processes

• Endochondral Ossification: Most bones form from cartilage models; ossification centers appear in the diaphysis and epiphyses.

• Intramembranous Ossification: Flat bones form within connective tissue membranes.

Bone Development in Infancy

At birth, the skeleton consists largely of cartilage and fibrous structures, which are gradually replaced by bone through ossification.

Divisions of the Skeleton

Axial and Appendicular Skeleton

• Axial Skeleton (80 bones): Skull, vertebral column, thorax.

• Appendicular Skeleton (126 bones): Upper and lower limbs, pectoral and pelvic girdles.

Skull

• 8 cranial bones, 14 facial bones, 6 middle ear bones.

• Sinuses lighten the skull and enhance resonance of voice.

Mastoiditis

Inflammation of the mastoid air cells in the temporal bone can lead to serious complications if untreated.

Hyoid Bone

The hyoid bone is unique as it does not articulate with any other bone and serves as an anchor for tongue muscles and supports the larynx.

Vertebral Column

• Composed of cervical (7), thoracic (12), lumbar (5), sacrum (1), and coccyx (1) bones.

• Normal curves provide strength and flexibility; abnormal curves include lordosis, kyphosis, and scoliosis.

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 vertebrae
Sacrum	1	Fused from 5 vertebrae in adults
Coccyx	1	Fused from 3-5 vertebrae in adults

Thorax

• Composed of 12 pairs of ribs, sternum, and thoracic vertebrae.

• True ribs (1-7), false ribs (8-10), floating ribs (11-12).

Name	Number	Description
True ribs	14	Upper seven pairs; attached to sternum by costal cartilages
False ribs	10	Lower five pairs; lowest two are floating ribs
Sternum	1	Breastbone; includes manubrium and xiphoid process

Appendicular Skeleton

• Upper Extremity: Clavicle, scapula, humerus, radius, ulna, carpals, metacarpals, phalanges.

• Lower Extremity: Coxal bone, femur, patella, tibia, fibula, tarsals, metatarsals, phalanges.

Name	Number	Description
Clavicle	2	Collar bones; only joints between shoulder girdle and axial skeleton
Scapula	2	Shoulder blades; forms shoulder girdle with clavicle
Humerus	2	Upper arm bone
Radius	2	Forearm bone on thumb side
Ulna	2	Forearm bone on little finger side
Carpals	16	Wrist bones
Metacarpals	10	Palm bones
Phalanges	28	Finger bones

Name	Number	Description
Coxal bone	2	Hip bones; each formed by fusion of ilium, ischium, and pubis
Femur	2	Thigh bones
Patella	2	Kneecap
Tibia	2	Shinbone
Fibula	2	Slender lateral bone 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: Broader and shallower pelvis, wider pelvic inlet and pubic angle for childbirth.

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

Joints (Articulations)

Classification by Movement

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

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

• Diarthrosis: Freely movable joints (e.g., shoulder, knee).

Types of Diarthrotic Joints

• Ball-and-Socket: Shoulder, hip.

• Hinge: Elbow, knee.

• Pivot: Atlas and axis of neck.

• Saddle: Thumb joint.

• Gliding: Intercarpal joints.

• Condyloid: Wrist joint.

Structure of a Diarthrotic Joint

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

• Articular Cartilage: Covers bone ends, absorbs shock.

• Synovial Membrane: Secretes lubricating fluid.

• Joint Cavity: Space between articulating bones.

Clinical Conditions of the Skeletal System

Bone and Cartilage Tumors

• Osteosarcoma: Malignant bone tumor, often in distal femur, proximal tibia, or humerus.

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

Metabolic Bone Diseases

• Osteoporosis: Loss of bone matrix and trabeculae, leading to fractures; treated with drugs, exercise, calcium, and vitamin D.

• Rickets: Loss of bone minerals in children, 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, causing deformities and fractures; may be genetic or viral in origin.

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

Bone Infection

• Osteomyelitis: Bacterial infection of bone, often requiring surgery and 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 Conditions

• Osteoarthritis (DJD): Degenerative, noninflammatory joint disease; most common cause for joint replacements.

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

• Sprain: Ligament injury.

• Strain: Injury to muscle or tendon.

• Arthritis: Inflammatory joint conditions, including rheumatoid arthritis (autoimmune), gouty arthritis (urate crystals), and infectious arthritis (bacterial).

Review Questions

1. What substance is stored in bones? Calcium

2. Red bone marrow fills in small spaces in the spongy bone composing the: Epiphyses

3. Osteocytes and chondrocytes lie within little spaces called: Lacunae

4. Bone-forming cells are called osteoblasts, whereas bone-resorbing cells are called osteoclasts.

5. Which of these bones would be found in the axial division of the skeleton? Ribs

6. The normal curves of the spine provide us with enough strength to support our weight and: Provide the balance necessary for us to stand upright

7. An example of an amphiarthrotic joint is: Symphysis pubis

8. A skeletal disorder that involves the demineralization of developing bones in infants and young children before skeletal maturity is: Rickets