Chapter 7: Bone (Osseous) Tissue – Structure, Function, and Histology

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7.1 Tissues and Organs of the Skeletal System

Introduction

The skeletal system forms the structural framework of the body and is composed of bones, cartilages, and ligaments. It provides support, protection, movement, and plays a role in mineral and blood homeostasis.

Osteology: The scientific study of bones. There are 206 major bones in the adult human body.
Cartilage: Precursor to most bones; covers many joint surfaces.
Ligaments: Connect bones at joints.
Tendons: Attach muscles to bones (not part of the skeletal system but closely associated).

7.1a Functions of the Skeleton

Support: Provides structural support for the body and organs.
Protection: Shields vital organs (brain, spinal cord, heart, lungs).
Movement: Acts as levers for muscles to produce movement.
Electrolyte and Acid-Base Balance: Regulates calcium and phosphate levels; buffers blood pH by altering phosphate and carbonate salt levels.
Blood Formation: Red bone marrow produces blood cells (hematopoiesis).
Hormone Secretion: Bone cells secrete hormones that influence insulin action and stress response.

7.1b Bones and Osseous Tissue

Bone (Osseous Tissue): A connective tissue with a hardened matrix due to calcium phosphate and other minerals.
Mineralization/Calcification: The process of hardening bone tissue by depositing minerals.
Each bone is an organ composed of bone tissue, marrow, cartilage, adipose tissue, nervous tissue, and fibrous connective tissue.
The term "bone" can refer to the organ or just the osseous tissue.

7.1c General Features of Bones

Bones are classified by shape and structure, each adapted for specific functions.

Flat Bones: Thin, curved plates that protect soft organs (e.g., parietal bones, sternum, scapula, ribs, coxal bones).
Long Bones: Longer than wide; act as rigid levers for movement (e.g., humerus, femur, tibia, metacarpals, phalanges).
Short Bones: Nearly equal in length and width; found in wrist and ankle (carpals, tarsals).
Irregular Bones: Complex shapes (e.g., vertebrae, some skull bones, coxal bones).

General Anatomy of a Long Bone

Compact (Dense/Cortical) Bone: Outer shell; encloses the marrow (medullary) cavity containing yellow bone marrow.
Spongy (Cancellous) Bone: Loosely organized tissue found at bone ends and inside flat bones; covered by compact bone.
Diaphysis: Shaft providing leverage.
Epiphysis: Enlarged ends; strengthen joints and anchor ligaments/tendons.
Epiphyseal Line/Plate: Remnant of growth zone (cartilage in children, ossified in adults).
Articular Cartilage: Hyaline cartilage covering joint surfaces for smooth movement.
Periosteum: External sheath; outer fibrous layer (collagen), inner osteogenic layer (bone-forming cells). Perforating (Sharpey's) fibers anchor periosteum to bone.
Endosteum: Thin reticular connective tissue lining marrow cavity and internal surfaces; contains osteogenic cells, osteoblasts, osteoclasts; active in bone growth and repair.
Flat Bones: "Sandwich" structure—compact bone layers (inner and outer tables) with spongy bone (diploë) in between.

7.2 Histology of Osseous Tissue

7.2a Bone Cells

Bone tissue contains four main types of cells, each with specialized functions in bone formation, maintenance, and remodeling.

Osteogenic/Osteoprogenitor Cells: Mesenchymal stem cells in endosteum and periosteum; give rise to osteoblasts.
Osteoblasts: Bone-forming cells; synthesize organic matrix (osteoid) and promote mineralization. Stimulated by stress to reinforce bone, using calcium and phosphate from blood.
Osteocytes: Former osteoblasts trapped in matrix; reside in lacunae, connected by canaliculi. Act as strain sensors, regulate bone remodeling, maintain matrix, and aid in repair.
Osteoclasts: Large, multinucleated cells from bone marrow stem cells (not osteogenic cells); perform bone resorption (osteolysis), releasing calcium and phosphate into blood.
Osteocalcin: Hormone secreted by osteoblasts and osteocytes; influences insulin secretion and sensitivity, part of the acute stress response.

7.2b The Matrix

The bone matrix is a composite material providing both strength and flexibility, composed of organic and inorganic components.

Organic Matter (1/3): Collagen fibers and carbohydrate-protein complexes (glycosaminoglycans, proteoglycans, glycoproteins); synthesized by osteoblasts.
Inorganic Matter (2/3): 85% hydroxyapatite (), 10% calcium carbonate (), and other ions (magnesium, fluoride, sodium).
Composite Function: Organic portion provides flexibility (collagen, sacrificial bonds dissipate shock); inorganic portion provides compressive strength (prevents sagging).

Clinical Correlations:

Rickets: Soft, deformed bones due to mineral deficiency (often vitamin D or calcium).
Osteogenesis Imperfecta: Brittle bone disease from defective collagen deposition.

7.2c Compact Bone

Compact bone is organized into structural units called osteons, which provide strength and resist stress.

Concentric Lamellae: Layers of matrix around a central (Haversian) canal.
Osteon (Haversian System): Central canal plus surrounding lamellae; basic structural unit of compact bone.
Perforating (Volkmann's) Canals: Transverse canals connecting central canals.
Cement Line: Separates osteons, prevents spread of microfractures.
Circumferential Lamellae: Encircle inner and outer regions of dense bone.
Interstitial Lamellae: Fill spaces between osteons.

7.2d Spongy Bone

Spongy bone consists of a lattice of trabeculae and spicules, providing strength with minimal weight and housing red bone marrow.

Structure: Trabeculae (plates) and spicules (slivers) covered with endosteum; canaliculi open onto trabecular surfaces.
Function: Supports bone marrow, resists stress from multiple directions.
Red Bone Marrow: Fills spaces; site of blood cell production.
Few Osteons: Most osteocytes are close to bone marrow; no central canals.
Mechanical Stress: Trabeculae align along lines of stress; spongy bone has greater surface area, making it more susceptible to osteoclastic activity.

7.2e Bone Marrow

Bone marrow is the soft tissue within bone cavities, essential for blood cell production and energy storage.

Red Bone Marrow (Myeloid Tissue): Hematopoietic tissue producing blood cells; found in nearly all bones in children, restricted to axial skeleton and proximal limb bones in adults.
Yellow Bone Marrow: Adipose tissue; replaces red marrow in adults, can revert to red marrow in severe anemia.

7.2f Bone Remodeling

Bone remodeling is a dynamic process balancing bone formation and resorption to maintain bone strength and mineral homeostasis.

Homeostasis: Balance between osteoblast (building) and osteoclast (breakdown) activity.
Imbalance: More breakdown leads to weaker bones (osteopenia, osteoporosis); more building leads to stronger bones.
Exercise: Weight-bearing activity stimulates osteoblasts, increasing bone mass.

Table: Types of Bone Cells and Their Functions

Cell Type	Origin	Main Function	Location
Osteogenic/Osteoprogenitor	Mesenchymal stem cells	Differentiate into osteoblasts	Endosteum, periosteum
Osteoblast	Osteogenic cells	Bone formation (osteogenesis)	Endosteum, periosteum
Osteocyte	Osteoblasts trapped in matrix	Maintain matrix, sense strain	Lacunae within bone
Osteoclast	Fusion of bone marrow stem cells	Bone resorption (osteolysis)	Bone surfaces

Key Equations

Hydroxyapatite Formula:

Additional info: The notes above are synthesized from textbook-style lecture slides and expanded with academic context for clarity and completeness. They cover the structure, function, and histology of bone tissue, suitable for college-level Anatomy & Physiology students.