The Skeletal System: Structure, Composition, and the Axial Skeleton

The Skeletal System

Overview and Functions

The skeletal system is a complex framework of bones and cartilages that provides structural support, protection, and movement for the human body. It is composed of two primary tissue types and is organized into distinct divisions and bone types.

• Primary Tissue Types: The skeleton is formed by bone tissue (osseous tissue) and cartilage.

• Functions of the Skeletal System:

  • Support: Provides a rigid framework for the body.

  • Protection: Shields vital organs (e.g., skull protects the brain).

  • Movement: Acts as levers for muscles to act upon.

  • Mineral Storage: Reservoir for calcium and phosphate.

  • Blood Cell Formation: Hematopoiesis occurs in red marrow.

  • Triglyceride Storage: Yellow marrow stores fat.

Classification of Bones

Divisions and Bone Types

Bones are classified by their location in the skeleton and by their shape and structure.

• Divisions of the Skeleton:

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

  • Appendicular Skeleton: Limbs and girdles (shoulder and pelvic).

• Bone Textures:

  • Compact Bone: Dense, smooth, and forms the outer layer of bones.

  • Spongy (Cancellous) Bone: Porous, contains trabeculae, found inside bones.

• Bone Shapes:

  • Long Bones: Longer than wide (e.g., femur, humerus).

  • Short Bones: Cube-shaped (e.g., carpals, tarsals).

  • Flat Bones: Thin, flattened, often curved (e.g., sternum, skull bones).

  • Irregular Bones: Complex shapes (e.g., vertebrae, hip bones).

  • Sesamoid Bones: Form within tendons (e.g., patella).

  • Wormian (Sutural) Bones: Small bones within cranial sutures.

Gross Anatomy of a Typical Long Bone

Major Structures and Regions

Long bones have distinct anatomical regions, each with specific functions.

• Diaphysis: The shaft; composed mainly of compact bone.

• Epiphysis: The ends of the bone; mostly spongy bone covered by compact bone.

• Medullary Cavity: Central cavity within the diaphysis; contains yellow marrow in adults.

• Red Marrow: Found in spongy bone of epiphyses; site of blood cell formation.

• Periosteum: Dense connective tissue membrane covering the outer surface of bone.

• Endosteum: Thin membrane lining the medullary cavity.

• Articular Cartilage: Hyaline cartilage covering joint surfaces of epiphyses.

• Epiphyseal Line: Remnant of the epiphyseal plate (growth cartilage) in adults.

Example: In a child's hand X-ray, the epiphyseal plates are visible as dark lines between the diaphysis and epiphyses, indicating sites of bone growth.

Chemical Composition of Bone

Organic and Inorganic Components

The unique properties of bone are due to its composite matrix of organic and inorganic materials.

• Organic Matrix: Primarily collagen fibers, providing flexibility and tensile strength.

• Inorganic Salts: Mainly hydroxyapatite (calcium phosphate crystals), providing hardness and resistance to compression.

Example: Baked bones (removal of organic matrix) become brittle, while acid-soaked bones (removal of minerals) become flexible.

Microscopic Structure of Bone

Compact Bone and Osteons

Compact bone is organized into structural units called osteons or Haversian systems.

• Osteon (Haversian System): Cylindrical structure containing concentric lamellae around a central canal.

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Lamellae: Concentric rings of bone matrix.

• Lacunae: Small spaces housing osteocytes (bone cells).

• Canaliculi: Tiny canals connecting lacunae, allowing communication and nutrient exchange between osteocytes.

• Periosteum: Outer fibrous layer of dense irregular connective tissue.

• Perforating (Sharpey's) Fibers: Collagen fibers anchoring periosteum to bone.

• Perforating (Volkmann's) Canals: Transverse channels for blood vessels and nerves from periosteum into bone.

Cartilages of the Skeleton

Types and Locations

Cartilage is a resilient connective tissue found in various parts of the skeleton, classified by fiber type and function.

• Hyaline Cartilage: Most abundant; found in articular surfaces, costal cartilages, nose, trachea, and larynx.

• Elastic Cartilage: Contains elastic fibers; found in external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers; found in intervertebral discs, pubic symphysis, and menisci.

Example: The trachea contains hyaline cartilage rings, while the external ear is supported by elastic cartilage.

The Axial Skeleton

Components and Major Features

The axial skeleton forms the central axis of the body and consists of the skull, vertebral column, and thoracic cage.

• Skull: Composed of cranial and facial bones.

  • Cranial Bones: Frontal, parietal, occipital, temporal, sphenoid, ethmoid.

  • Facial Bones: Mandible, maxilla, palatine, zygomatic, lacrimal, nasal, vomer, inferior nasal concha.

  • Hyoid Bone: Not attached to other bones; supports tongue and neck muscles.

  • Paranasal Sinuses: Air-filled spaces in certain skull bones (e.g., frontal sinus).

• Vertebral Column: Consists of 33 vertebrae in five regions.

  • Cervical (C1–C7): C1 (Atlas) and C2 (Axis) are specialized for head movement.

  • Thoracic (T1–T12): Articulate with ribs.

  • Lumbar (L1–L5): Large, block-like bodies for weight-bearing.

  • Sacral (S1–S5): Fused to form the sacrum.

  • Coccyx: 3–5 fused vertebrae forming the tailbone.

• Thoracic Cage: Protects thoracic organs; includes sternum and 12 pairs of ribs.

  • True Ribs (1–7): Attach directly to sternum via costal cartilage.

  • False Ribs (8–12): Attach indirectly or not at all to sternum.

  • Floating Ribs (11–12): No anterior attachment.

  • Sternum: Manubrium, body, xiphoid process.

Vertebral Structure and Identification

Each vertebra has characteristic features that allow for identification and function.

• Body: Weight-bearing anterior portion.

• Lamina: Part of the vertebral arch.

• Vertebral Foramen: Canal for the spinal cord.

• Transverse Process: Lateral projections for muscle/ligament attachment.

• Spinous Process: Posterior projection for muscle/ligament attachment.

• Superior and Inferior Articular Processes: Form joints with adjacent vertebrae.

Spinal Curvatures

• Normal Curvatures: Cervical, thoracic, lumbar, and sacral curves provide flexibility and distribute mechanical stress.

• Abnormal Curvatures:

  • Scoliosis: Lateral curvature.

  • Kyphosis: Exaggerated thoracic curvature (hunchback).

  • Lordosis: Exaggerated lumbar curvature (swayback).

The Fetal Skeleton

Developmental Features

The fetal skeleton differs from the adult skeleton in both structure and number of bones.

• Fontanelles: Soft membranous gaps between cranial bones in infants; allow for brain growth and skull flexibility during birth.

• Ossification Centers: Sites where bone formation begins in the fetal skull.

• Epiphyseal Plates: Cartilaginous growth plates that later ossify to form a single bone.

Example: The anterior fontanelle is the largest and most clinically significant, closing by 18–24 months of age.

Supportive Connective Tissues: Bone and Cartilage

Classification and Microscopic Features

Supportive connective tissues include bone and three types of cartilage, each with unique matrix and cellular components.

Type	Matrix	Fibers	Cells	Location
Bone	Hard, calcified	Collagen	Osteocytes in lacunae	Skeletal bones
Hyaline Cartilage	Firm, glassy	Fine collagen	Chondrocytes in lacunae	Articular surfaces, nose, trachea
Elastic Cartilage	Flexible	Elastic fibers	Chondrocytes in lacunae	External ear, epiglottis
Fibrocartilage	Tough	Thick collagen	Chondrocytes in lacunae	Intervertebral discs, menisci

Additional info: In histological slides, ground matrix appears as the background substance, fibers are visible as thread-like structures, and cells (osteocytes or chondrocytes) are found within lacunae.