The Skeletal System: Structure, Function, and Bone Physiology

The Skeletal System

Components of the Skeletal System

The skeletal system is primarily composed of connective tissues and serves as the structural framework for the human body. Its main components include bones, cartilage, ligaments, and tendons.

• Bones: Provide support, protection, and leverage for movement.

• Cartilage: Acts as a precursor to most bones and covers joint surfaces.

• Ligaments: Dense connective tissue bands that connect bone to bone at joints.

• Tendons: Dense connective tissue bands that attach muscle to bone.

Functions of the Skeleton

The skeleton performs several essential functions for the body:

• Support: Provides structural support for the body and organs.

• Protection: Shields vital organs (e.g., brain, heart, lungs).

• Movement: Facilitates movement through muscle attachment.

• Mineral Storage: Stores minerals such as calcium and phosphate.

• Acid–Base Balance: Buffers blood pH by altering mineral levels.

• Hemopoiesis: Produces blood cells in bone marrow.

• Fat Storage: Yellow marrow stores fat in long bones.

Osseous Tissue: Structure and Cells

Bone Matrix

The matrix of osseous tissue is a composite material consisting of organic and inorganic components:

• Organic Matter: Synthesized by osteoblasts; includes collagen and carbohydrate–protein complexes (glycosaminoglycans, proteoglycans, glycoproteins).

• Inorganic Matter: Mainly hydroxyapatite (crystallized calcium phosphate), calcium carbonate, and other minerals.

This combination provides both strength and flexibility. Deficiencies in minerals or collagen can lead to diseases such as rickets or osteogenesis imperfecta.

Bone Cells

Four principal types of bone cells regulate bone formation and remodeling:

• Osteogenic Cells: Stem cells found in endosteum and periosteum; give rise to other bone cells.

• Osteoblasts: Bone-forming cells; secrete matrix and promote mineralization.

• Osteocytes: Mature bone cells trapped in matrix; reside in lacunae and act as strain sensors.

• Osteoclasts: Bone-dissolving cells; large, multinucleated, derived from white blood cells.

Bone Structure: Compact vs. Spongy Bone

Compact Bone

Compact bone forms the dense outer shell of bones and is organized into osteons (haversian systems):

• Osteons: Cylindrical structures with concentric lamellae surrounding a central canal.

• Lamellae: Layers of bone matrix.

• Canaliculi: Small channels connecting lacunae for nutrient exchange.

• Interstitial Lamellae: Fill spaces between osteons.

Spongy (Cancellous) Bone

Spongy bone is found at the ends of long bones and in the interior of other bones. It consists of a lattice of trabeculae and spicules, with spaces filled by red bone marrow.

• Trabeculae: Thin plates aligned along lines of stress.

• Red Bone Marrow: Site of hemopoiesis.

• Provides strength with minimal weight.

Bone Marrow

Types of Bone Marrow

Bone marrow is the soft tissue found in marrow cavities and spongy bone spaces:

• Red Marrow: Hemopoietic tissue; produces blood cells; found in most bones of children and select bones in adults.

• Yellow Marrow: Fatty tissue; does not produce blood cells but can revert to red marrow during chronic anemia.

Bone Development and Growth

Ossification Methods

Bone formation occurs via two primary methods:

• Endochondral Ossification: Bone develops from a pre-existing hyaline cartilage model (e.g., long bones).

• Intramembranous Ossification: Bone develops within a fibrous connective tissue membrane (e.g., flat bones of the skull).

Bone Growth in Length and Width

Bones grow in length via the epiphyseal plate (growth plate) and in width via appositional growth:

• Epiphyseal Plate: Zone of hyaline cartilage where bone elongation occurs; replaced by bone as growth ends.

• Appositional Growth: Osteoblasts deposit new bone at the surface, increasing diameter; osteoclasts enlarge the marrow cavity.

Bone Classification

By Texture

• Compact Bone: Dense, strong, forms outer layer.

• Spongy Bone: Porous, lightweight, forms inner layer.

By Development

• Endochondral: From cartilage.

• Intramembranous: From membrane.

By Shape and Size

• Flat Bones: Thin, protect organs (e.g., skull).

• Long Bones: Longer than wide, crucial for movement (e.g., femur).

• Short Bones: Nearly equal in length and width (e.g., carpals).

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

• Sutural Bones: Small bones within sutures of the skull.

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

General Features of Bones

Long Bone Anatomy

Long bones have distinct structural features:

• Diaphysis: Shaft providing leverage.

• Epiphyses: Enlarged ends for joint strength and attachment.

• Medullary Cavity: Contains bone marrow.

• Articular Cartilage: Covers joint surfaces.

• Periosteum: External sheath with fibrous and osteogenic layers.

• Endosteum: Lines marrow cavity; contains bone cells.

• Epiphyseal Plate/Line: Growth plate in children; line in adults.

Flat Bone Anatomy

Flat bones have a sandwich-like structure:

• Two layers of compact bone enclosing a middle layer of spongy bone (diploe).

• Both surfaces covered with periosteum.

• Diploe absorbs shock; marrow spaces lined with endosteum.

Bone Growth and Remodeling

Bone Remodeling

Bone remodeling is a continuous process involving absorption and deposition of bone tissue:

• Repairs microfractures, releases minerals, and adapts bone shape to mechanical stress (Wolff’s law).

• Involves coordinated activity of osteoblasts and osteoclasts.

Calcium Homeostasis and Hormonal Regulation

Calcium Regulation

Calcium homeostasis is vital for physiological functions and is regulated by three hormones:

• Calcitriol: Most active form of vitamin D; increases calcium absorption in the intestine, resorption from bone, and weakly promotes kidney reabsorption.

• Calcitonin: Secreted by the thyroid when blood calcium is high; inhibits osteoclasts and stimulates osteoblasts.

• Parathyroid Hormone (PTH): Secreted when blood calcium is low; stimulates osteoclasts, increases kidney reabsorption, promotes calcitriol synthesis, and inhibits osteoblasts.

Normal blood calcium concentration: 9.2–10.4 mg/dL.

Calcium Disorders

• Hypocalcemia: Deficient calcium; causes nervous system excitability and muscle spasms.

• Hypercalcemia: Excess calcium; causes muscle weakness and cardiac issues.

Bone Disorders

Fractures

Fractures are breaks in bone and are treated by closed or open reduction, casting, traction, or electrical stimulation.

Osteoporosis

Osteoporosis is the most common bone disease, characterized by severe loss of bone density and increased risk of fractures. Estrogen maintains bone density; risk factors include age, gender, diet, and lifestyle. Treatments include hormone therapy, medications, and prevention through diet and exercise.

Summary Table: Bone Cell Types

Cell Type	Function	Location
Osteogenic Cells	Stem cells; produce other bone cells	Endosteum, periosteum
Osteoblasts	Bone formation; secrete matrix	Bone surface
Osteocytes	Maintain bone tissue; sense strain	Lacunae within matrix
Osteoclasts	Bone resorption; dissolve matrix	Bone surface

Summary Table: Bone Classification

Classification	Description	Example
Flat Bone	Thin, curved, protective	Skull, ribs
Long Bone	Longer than wide, movement	Femur, humerus
Short Bone	Equal length and width, glide	Carpals, tarsals
Irregular Bone	Complex shape	Vertebrae
Sutural Bone	Small, within sutures	Skull
Sesamoid Bone	Develop in tendons	Patella

Additional info: Academic context and expanded explanations were added to ensure completeness and clarity for college-level study.