Skeletal System Overview

Main Components of the Skeletal System

The skeletal system provides the framework for the body and is essential for movement, protection, and mineral storage. It 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

• Support – Provides structural support for the body.

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

• Movement – Acts as levers for muscles.

• Storage – Stores 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. Examples: Femur, Humerus.

• Flat bones – Thin, flattened, usually curved; two layers of compact bone with spongy bone between. Examples: Skull bones, Ribs, Sternum.

• Short bones – Cube-shaped; mostly spongy bone. Includes sesamoid bones (e.g., Patella). Examples: Carpals, Tarsals.

• Irregular bones – Complex shapes. Examples: 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; line in adults (remnant).

• Endosteum – Connective tissue lining inner shaft surface.

• Medullary cavity – Central cavity; 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., fossa, foramen, meatus).

Common Bone Markings

Microscopic Anatomy of Bone

• Osteocytes – Mature bone cells in bone matrix.

• Lacunae – Cavities housing osteocytes.

• Lamellae – Concentric rings of bone matrix.

• Central (Haversian) canal – Center of osteon; contains blood vessels and nerves.

• Osteon (Haversian system) – Structural unit of compact bone.

• Canaliculi – Tiny canals connecting lacunae to central canal; transport nutrients.

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

Bone Composition

• Organic parts (collagen fibers) – Provide flexibility and tensile strength.

• Inorganic parts (calcium salts) – Provide hardness and resistance to compression.

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.

Bone Growth

• Lengthwise growth – Occurs at epiphyseal plates.

• 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 calcium storage by osteoblasts.

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.

Fracture Treatment and Healing

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

• Immobilization – Prevents movement during healing.

• Healing time: 6–8 weeks.

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; joined by sutures (immovable joints).

• Facial bones – Hold eyes, house nasal/oral cavities, allow facial expression.

• Mandible is the only freely movable bone in the skull.

Cranial Bones

Facial Bones

Paranasal Sinuses

• Hollow portions of bones around the 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)

Spinal Curvatures

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

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

Common Vertebral 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; 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, part of large intestine.

Female vs. Male Pelvis

• Female pelvis: larger, more circular inlet; shallower, lighter, thinner bones; flared ilia; shorter, less curved sacrum; wider outlet; more rounded pubic arch.

Lower Limbs

• Femur – Thigh bone; strongest bone; articulates with hip and tibia.

• Lower leg – Tibia (medial, weight-bearing), fibula (lateral, thin).

• Foot – Tarsals (7; calcaneus and 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 Classification

Functional Classification

• Synarthroses – Immovable joints.

• Amphiarthroses – Slightly movable joints.

• Diarthroses – Freely movable joints.

Structural Classification

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

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

• Synovial joints – Freely movable; separated by joint cavity with synovial fluid.

Features of Synovial Joints

• Articular cartilage

• Articular capsule

• Joint cavity

• Reinforcing ligaments

• Bursae (fluid-filled sacs) and tendon sheaths (elongated bursae)

Types of Synovial Joints (by shape)

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 (esp. women over 65, men over 70).

• Leads to fragile bones and increased fracture risk.

• Vertebral collapse causes kyphosis (“dowager’s hump”).

• Estrogen helps maintain bone density in females.

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