The Skeletal System: Structure, Function, and Bone Tissue

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The Skeletal System

Overview

The skeletal system is a fundamental component of the human body, providing structure, protection, and support for movement. It consists of bones, joints (articulations), and associated cartilages. Understanding its organization and function is essential for the study of anatomy and physiology.

Functions of the Skeletal System

Support: Provides a framework that supports the body and cradles soft organs.
Protection: Encloses and protects vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).
Movement: Serves as levers for muscles to act upon, enabling movement.
Mineral Storage: Reservoir for minerals, especially calcium and phosphate.
Triglyceride Storage: Stores fat in yellow bone marrow.
Hematopoiesis: Blood cell formation occurs in red bone marrow.
Hormone Production: Produces osteocalcin, which helps regulate bone formation and metabolism.

Bone Classification

Axial vs. Appendicular Skeleton

Axial Skeleton: Forms the long axis of the body; includes the skull, vertebral column, and rib cage.
Appendicular Skeleton: Includes the bones of the upper and lower limbs, and the girdles (shoulder and pelvic) that attach them to the axial skeleton.

Classification by Shape

Class	Description	Examples
Long	Longer than they are wide	Femur, humerus
Short	Somewhat cube-shaped	Carpals, tarsals
Flat	Thin with two parallel sides; often slightly curved	Sternum, ribs, skull bones
Irregular	Complex shapes that do not fit other categories	Vertebrae, hip bones

Sesamoid: Short bone embedded within a tendon (e.g., patella).
Wormian (Sutural): Bone located within a skull suture line.
Pneumatic: Bone that contains a sinus (e.g., maxilla).
Heterotrophic: Bone that develops in soft tissue (e.g., scar tissue).
Accessory: Surplus bones.

Bone Tissue Structure

Cellular and Matrix Components

Cells	Extracellular Matrix
Osteoprogenitor cells: Stem cells that give rise to osteoblasts. Osteoblasts: Bone-forming cells; secrete organic matrix (osteoid). Bone lining cells: Found on bone surfaces where remodeling is not occurring. Osteocytes: Mature bone cells; maintain the matrix. Osteoclasts: Bone-resorbing cells; break down bone matrix.	Osteoid: Organic component; mainly collagen, provides tensile strength. Hydroxyapatite: Inorganic component; calcium phosphate crystals, provide compressional strength.

Lamellar Bone Structure

Concentric lamellae: Circular rings of bone tissue.
Interstitial lamellae: Bone tissue between groups of rings.
Canaliculi: Small channels connecting osteocytes, allowing nutrient and waste exchange.

Types of Bone Tissue

Spongy (Cancellous) Bone: Composed of trabeculae (lattice-like structures); spaces filled with red marrow; lined by endosteum.
Compact Bone: Dense, solid; contains osteons (Haversian systems) as functional units.

Osteon Structure

Concentric lamellae: Layers of bony matrix around a central canal.
Lacuna: Small spaces housing osteocytes.
Canaliculi: Channels for communication and nutrient transfer.
Central canal: Contains blood vessels and nerves.
Perforating (Volkmann's) canals: Connect central canals of adjacent osteons.
Circumferential lamellae: Encircle the entire bone beneath the periosteum.
Interstitial lamellae: Fill spaces between osteons.

Gross Anatomy of Bones

Long Bone Structure

Diaphysis: Shaft of the bone.
Epiphysis: Ends of the bone; proximal and distal.
Metaphysis: Region between diaphysis and epiphysis; contains epiphyseal plate in growing bones.
Epiphyseal plate/line: Site of bone growth in length during childhood; becomes epiphyseal line after growth ceases.
Articular cartilage: Hyaline cartilage covering joint surfaces.
Medullary cavity: Central cavity containing yellow marrow (fat storage).
Endosteum: Thin connective tissue lining the medullary cavity and trabeculae.
Periosteum: Double-layered membrane covering external bone surface; outer fibrous layer and inner osteogenic layer.
Sharpey's fibers: Collagen fibers anchoring periosteum to bone.

Short, Flat, and Irregular Bones

Composed of thin layers of compact bone surrounding a core of spongy bone.
In flat bones, the spongy bone is called diploë.
The exterior is covered by periosteum; spaces between trabeculae are filled with marrow.

Summary Table: Bone Types and Features

Bone Type	Key Features
Long	Diaphysis, epiphyses, medullary cavity, periosteum, endosteum
Short	Cube-shaped, mostly spongy bone, thin compact bone layer
Flat	Thin, parallel surfaces, spongy bone (diploë) between compact bone
Irregular	Complex shapes, variable composition

Key Terms and Definitions

Osteon (Haversian system): Structural unit of compact bone.
Trabeculae: Lattice-like structures in spongy bone.
Lamellae: Layers of bone matrix.
Lacunae: Small spaces containing osteocytes.
Canaliculi: Channels connecting lacunae.
Periosteum: Membrane covering bone surface.
Endosteum: Membrane lining internal bone surfaces.
Epiphyseal plate: Growth plate in long bones.
Diploë: Spongy bone in flat bones.

Additional info:

Bone remodeling and repair, bone growth (appositional and longitudinal), and joint classification are also key topics in the skeletal system, though not fully detailed in these notes.
For further study, refer to the textbook "Human Anatomy & Physiology" by Elaine Marieb and Katja Hoehn, 10th edition.