Skeletal System: Structure, Function, and Development

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.

Functions of Bones

• Support – Provides structural support for the body.

• Protection – Shields soft 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 sites for muscle, tendon, and ligament attachment, and passages for nerves and blood vessels. They are categorized as:

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

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

Common Bone Markings Table

Marking	Description	Example
Crest	Narrow ridge of bone	Iliac crest
Spine	Sharp, slender projection	Spinous process of vertebra
Foramen	Round or oval opening	Foramen magnum
Meatus	Canal-like passageway	External auditory meatus
Fossa	Shallow depression	Mandibular fossa
Process	Any bony prominence	Mastoid process
Additional info:	Table entries inferred from context and standard anatomy.

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) – 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 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 with bone matrix.

• In fetuses, cartilage is replaced by bone, forming the 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 to periosteum, osteoclasts remove bone from endosteum.

• Growth controlled 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 – 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 (6-8 weeks typical).

Steps in Bone Repair

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; bones: 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).

Common Vertebra Features

• Body (centrum)

• Vertebral arch (pedicle, lamina)

• Vertebral foramen

• Transverse and spinous processes

• Articular processes (superior/inferior)

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.

• Bones: clavicle, 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 pelvis – Larger, more circular inlet; shallower, lighter bones; wider pubic arch.

Lower Limbs

• Femur – Thigh bone; strongest in body.

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

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

• Bones arranged in three arches (two longitudinal, one transverse).

Joints (Articulations)

Functions of Joints

• Hold bones together securely.

• Allow for mobility.

Joint Classification

• Functional – Based on movement:

  • Synarthroses – Immovable

  • Amphiarthroses – Slightly movable

  • Diarthroses – Freely movable

• Structural – Based on tissue:

  • Fibrous joints – Generally immovable

  • Cartilaginous joints – Immovable or slightly movable

  • Synovial joints – Freely movable

Summary Table: Joint Classes

Type	Structure	Movement	Examples
Fibrous	Fibrous tissue	Immovable	Sutures, syndesmoses, gomphoses
Cartilaginous	Cartilage	Immovable/slightly movable	Epiphyseal plate, pubic symphysis
Synovial	Joint cavity with synovial fluid	Freely movable	Knee, shoulder, hip
Additional info:	Table entries inferred from context and standard anatomy.

Synovial Joint Features

• Articular cartilage covers bone ends.

• Articular capsule encloses joint cavity.

• Joint cavity contains synovial fluid.

• Reinforcing ligaments strengthen joint.

• Bursae – Fluid-filled sacs reducing friction.

• Tendon sheath – Elongated bursa around tendon.

Types of Synovial Joints (by Shape)

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

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

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

• Condylar joint – Flexion, extension, adduction, abduction (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 grows rapidly in early years; facial skeleton growth follows tooth and respiratory passage development.

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

• Puberty: female pelvis broadens; male skeleton becomes more robust.

• Epiphyseal plates ossify by end of adolescence.

Aging and Skeletal Health

• Osteoporosis – Bone-thinning disease; common in postmenopausal women and elderly men.

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

• Estrogen helps maintain bone density in females.

Key Equations

• Calcium Regulation:

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

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

Additional info: Some tables and bone marking details were inferred and expanded for clarity and completeness.