Bone Structure and Function: Anatomy & Physiology Study Notes

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

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

Overview of Skeletal Functions

Protection: The skeleton protects vital organs, such as the brain (protected by the skull) and the 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 (Hematopoiesis): Red bone marrow is the site of blood cell formation, producing red blood cells, white blood cells, and platelets.
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 provides a framework for soft tissues.

Classification of Bones by Shape

Main Bone Types

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, round 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; collagen fibers (Sharpey's fibers) anchor it to bone.
Nutrient Foramina: Small holes allowing blood vessels to penetrate bone tissue.
Endosteum: Reticular connective tissue lining marrow cavities and covering spongy bone surfaces.
Medullary Cavity: Central cavity containing bone marrow.
Epiphyseal Plate/Line: Site of bone growth in length (plate in children, line in adults).

Other Bone Types

Short, Flat, Irregular, and Sesamoid Bones: Generally lack a medullary cavity; composed of a layer of spongy bone (diploë in flat bones) sandwiched between compact bone layers.
Trabeculae: Bony struts in spongy bone, surrounded by 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, sternum).
Yellow Bone Marrow: Contains adipocytes that store triglycerides; found in the medullary cavity of long bones in adults.

Organic and Inorganic Components of Bone Matrix

Bone Matrix Composition

Organic Matrix (Osteoid): Makes up about 35% of extracellular matrix (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: Ca10(PO4)6(OH)2), 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. Some become trapped in the matrix and mature into osteocytes.
Osteocytes: Mature bone cells residing in lacunae; maintain bone tissue and communicate via canaliculi.
Osteoclasts: Large, multinucleated cells derived from blood cell lineages; responsible for bone resorption by secreting acids and enzymes.

Structures of Compact and Spongy Bone

Comparison Table

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

Bone Development: Primary vs. Secondary Bone

Bone Formation (Ossification/Osteogenesis)

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

Bone Formation Processes

Intramembranous Ossification

Forms flat bones of the skull and clavicle.
Begins with a membrane of embryonic connective tissue (mesenchyme).
Mesenchymal cells differentiate into osteogenic cells, then osteoblasts, which secrete osteoid and form bone tissue.
Osteocytes become trapped in the matrix; bone hardens by mineral deposition.

Endochondral Ossification

Forms most bones of the skeleton.
Begins with a hyaline cartilage model.
Chondroblasts in the perichondrium differentiate into osteoblasts, forming a bone collar around the shaft.
Primary ossification center develops in the diaphysis; secondary centers in the epiphyses.
Cartilage is replaced by bone; growth continues at the epiphyseal plates until adulthood.

Longitudinal vs. Appositional Bone Growth

Growth Mechanisms

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

Hormonal Regulation of Bone Growth

Key Hormones

Growth Hormone: Stimulates longitudinal and appositional growth by increasing mitosis of chondrocytes and osteogenic cells.
Testosterone: Accelerates bone growth and epiphyseal plate closure.
Estrogen: Potently stimulates bone growth and epiphyseal plate closure.

Bone Remodeling and Homeostasis

Why Bones Continuously Remodel

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

Osteoporosis and Healthy Bones

Overview

Osteoporosis: Common bone disease characterized by weak and brittle bones due to inadequate inorganic matrix, increasing fracture risk.
Risk Factors: Dietary deficiencies (calcium, vitamin D), female gender, advanced age, lack of exercise, hormonal changes (e.g., menopause), genetics, and other diseases.
Prevention: Balanced diet, supplementation, weight-bearing exercise, estrogen replacement therapy (if appropriate), and medications that inhibit osteoclasts or stimulate osteoblasts.

Key Equations and Terms

Hydroxyapatite Formula:

Bone Remodeling Equation (simplified):

Additional info: These notes expand on brief points from the original materials, providing academic context and definitions for key terms and processes in bone anatomy and physiology.