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 processes.

Functions of the Skeletal System

Overview of Skeletal Functions

• Protection: The skeleton protects vital organs, such as the brain (protected by the skull) and heart/lungs (protected by the rib cage).

• Mineral Storage and Acid-Base Homeostasis: Bones store minerals like calcium (Ca2+) and phosphate (PO43-), which are released into the blood as needed to maintain electrolyte and acid-base balance.

• Blood Cell Formation: Hematopoiesis occurs in red bone marrow, producing red blood cells, white blood cells, and platelets.

• Fat Storage: Yellow bone marrow stores triglycerides, serving as an energy reserve.

• Movement: Muscles attach to bones, and their contraction enables movement at joints.

• Support: The skeleton provides a rigid framework that supports the body’s weight.

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 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 located within the sutures of the skull.

Structure of Bone Types

Comparison of Long, Short, Flat, Irregular, and Sesamoid Bones

• Long Bones:

  • Consist of a diaphysis (shaft) and two epiphyses (ends).

  • Medullary cavity contains bone marrow.

  • Covered by periosteum (dense connective tissue) and endosteum (lines marrow cavity).

  • Articular cartilage covers joint surfaces.

  • Compact bone forms the outer layer; spongy bone is found at the ends.

• Short, Flat, Irregular, and Sesamoid Bones:

  • Do not have a shaft or epiphyses.

  • Consist of spongy bone (diploë in flat bones) sandwiched between layers of compact bone.

  • Contain bone marrow but no medullary cavity.

  • Covered by periosteum and lined by endosteum.

Red vs. Yellow Bone Marrow

Bone Marrow Types and Functions

• Red Bone Marrow:

  • Contains hematopoietic cells that produce blood cells.

  • Found in spongy bone of flat bones and epiphyses of long bones in adults.

• Yellow Bone Marrow:

  • Contains adipocytes that store triglycerides (fat).

  • Found in the medullary cavity of long bones in adults.

Bone Matrix Composition

Organic and Inorganic Components

• Organic Matrix (Osteoid):

  • Composed of collagen fibers, proteoglycans, glycosaminoglycans (GAGs), and glycoproteins.

  • Provides flexibility and tensile strength.

• Inorganic Matrix:

  • Makes up about 65% of bone extracellular matrix.

  • Consists mainly of hydroxyapatite crystals (), bicarbonate, potassium, magnesium, and sodium.

  • Provides hardness and resistance to compression.

Bone Tissue Cells

Types and Functions

• Osteogenic Cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-building cells that secrete osteoid and initiate mineralization.

• Osteocytes: Mature bone cells trapped in lacunae; maintain bone matrix and communicate via canaliculi.

• Osteoclasts: Large, multinucleated cells that resorb (break down) bone tissue; derived from monocyte lineage.

Compact vs. Spongy Bone Structure

Comparison Table

Bone Development and Ossification

Primary vs. Secondary Bone

• Primary (Woven) Bone:

  • First bone formed during embryonic development and fracture repair.

  • Irregular arrangement of collagen fibers; less mineralized.

• Secondary (Lamellar) Bone:

  • Mature bone with regular, parallel collagen arrangement.

  • Higher mineral content; stronger and more resilient.

Intramembranous Ossification

• Occurs in flat bones of the skull and clavicle.

• Mesenchymal cells differentiate into osteogenic cells, then osteoblasts.

• Osteoblasts secrete osteoid, which mineralizes to form bone.

• Trabeculae form, and periosteum develops on the surface.

Endochondral Ossification

• Forms most bones of the skeleton (long, short, irregular).

• Bone develops from a hyaline cartilage model.

• Primary ossification center forms in the diaphysis; secondary centers in epiphyses.

• Cartilage is replaced by bone tissue, except at articular surfaces and epiphyseal plates.

Bone Growth

Longitudinal vs. Appositional Growth

• Longitudinal Growth:

  • Increase in bone length via chondrocyte division in epiphyseal plates.

  • New cartilage is replaced by bone tissue.

• Appositional Growth:

  • Increase in bone width via osteoblasts depositing new compact bone under periosteum.

  • Remodeling occurs as osteoclasts resorb bone from the endosteal surface.

Hormonal Regulation of Bone Growth

Key Hormones

• Growth Hormone: Stimulates longitudinal and appositional growth by increasing chondrocyte and osteoblast activity.

• Testosterone: Promotes bone growth and accelerates closure of epiphyseal plates.

• Estrogen: Potently accelerates epiphyseal plate closure and maintains bone density.

Bone Remodeling

Why Bones Continuously Remodel

• Maintains calcium homeostasis.

• Repairs microdamage and replaces primary bone with secondary bone.

• Adapts to mechanical stress and changing functional demands.

Osteoporosis and Healthy Bones

Overview

• Osteoporosis: Common bone disease characterized by decreased bone density and increased fracture risk due to inadequate inorganic matrix.

• Risk Factors: Calcium/vitamin D deficiency, female gender, advanced age, lack of exercise, hormonal changes (e.g., menopause), genetics, and other diseases.

• Prevention: Balanced diet, supplementation, weight-bearing exercise, and estrogen replacement therapy if indicated.

• Treatment: Drugs that inhibit osteoclasts or stimulate osteoblasts.

Additional info: The notes above expand on brief points and fill in context for bone structure, function, and development, suitable for college-level Anatomy & Physiology students.