Chapter 6: Bones and Skeletal Tissue

Introduction

This chapter explores the anatomy, composition, and physiological roles of bones and skeletal tissue. Understanding these concepts is essential for recognizing bone diseases and the process of bone remodeling.

Principal Constituents of Bone Tissue

Cartilage Structure and Types

Cartilage is a resilient, molded tissue that forms the initial skeleton and is later replaced by bone, except in specific areas. It is avascular and lacks nerves.

• Perichondrium: Dense connective tissue layer surrounding cartilage, providing support and nutrient delivery.

• Chondrocytes: Cells encased in lacunae within the cartilage matrix.

Types of Cartilage

• Hyaline Cartilage: Most abundant; provides support, flexibility, and resilience. Contains collagen fibers. Found in articular joints, costal (ribs), respiratory tract, and nasal cartilage.

• Elastic Cartilage: Similar to hyaline but contains more elastic fibers. Found in the external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers; provides great tensile strength. Found in menisci of the knee and intervertebral discs.

Cartilage Growth

• Appositional Growth: Cartilage-forming cells secrete new matrix on the external surface.

• Interstitial Growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

Functions of Bones

Bones serve multiple vital functions in the human body:

• Support: Framework for the body and soft organs.

• Protection: Shields the brain, spinal cord, and vital organs.

• Movement: Provides levers for muscle action.

• Mineral and Growth Factor Storage: Reser`voir for calcium, phosphorus, and growth factors.

• Blood Cell Formation (Hematopoiesis): Occurs in red marrow cavities.

• Triglyceride (Fat) Storage: Energy source stored in bone cavities.

• Hormone Production: Osteocalcin regulates insulin secretion and glucose metabolism.

Classification of Bones

Bones are classified by location and shape:

• Axial Skeleton: Long axis of the body (skull, vertebral column, rib cage).

• Appendicular Skeleton: Limbs and girdles attaching limbs to the axial skeleton.

Bone Shapes

• Long Bones: Longer than wide (e.g., humerus).

• Short Bones: Cube-shaped (e.g., wrist, ankle); includes sesamoid bones (e.g., patella).

• Flat Bones: Thin, slightly curved (e.g., scapulae, ribs, skull bones).

• Irregular Bones: Complex shapes (e.g., vertebrae, hip bones).

Major Structures of Long Bone

General Structure

Long bones contain several tissue types, but osseous tissue predominates. They have a shaft (diaphysis), bone ends (epiphyses), and membranes.

• Diaphysis: Tubular shaft forming the long axis; consists of compact bone surrounding the medullary cavity (yellow marrow in adults).

• Epiphyses: Bone ends; consist of compact bone externally and spongy bone internally. Articular cartilage covers joint surfaces.

• Epiphyseal Line: Remnant of the childhood epiphyseal plate where bone growth occurred.

Membranes

• Periosteum: Double-layered membrane covering external bone surfaces except joints. Outer fibrous layer secures to bone; inner osteogenic layer gives rise to bone cells. Contains nerve fibers and blood vessels.

• Endosteum: Connective tissue membrane covering internal bone surfaces and trabeculae of spongy bone; contains osteogenic cells.

Bone Marrow

• Red Marrow: Found in trabecular cavities of spongy bone and flat bones (e.g., sternum, hip bone); site of hematopoiesis.

• Yellow Marrow: Fat storage; can convert to red marrow if needed (e.g., anemia).

Bone Markings

Sites for muscle, ligament, and tendon attachment, joint formation, and passage of blood vessels and nerves.

• Tuberosity: Large, rounded projection; may be roughened.

• Crest: Prominent ridge of bone.

• Trochanter: Very large, blunt, irregularly shaped process (only on femur).

• Epicondyle: Raised area on or above a condyle.

• Spine: Slender, pointed projection.

• Process: Any bony prominence.

Types and Functions of Bone Cells

Overview of Bone Cells

Bone tissue contains five major cell types, each with specialized functions:

• Osteogenic (Osteoprogenitor) Cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix (osteoid).

• Osteocytes: Mature bone cells that maintain bone matrix and act as stress/strain sensors.

• Bone-Lining Cells: Flat cells on bone surfaces; help maintain bone matrix.

• Osteoclasts: Multinucleated cells responsible for bone resorption (breakdown).

Functions of Bone Cells

• Osteogenic Cells: Mitotically active stem cells found in periosteum and endosteum.

• Osteoblasts: Secrete osteoid, which is made of collagen and calcium-binding proteins; responsible for bone growth.

• Osteocytes: Monitor and maintain bone matrix; communicate with osteoblasts and osteoclasts to direct bone remodeling.

• Bone-Lining Cells: Maintain bone matrix; called periosteal cells (external) and endosteal cells (internal).

• Osteoclasts: Derived from hematopoietic stem cells; break down bone matrix via enzyme degradation and acid secretion.

Bone Tissue Composition

Organic and Inorganic Components

• Organic: Includes bone cells and osteoid (ground substance and collagen fibers), providing tensile strength and flexibility.

• Inorganic: Mineral salts (mainly hydroxyapatites, calcium phosphate crystals) make up 65% of bone mass, providing hardness and resistance to compression.

Mechanism of Bone Development/Ossification

Overview of Ossification

Ossification (osteogenesis) is the process of bone tissue formation. It begins in the embryo and continues through childhood and adolescence, with remodeling and repair occurring throughout life.

Types of Bone Formation

• Endochondral Ossification: Bone forms by replacing hyaline cartilage; forms most bones below the skull.

• Intramembranous Ossification: Bone develops from fibrous membrane; forms flat bones (e.g., cranial bones, clavicles).

Bone Growth

• Interstitial (Longitudinal) Growth: Increases bone length at the epiphyseal plate.

• Appositional Growth: Increases bone thickness by adding new bone to the surface.

Zones of Epiphyseal Plate (for Longitudinal Growth)

• Proliferation zone

• Hypertrophic zone

• Calcification zone

• Ossification (osteogenic) zone

Bone Remodeling and Repair

• Bone remodeling is a continuous process regulated by hormonal and mechanical factors.

• Bone repair involves hematoma formation, fibrocartilaginous callus formation, bony callus formation, and bone remodeling.

Homeostatic Functions of Bones

Calcium Homeostasis

Bones play a critical role in maintaining blood calcium levels through a negative feedback loop involving hormones.

• Parathyroid Hormone (PTH): Released in response to low blood calcium; stimulates osteoclasts to resorb bone, increasing blood calcium.

• Calcitonin: Produced by the thyroid gland in response to high blood calcium; inhibits bone resorption.

Calcium is essential for nerve transmission, muscle contraction, blood coagulation, gland and nerve function, and cell division. Absorption of calcium requires vitamin D.

Hormonal Regulation of Bone Growth

• Growth Hormone: Stimulates epiphyseal plate activity during infancy and childhood.

• Thyroid Hormone: Modulates growth hormone activity.

• Sex Hormones (Testosterone, Estrogen): Promote growth spurts and induce epiphyseal plate closure at puberty.

Mechanical Stress and Bone Remodeling

Bones remodel in response to mechanical stress (Wolff's Law): bones grow or remodel according to the demands placed on them.

Clinical Implications

Bone Diseases

• Osteoporosis: Bone resorption exceeds deposition, leading to fragile bones and increased fracture risk.

• Osteomalacia/Rickets: Bones are poorly mineralized due to vitamin D deficiency, resulting in soft, weak bones and deformities.

• Fractures: Classified by severity and whether the skin is penetrated (open vs. closed).

Summary Table: Types of Bone Cells

Key Equations

• Calcium Homeostasis (Negative Feedback):

• Bone Mass Balance:

Example

Application: In osteoporosis, bone resorption by osteoclasts outpaces bone formation by osteoblasts, resulting in decreased bone density and increased fracture risk.

Additional info: Academic context and terminology have been expanded for clarity and completeness.