Bones and Bone Tissue: Study Guide for ANP College Students

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Chapter 6: Bones and Bone Tissue

I. Bones as Organs

Bones are complex organs composed of several tissue types, not just osseous (bone) tissue. The skeletal system includes bones, joints, cartilage, ligaments, and bone marrow, all working together to support body structure and function.

Bone as an organ: Contains bone tissue, cartilage, connective tissue, blood vessels, and nerves.
Skeletal system components: Bones, joints (articulations), cartilage, ligaments, and bone marrow.
Key point: Bone is more than just osseous tissue; it is an organ system component.

II. Functions of the Skeletal System

The skeletal system performs several essential functions necessary for survival and movement.

Support: Provides structural framework for the body.
Protection: Shields vital organs (e.g., skull protects brain, ribs protect heart and lungs).
Movement: Acts as levers for muscles to produce movement.
Mineral storage: Stores calcium and phosphate ions, releasing them as needed.
Blood cell formation (hematopoiesis): Occurs in red bone marrow.
Fat storage: Yellow bone marrow stores triglycerides.

III. Classification of Bones by Shape

Bones are classified based on their shapes, which relate to their functions.

Long bones: Longer than wide (e.g., femur, humerus).
Short bones: Cube-shaped (e.g., carpals, tarsals).
Flat bones: Thin and broad (e.g., sternum, ribs, skull).
Irregular bones: Complex shapes (e.g., vertebrae).
Sesamoid bones: Embedded in tendons (e.g., patella).

IV. Gross Anatomy of a Long Bone

Long bones have distinct anatomical regions, each with specific functions.

Diaphysis: The shaft of the bone.
Epiphyses: The ends of the bone.
Articular cartilage: Hyaline cartilage covering joint surfaces.
Medullary cavity: Central cavity containing bone marrow.
Periosteum: Dense irregular connective tissue covering the outer surface.
Endosteum: Thin membrane lining the medullary cavity (inner surface).

Common mistake: Mixing up periosteum (outer) and endosteum (inner).

V. Compact vs Spongy Bone

Bones contain two types of osseous tissue, each with unique structure and function.

Compact bone: Dense outer layer; provides strength and resists stress. Contains osteons (Haversian systems).
Spongy bone: Internal lattice of trabeculae; houses bone marrow and reduces bone weight. Does not contain osteons.

VI. Bone Marrow

Bone marrow is a soft tissue found in the medullary cavities and spaces of spongy bone.

Red bone marrow: Site of hematopoiesis (blood cell production). More abundant in children; in adults, found in axial skeleton, hip bones, and proximal femur/humerus.
Yellow bone marrow: Stores fat (triglycerides); increases with age.

VII. Bone Matrix

The bone matrix is composed of inorganic and organic components, each contributing to bone properties.

Inorganic matrix (~65%): Mainly hydroxyapatite crystals (calcium phosphate); provides hardness and strength.
Organic matrix (~35%): Primarily collagen fibers; provides flexibility and tensile strength.

Key point: Collagen gives flexibility, not hardness.

VIII. Bone Cells

Bone tissue contains several specialized cell types, each with distinct roles.

Osteoblasts: Build new bone matrix (bone-forming cells).
Osteocytes: Mature bone cells that maintain bone tissue; reside in lacunae.
Osteoclasts: Break down (resorb) bone matrix; large, multinucleated cells.

Mnemonic: 'Blast builds, clast clears.'

IX. Histology of Compact Bone

Compact bone is organized into structural units called osteons.

Osteon (Haversian system): Cylindrical structure with concentric lamellae (rings) around a central canal.
Central canal: Contains blood vessels and nerves.
Osteocytes: Located in lacunae between lamellae; connected by canaliculi (tiny channels).

Common mistake: Confusing osteons (compact bone) with trabeculae (spongy bone).

X. Histology of Spongy Bone

Spongy bone has a porous, lattice-like structure.

Trabeculae: Thin, bony plates forming a network; provide strength with minimal weight.
No osteons or central canals: Nutrients diffuse from bone marrow.

XI. Ossification (Bone Formation)

Ossification is the process of bone tissue formation, occurring via two main mechanisms.

Intramembranous ossification: Forms flat bones (e.g., skull, clavicles) directly from mesenchymal tissue.
Endochondral ossification: Forms most bones; bone develops from a cartilage model.

Key point: Cartilage model = endochondral ossification.

XII. Bone Growth

Bones grow in length and width through different processes.

Length growth: Occurs at epiphyseal (growth) plates via endochondral ossification.
Width growth (appositional): New bone added to surface by osteoblasts.
Epiphyseal plate closure: Marks end of lengthwise growth (late teens).

XIII. Bone Remodeling

Bone is a dynamic tissue, constantly being remodeled throughout life.

Osteoblasts: Deposit new bone matrix.
Osteoclasts: Resorb (break down) bone matrix.
Remodeling triggers: Mechanical stress, hormones, and calcium levels.

XIV. Hormonal Regulation of Bone

Bone metabolism is regulated by several hormones that control calcium homeostasis.

Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts to resorb bone.
Calcitonin: Decreases blood calcium by inhibiting osteoclast activity.
Vitamin D: Promotes calcium absorption from the gut.

Key point: PTH pulls calcium out of bone into blood.

XV. Fracture Repair

Bone repair after fracture occurs in a series of well-defined stages.

Hematoma formation: Blood clot forms at fracture site.
Soft callus formation: Fibrocartilaginous tissue bridges the gap.
Hard callus formation: New bone replaces soft callus.
Remodeling: Bone is reshaped to original form.

XVI. Practice Questions (Lab & Lecture)

Identify compact vs spongy bone on images.
Name the bone cell responsible for bone resorption.
Identify an osteon and its components.
Differentiate intramembranous and endochondral ossification.
Explain how bone structure supports its function.

Summary Table: Bone Types and Features

Bone Type	Shape	Example	Main Function
Long	Longer than wide	Femur, humerus	Movement, support
Short	Cube-shaped	Carpals, tarsals	Stability, support
Flat	Thin, broad	Sternum, ribs, skull	Protection, muscle attachment
Irregular	Complex shape	Vertebrae	Protection, support
Sesamoid	Embedded in tendons	Patella	Reduce friction, modify pressure

Key Equations and Concepts

Hydroxyapatite formula:

Bone remodeling balance:

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