Skeletal System: Structure, Function, and Development

Skeletal System

Components of the Skeletal System

The skeletal system is a complex framework that supports and protects the body, enables movement, and serves as a reservoir for minerals and fat. It consists of several key components:

• 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 fibrous tissue connecting bones.

The skeleton is subdivided into:

• Axial skeleton – Includes the skull, vertebral column, and thoracic cage.

• Appendicular skeleton – Comprises the limbs and girdles.

Functions of Bones

Bones perform several vital functions:

• Support – Provide structural framework for the body.

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

• Movement – Serve as levers for 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 open spaces.

Bone Shapes

Bones are classified by shape into four groups:

• Long bones – Longer than wide, shaft with enlarged ends, 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, outer layer of compact bone; includes sesamoid bones (e.g., carpals, tarsals, patella).

• Irregular bones – Complex shapes, do not fit other categories (e.g., vertebrae, hip bones, facial 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 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 for nerves and vessels. They are categorized as:

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

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

Microscopic Anatomy of Bone

• Osteocytes – Mature bone cells in bone matrix.

• Lacunae – Cavities housing osteocytes.

• Lamellae – Concentric rings of matrix around central canal.

• Central (Haversian) canal – Runs lengthwise, contains blood vessels and nerves.

• Osteon (Haversian system) – Functional unit of compact bone, includes central canal and lamellae.

• Canaliculi – Tiny canals connecting lacunae to central canal, facilitating nutrient transport.

• Perforating (Volkmann’s) canals – Perpendicular canals carrying blood vessels and nerves.

Bone matrix consists of organic (collagen fibers for flexibility and tensile strength) and inorganic (calcium salts for hardness and compression resistance) components.

Bone Formation, Growth, and Remodeling

Ossification

Ossification is the process of bone formation, occurring on hyaline cartilage models or fibrous membranes. Long bone growth involves two major phases:

1. Osteoblasts cover cartilage model with bone matrix.

2. Cartilage is digested away, forming the medullary cavity.

After birth, cartilage remains only at articular surfaces and epiphyseal plates. Bones grow in length at the epiphyseal plate and in width via appositional growth (osteoblasts add bone to periosteum, osteoclasts remove bone from endosteum).

Bone growth is regulated by hormones (growth hormone, sex hormones).

Bone Remodeling

Bones are remodeled throughout life in response to:

• Calcium ion levels in blood – Determines when bone is broken down or formed.

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

Calcium Regulation:

• Parathyroid hormone (PTH) – Released when blood calcium is low; activates osteoclasts to release calcium from bone.

• 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 – Bone breaks and penetrates skin.

Treatment and Healing

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

• Immobilization – Prevents movement during healing (typically 6-8 weeks).

Repair Process

1. Hematoma formation (blood-filled swelling).

2. Fibrocartilage callus formation (cartilage and collagen stabilize break).

3. Bony callus formation (osteoblasts/osteoclasts replace callus with bone).

4. Bone remodeling (restores original shape).

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.

• Sutures – Immovable joints between skull bones.

• Mandible – Only freely movable skull bone.

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

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

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

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

• Attaches upper limb to axial skeleton.

• Composed of clavicle and scapula.

• Allows exceptional mobility.

Upper Limbs

• Humerus – Arm bone; articulates proximally with scapula, distally with 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, large intestine.

• Female pelvis – Larger, more circular inlet; shallower, lighter, thinner bones; wider pubic arch.

Lower Limbs

• Femur – Thigh bone; strongest bone.

• Lower leg – Tibia (medial, forms knee and inner ankle), fibula (lateral, forms outer ankle).

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

• Foot forms three arches (two longitudinal, one transverse).

Joints (Articulations)

Functions of Joints

• Hold bones together securely.

• Allow for mobility.

Classification of Joints

• Functional – Based on movement:

  • Synarthroses – Immovable

  • Amphiarthroses – Slightly movable

  • Diarthroses – Freely movable

• Structural – Based on material:

  • Fibrous joints – Generally immovable (e.g., sutures, syndesmoses, gomphoses)

  • Cartilaginous joints – Immovable or slightly movable (e.g., synchondroses, symphyses)

  • Synovial joints – Freely movable; have joint cavity with synovial fluid

Synovial Joint Features

• Articular cartilage

• Articular capsule

• Joint cavity

• Reinforcing ligaments

• Bursae – Fluid-filled sacs reducing friction

• Tendon sheath – Elongated bursa around tendon

Types of Synovial Joints

• Plane joint – Gliding movements (e.g., intercarpal joints)

• Hinge joint – Flexion/extension (e.g., elbow, knee)

• Pivot joint – Rotation (e.g., proximal radioulnar joint)

• Condylar joint – Flexion/extension, abduction/adduction (e.g., metacarpophalangeal joints)

• Saddle joint – Similar to condylar, allows more movement (e.g., thumb)

• Ball-and-socket joint – Multiaxial movement (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 3/4 adult size by age 2; near adult size by 8-9 years.

• 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 ossify by end of adolescence.

Age-Related Changes

• Osteoporosis – Bone-thinning disease, common in older adults (esp. women); increases fracture risk and can cause vertebral collapse (kyphosis).

• Estrogen helps maintain bone density in females.

Summary Table: Joint Classes

Structural Class	Examples	Mobility
Fibrous	Sutures, syndesmoses, gomphoses	Immovable or slightly movable
Cartilaginous	Synchondroses, symphyses	Immovable or slightly movable
Synovial	Plane, hinge, pivot, condylar, saddle, ball-and-socket	Freely movable

Key Equations

• Calcium Regulation:

$\text{PTH} \uparrow \Rightarrow \text{Osteoclast activity} \uparrow \Rightarrow \text{Blood Ca}^{2+} \uparrow$

$\text{Blood Ca}^{2+} \uparrow \Rightarrow \text{Osteoblast activity} \uparrow \Rightarrow \text{Bone Ca}^{2+} \uparrow$

Example: Bone Remodeling

When blood calcium levels drop, parathyroid hormone stimulates osteoclasts to break down bone and release calcium into the bloodstream. Conversely, when calcium levels are high, osteoblasts deposit calcium into bone matrix.

Additional info: Some details (e.g., specific bone markings, fracture types, and developmental timelines) were expanded for clarity and completeness.