Bone Structure and Function: Anatomy & Physiology Study Notes

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Bone Structure and Function

Introduction

Bones are a fundamental component of the skeletal system, providing structure, protection, and support for the human body. This study guide covers the classification, structure, composition, and functions of bone tissue, as well as bone development and growth.

Functions of the Skeletal System

Main Functions

Protection: The skeleton protects vital organs, such as the brain (protected by the skull), heart and lungs (protected by the rib cage).
Mineral Storage and Acid-Base Homeostasis: Bones store minerals such as calcium (Ca2+) and phosphate (PO43-), which are necessary for electrolyte and acid-base balance.
Blood Cell Formation: Red bone marrow is the site of blood cell formation (hematopoiesis).
Fat Storage: Yellow bone marrow stores triglycerides (fat), serving as an energy reserve.
Movement: Muscles attach to bones, and their contraction moves the body via joints.
Support: The skeleton supports the weight of the body and maintains its shape.

Classification of Bones by Shape

Types of Bones

Long Bones: Longer than they are wide (e.g., humerus, femur). Function in movement and support.
Short Bones: Approximately equal in length and width (e.g., carpals, tarsals). Provide stability and support with limited motion.
Flat Bones: Thin and broad (e.g., skull bones, sternum, ribs). Protect internal organs and provide surfaces for muscle attachment.
Irregular Bones: Complex shapes that do not fit other categories (e.g., vertebrae).
Sesamoid Bones: Small bones embedded within tendons (e.g., patella).
Sutural (Wormian) Bones: Small bones found within the sutures of the skull.

Structure of Long Bones vs. Other Bone Types

Long Bone Structure

Articular Cartilage: Hyaline cartilage covering joint surfaces for smooth movement.
Periosteum: Dense irregular collagenous layer with an osteogenic layer; contains Sharpey's fibers anchoring it to bone.
Medullary Cavity: Central cavity containing bone marrow.
Endosteum: Reticular connective tissue lining marrow cavities and covering spongy bone surfaces.
Compact Bone: Dense tissue forming the outer layer and enclosing the medullary cavity.
Spongy (Cancellous) Bone: Found at the ends of long bones and inside short, flat, and irregular bones; contains trabeculae and bone marrow.

Other Bone Types

Short, Flat, Irregular, and Sesamoid Bones: Generally lack a medullary cavity; consist of a layer of spongy bone sandwiched between layers of compact bone.

Red vs. Yellow Bone Marrow

Bone Marrow Types

Red Bone Marrow: Contains hematopoietic cells that produce blood cells; found in children and in certain adult bones (e.g., pelvis, ribs, sternum).
Yellow Bone Marrow: Contains adipocytes that store triglycerides; found in the medullary cavity of long bones in adults.

Extracellular Bone Matrix: Organic and Inorganic Components

Bone Matrix Composition

Organic Matrix (Osteoid): Makes up about 35% of ECM; consists of collagen fibers, proteoglycans, glycosaminoglycans (GAGs), and glycoproteins. Provides flexibility and tensile strength.
Inorganic Matrix: Makes up about 65% of ECM; primarily hydroxyapatite crystals (calcium phosphate), bicarbonate, potassium, magnesium, and sodium. Provides hardness and resistance to compression.

Bone Tissue Cells and Their Functions

Types of Bone Cells

Osteogenic Cells: Stem cells that differentiate into osteoblasts.
Osteoblasts: Bone-building cells; secrete osteoid and promote mineralization.
Osteocytes: Mature osteoblasts trapped in lacunae; maintain bone matrix and communicate via canaliculi.
Osteoclasts: Large, multinucleated cells derived from monocytes; responsible for bone resorption by secreting acids and enzymes.

Compact vs. Spongy Bone Structure

Comparison Table

Feature	Compact Bone	Spongy Bone
Location	Outer layer of bones	Inner layer, ends of long bones
Structural Unit	Osteon (concentric lamellae)	Trabeculae (bony struts)
Blood Supply	Central and perforating canals	From vessels in compact bone
Function	Strength, support	Lightweight, supports marrow

Bone Development: Primary vs. Secondary Bone

Bone Types

Primary (Woven) Bone: First bone formed during development and repair; irregular arrangement of collagen fibers, lower mineral content.
Secondary (Lamellar) Bone: Mature bone; regular arrangement of collagen fibers, higher mineral content, organized into lamellae.

Bone Formation (Ossification)

Types of Ossification

Intramembranous Ossification: Formation of flat bones (skull, clavicle) from a membrane of embryonic connective tissue. Mesenchymal cells differentiate into osteoblasts, which secrete osteoid and form bone.
Endochondral Ossification: Formation of most bones from a hyaline cartilage model. Chondroblasts form cartilage, which is gradually replaced by bone tissue.

Bone Growth: Longitudinal vs. Appositional

Growth Mechanisms

Longitudinal Growth: Increase in length via chondrocyte division in the epiphyseal plate (growth plate).
Appositional Growth: Increase in width via osteoblasts depositing new compact bone beneath the periosteum.

Hormonal Regulation of Bone Growth

Key Hormones

Growth Hormone: Stimulates longitudinal and appositional bone growth.
Testosterone: Accelerates bone growth and epiphyseal plate closure.
Estrogen: Potently stimulates bone growth and epiphyseal plate closure.

Bone Remodeling

Why Bones Continuously Remodel

Maintains calcium homeostasis
Repairs microdamage
Replaces primary bone with secondary bone
Adapts to sustained tension and stresses

Osteoporosis and Healthy Bones

Osteoporosis

Common bone disease characterized by weak and brittle bones due to inadequate inorganic matrix.
Risk factors: calcium/vitamin D deficiency, female gender, advanced age, lack of exercise, hormonal changes, genetics.
Prevention: balanced diet, supplementation, weight-bearing exercise, estrogen replacement (if appropriate).
Treatment: drugs that inhibit osteoclasts or stimulate osteoblasts.

Key Equations and Terms

Hydroxyapatite Formula:
Hematopoiesis: Formation of blood cells in red bone marrow.
Osteogenesis: Formation of bone tissue.

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