Skeletal System Overview

Components of the Skeletal System

The skeletal system provides the framework for the body and is essential for movement, protection, and support. It consists of bones, joints, cartilages, and ligaments.

• Bones (skeleton): Rigid organs that form the structure of the body.

• Joints: Articulations where two or more bones meet.

• Cartilages: Flexible connective tissue found in joints and other areas.

• Ligaments: Bands of connective tissue connecting bones at joints.

The skeleton is subdivided into:

• Axial skeleton: Skull, vertebral column, and thoracic cage.

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

Functions of Bones

Bones serve several vital functions in the body:

• Support: Provide structural support for the body.

• Protection: Protect soft organs (e.g., skull protects brain, rib cage protects thoracic organs).

• Movement: Facilitate movement via attached muscles.

• Storage: Store minerals (calcium, phosphorus) and fats (yellow marrow).

• Blood cell formation (hematopoiesis): Occurs in red marrow.

Classification of Bones

Types of Osseous Tissue

• Compact bone: Dense, smooth, homogeneous tissue.

• Spongy bone: Composed of small needlelike pieces (trabeculae) and many open spaces.

Bone Shapes

Bones are classified by shape into four categories:

• Long bones: Longer than wide, shaft with enlarged ends, mostly compact bone. Examples: Femur, Humerus.

• Flat bones: Thin, flattened, usually curved, two layers of compact bone sandwiching spongy bone. Examples: Skull bones.

• Short bones: Cube-shaped, mostly spongy bone with outer compact layer. Sesamoid bones (e.g., patella) form within tendons. Examples: Carpals, Tarsals.

• Irregular bones: Complex shapes, do not fit other categories. Examples: Vertebrae.

Structure of Bone

Gross Anatomy

• Diaphysis: Shaft, mostly compact bone, provides length.

• Periosteum: Outer covering, fibrous connective tissue, secured by Sharpey's fibers.

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

• Articular cartilage: Hyaline cartilage covering epiphyses, reduces friction and absorbs shock.

• Epiphyseal plate/line: Plate of hyaline cartilage for growth (children); line is remnant in adults.

• Endosteum: Lines inner surface of shaft, made of connective tissue.

• Medullary cavity: Central cavity in shaft, contains yellow marrow (fat) in adults, red marrow (hematopoiesis) in children.

Bone Markings

Bone markings are features on bones that serve as attachment sites or passageways.

• Projections/processes: Grow out from bone surface (e.g., crest, spine).

• Depressions/cavities: Indentations (e.g., foramen, fossa).

Microscopic Anatomy

• Osteocytes: Mature bone cells in bone matrix.

• Lacunae: Cavities housing osteocytes.

• Lamellae: Concentric rings of bone matrix around central canal.

• Osteon (Haversian system): Structural unit containing central canal and lamellae.

• Central (Haversian) canal: Runs lengthwise, carries blood vessels and nerves.

• Canaliculi: Tiny canals connecting lacunae to central canal, allow nutrient transport.

• Perforating (Volkmann's) canals: Perpendicular to central canal, carry blood vessels and nerves.

Bone Composition

• Organic parts: Collagen fibers provide flexibility and tensile strength.

• Inorganic parts: Calcium salts make bone hard and resistant to compression.

Bone Formation, Growth, and Remodeling

Ossification

Ossification is the process of bone formation, occurring on hyaline cartilage models or fibrous membranes.

• Phase 1: Osteoblasts cover cartilage model with bone matrix.

• Phase 2: Cartilage is digested away, opening up medullary cavity.

At birth, cartilage remains only at articular cartilages and epiphyseal plates.

Bone Growth

• Lengthwise growth: Occurs at epiphyseal plates.

• Appositional growth: Bones grow in width; osteoblasts add bone matrix to outside, osteoclasts remove from inside.

• Hormonal control: Growth hormone and sex hormones regulate growth.

Bone Remodeling

• Calcium ion level in blood: Determines when bone matrix is broken down or formed.

• Mechanical stress: Pull of gravity and muscles determines where bone matrix is remodeled.

Calcium Ion Regulation

• Parathyroid hormone (PTH): Released when blood calcium is low, activates osteoclasts to release calcium.

• Hypercalcemia: High blood calcium prompts storage in bones by osteoblasts.

Equation:

Bone Fractures and Repair

Types of Fractures

• Closed (simple) fracture: Break does not penetrate skin.

• Open (compound) fracture: Broken bone penetrates skin.

Fracture Treatment

• Reduction: Realignment of bone ends (closed: manual, open: surgical with pins/wires).

• Immobilization: Prevents movement during healing (6-8 weeks typical).

Repair Process

1. Hematoma formation (blood-filled swelling).

2. Fibrocartilage callus forms (cartilage matrix, bony matrix, collagen fibers).

3. Bony callus replaces fibrocartilage callus (osteoblasts and osteoclasts migrate in).

4. Bone remodeling occurs in response to mechanical stress.

Axial Skeleton

Skull

• Cranium: Encloses brain (frontal, occipital, ethmoid, sphenoid, parietal, temporal bones).

• Facial bones: Maxillae, palatine, lacrimal, zygomatic, nasal, vomer, inferior nasal conchae, mandible.

• Sutures: Immovable joints between skull bones.

• Mandible: Only freely movable joint in skull.

Paranasal Sinuses

• Hollow portions of bones surrounding nasal cavity.

• Functions: Lighten skull, amplify sounds.

Hyoid Bone

• Does not articulate with other bones.

• Movable base for tongue, aids swallowing and speech.

Vertebral Column

• Axial support from skull to pelvis.

• Separated by intervertebral discs.

• Regions: 7 cervical, 12 thoracic, 5 lumbar, sacrum (5 fused), coccyx (3-5 fused).

Spinal Curvatures

• Primary curvatures: Thoracic and sacral, present from birth (C-shaped).

• Secondary curvatures: Cervical and lumbar, develop after birth (S-shaped).

Vertebrae Features

• Body (centrum), vertebral arch (pedicle, lamina), vertebral foramen, transverse and spinous processes, articular processes.

Thoracic Cage

• Protects thoracic organs.

• Consists of sternum, ribs (true 1-7, false 8-12, floating 11-12), thoracic vertebrae.

Appendicular Skeleton

Pectoral (Shoulder) Girdle

• Clavicle and scapula attach upper limb to axial skeleton.

• Allows exceptional mobility.

Upper Limbs

• Humerus: Arm bone, articulates with scapula and forearm.

• Forearm: Ulna (medial), radius (lateral).

• Hand: Carpals (8), metacarpals (5), phalanges (14).

Pelvic Girdle

• Formed by two coxal bones (ilium, ischium, pubis).

• Pelvis = coxal bones, sacrum, coccyx.

• Supports upper body weight, protects reproductive organs, bladder, part of large intestine.

Female vs. Male Pelvis

• Female pelvis: Larger, more circular inlet; shallower, lighter, thinner bones; sacrum shorter and less curved; ischial spines shorter and farther apart; pubic arch more rounded.

Lower Limbs

• Femur: Thigh bone, heaviest and strongest.

• Lower leg: Tibia (medial, shinbone), fibula (lateral, thin).

• Foot: Tarsals (7, including calcaneus and talus), metatarsals (5), phalanges (14).

• Bones of foot form three arches (two longitudinal, one transverse).

Joints (Articulations)

Functions of Joints

• Hold bones together securely.

• Allow for mobility.

Joint Classification

• Functional: Synarthroses (immovable), amphiarthroses (slightly movable), diarthroses (freely movable).

• Structural: Fibrous (immovable), cartilaginous (immovable/slightly movable), synovial (freely movable).

Fibrous Joints

• Bones united by fibrous tissue.

• Types: Sutures (immobile), syndesmoses (slightly movable), gomphoses (immobile, teeth in sockets).

Cartilaginous Joints

• Bones connected by cartilage.

• Types: Synchondrosis (immobile, e.g., epiphyseal plates), symphysis (slightly movable, e.g., pubic symphysis, intervertebral joints).

Synovial Joints

• Articulating bones separated by joint cavity filled with synovial fluid.

• Features: Articular cartilage, articular capsule, joint cavity, reinforcing ligaments.

• Bursae (fibrous sacs with synovial fluid) and tendon sheaths (elongated bursae) reduce friction.

Types of Synovial Joints

Developmental Aspects of the Skeleton

Fetal Skeleton

• Initially composed of hyaline cartilage and fibrous membranes.

• Ossification centers develop as fetus grows.

Fontanels

• Fibrous membranes connecting cranial bones in infants (“soft spots”).

• Allow skull compression during birth and brain growth.

• Ossify by age 2.

Growth and Proportions

• Skull is three-fourths adult size by age 2; near adult size by age 8-9.

• Face grows out from skull between ages 6-11.

• At birth, head and trunk are proportionally larger than limbs.

• During puberty, female pelvis broadens, male skeleton becomes more robust.

• Epiphyseal plates fully ossify by end of adolescence.

Osteoporosis

• Bone-thinning disease, common in older adults (50% women over 65, 20% men over 70).

• Bones become fragile, prone to fracture; vertebral collapse leads to kyphosis (“dowager’s hump”).

• Estrogen helps maintain bone density in females.

Example: Osteoporosis can result in vertebral compression fractures, leading to abnormal spinal curvature.

Additional info: Bone health can be maintained by adequate calcium intake, weight-bearing exercise, and hormone regulation.