Skeletal System: Structure, Function, and Development

Skeletal System Overview

Components of the Skeletal System

The skeletal system provides the framework for the body and consists of several key structures.

• Bones: The rigid organs forming the skeleton.

• 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.

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 multiple essential functions in the body:

• Support: Provide structural support for the body.

• Protection: Shield 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 (in marrow).

• Blood cell formation: Hematopoiesis occurs in bone marrow.

Classification of Bones

Types of Osseous Tissue

• Compact bone: Dense, smooth, homogeneous tissue.

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

Bone Shapes

Bones are classified by shape into four groups:

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

• Flat bones: Thin, flattened, usually curved; two layers of compact bone with spongy bone between (e.g., skull, ribs, sternum).

• Short bones: Cube-shaped, mostly spongy bone (e.g., carpals, tarsals). Sesamoid bones (e.g., patella) form within tendons.

• Irregular bones: Complex shapes not fitting other categories (e.g., vertebrae, hip bones).

Structure of Bone

Gross Anatomy of Long Bones

• Diaphysis: Shaft, composed of compact bone.

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

• Periosteum: Outer fibrous membrane covering diaphysis, secured by Sharpey's fibers.

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

• Epiphyseal plate/line: Plate of hyaline cartilage in growing bones (site of lengthwise growth); line is remnant in adults.

• Endosteum: Connective tissue lining the inner shaft surface.

• 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 serving as attachment sites or passageways.

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

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

Marking	Description
Crest	Narrow ridge of bone
Spine	Sharp, slender, pointed projection
Foramen	Round or oval opening through bone
Fossa	Shallow, basinlike depression
Meatus	Canal-like passageway
Process	Any bony prominence
Additional info:	Other markings include condyle, tubercle, and trochanter.

Microscopic Anatomy of Bone

• 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, facilitating nutrient transport.

• Perforating (Volkmann's) canals: Perpendicular canals carrying blood vessels and nerves.

• Trabeculae: Needlelike structures in spongy bone, surrounded by marrow and vessels.

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.

• In embryos, osteoblasts cover cartilage models with bone matrix.

• In fetuses, cartilage is replaced by bone, forming the medullary cavity.

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

Bone Growth

• Lengthwise growth: Occurs at epiphyseal plates via cartilage formation and replacement by bone.

• Appositional growth: Bones grow in width; osteoblasts add bone to periosteum, osteoclasts remove bone from endosteum.

• Growth is regulated by hormones (growth hormone, sex hormones).

Bone Remodeling

• Occurs throughout life in response to blood calcium levels and mechanical stress.

• Calcium ion regulation:

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

  • Hypercalcemia: High blood calcium prompts osteoblasts to store calcium in bone.

Bone Fractures and Repair

Types of Fractures

• Closed (simple) fracture: Bone breaks but does not penetrate skin.

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

Fracture Treatment and Healing

• Reduction: Realignment of bone (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 formation (cartilage, bone matrix, collagen fibers).

3. Bony callus formation (osteoblasts and osteoclasts migrate in).

4. Bone remodeling (in response to mechanical stress).

Axial Skeleton

Skull

• Cranium: Encloses brain (frontal, occipital, ethmoid, sphenoid, parietal (2), temporal (2)).

• Facial bones: Maxillae (2), palatine (2), lacrimal (2), zygomatic (2), nasal (2), vomer, inferior nasal conchae (2), mandible.

• Bones joined by sutures (immovable joints); mandible is freely movable.

Paranasal Sinuses

• Hollow portions of bones around nasal cavity.

• Functions: Lighten skull, amplify voice.

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 (neck)

  • 12 thoracic (chest)

  • 5 lumbar (lower back)

  • Sacrum (5 fused)

  • Coccyx (3-5 fused)

Spinal Curvatures

• Primary curvatures: Thoracic and sacral (C-shaped in newborns).

• Secondary curvatures: Cervical and lumbar (S-shaped in adults).

General 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, thoracic vertebrae.

• Rib types:

  • True ribs (1-7)

  • False ribs (8-12)

  • Floating ribs (11-12)

Appendicular Skeleton

Pectoral (Shoulder) Girdle

• Attaches upper limb to axial skeleton.

• Composed of clavicle and scapula.

• Allows exceptional mobility.

Upper Limbs

• Humerus: Arm bone; head articulates with scapula, distal end with forearm.

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

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

Pelvic Girdle

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

• Pelvic girdle: coxal bones + sacrum; pelvis: coxal bones + sacrum + coccyx.

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

• Female pelvis: Larger inlet, shallower, lighter, more flared ilia, shorter sacrum, wider outlet, more rounded pubic arch.

Lower Limbs

• Femur: Thigh bone; strongest in body.

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

• Foot: Tarsals (7; calcaneus, talus largest), metatarsals (5), phalanges (14).

• Foot arches: Two longitudinal, one transverse.

Joints (Articulations)

Functions of Joints

• Hold bones together securely.

• Allow for mobility.

Joint Classifications

• Functional:

  • Synarthroses: Immovable

  • Amphiarthroses: Slightly movable

  • Diarthroses: Freely movable

• Structural:

  • Fibrous: Generally immovable

  • Cartilaginous: Immovable or slightly movable

  • Synovial: Freely movable

Fibrous Joints

• Bones united by fibrous tissue.

• Types:

  • Sutures: Immobile (skull)

  • Syndesmoses: Slight movement (distal tibia/fibula)

  • Gomphoses: Immobile (teeth in sockets)

Cartilaginous Joints

• Bones connected by cartilage.

• Types:

  • Synchondrosis: Immobile (epiphyseal plates)

  • Symphysis: Slightly movable (pubic symphysis, intervertebral joints)

Synovial Joints

• Articulating bones separated by joint cavity with synovial fluid.

• Features:

  1. Articular cartilage

  2. Articular capsule

  3. Joint cavity

  4. Reinforcing ligaments

• Bursae: Flattened sacs lined with synovial membrane, filled with fluid (not part of joint).

• Tendon sheath: Elongated bursa wrapping around tendon.

Joint Type	Structural Features	Functional Classification
Fibrous	Sutures, syndesmoses, gomphoses	Immobile or slightly movable
Cartilaginous	Synchondrosis, symphysis	Immobile or slightly movable
Synovial	Plane, hinge, pivot, condylar, saddle, ball-and-socket	Freely movable

Types of Synovial Joints

• Plane joint: Nonaxial, gliding movements (e.g., carpals).

• Hinge joint: Uniaxial, flexion/extension (e.g., elbow).

• Pivot joint: Uniaxial, rotation (e.g., proximal radioulnar joint).

• Condylar joint: Biaxial, flexion/extension and abduction/adduction (e.g., knuckle).

• Saddle joint: Biaxial, similar to condylar (e.g., thumb).

• Ball-and-socket joint: Multiaxial, most freely movable (e.g., shoulder, hip).

Developmental Aspects of the Skeleton

Fetal and Infant Skeleton

• Fetal skeleton is mostly hyaline cartilage; flat bones are fibrous membranes.

• Ossification centers develop as fetus grows.

• Fontanels: Soft spots in infant skull, allow 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 outward 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 ossify by end of adolescence.

Adult Skeleton and Aging

• Osteoporosis: Bone-thinning disease, common in older adults (especially women).

• Bones become fragile, prone to fracture; vertebral collapse leads to kyphosis ("dowager's hump").

• Estrogen helps maintain bone density in females.

Key Equations

• Calcium Homeostasis: $ext{Ca}^{2+} ext{ regulation:}~ ext{PTH} ightarrow ext{Osteoclast activation} ightarrow ext{Bone resorption} ightarrow ext{Increase blood Ca}^{2+}$

Example

When blood calcium levels drop, parathyroid hormone is released, stimulating osteoclasts to break down bone and release calcium into the bloodstream.

Additional info: Some tables and diagrams were inferred and expanded for clarity. All major topics from the original notes are included and explained in academic context.