Bones and Skeletal Tissue: Structure, Function, and Development

Notes Practice Video lessons

Study Guide - Smart Notes

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

Chapter 6: Bones and Skeletal Tissue

Learning Objectives

Describe the major structures of long bone.
Identify the principal constituents of bone tissue.
Explain the function of five types of bone cells.
Discuss the homeostatic functions of bones.
Outline the mechanism of bone development/ossification.

Importance of Bone Anatomy and Remodeling

Clinical Relevance

Understanding bone anatomy and the process of bone remodeling is essential for diagnosing and treating bone diseases such as osteoporosis and fractures. Knowledge of these topics enables effective clinical practice and patient care.

Principal Constituents of Bone Tissue

Cartilage in Bone Development

Bone formation initially begins with cartilage, which is gradually replaced by bone tissue except in specific areas. Cartilage is a resilient, molded tissue composed mainly of water and chondrocytes, and lacks blood vessels and nerves.

Perichondrium: A dense connective tissue layer surrounding cartilage, providing support and nutrient delivery.
Chondrocytes: Specialized cells encased in lacunae within the cartilage matrix.
Types of Cartilage:
- Hyaline cartilage: Most abundant; provides support, flexibility, and resilience. Found in joints, ribs, respiratory tract, and nose tip.
- Elastic cartilage: Similar to hyaline but contains elastic fibers; found in the external ear and epiglottis.
- Fibrocartilage: Contains thick collagen fibers; provides tensile strength. Located in intervertebral discs and menisci of the knee.

Bone Growth Mechanisms

Appositional Growth: Growth in width; new matrix is secreted on the external face of existing cartilage.
Interstitial Growth: Chondrocytes divide and secrete matrix from within, expanding cartilage internally.
Cartilage growth occurs during normal bone development and can continue in old age.

Functions of Bones

Major Functions

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: Reservoir for calcium, phosphorus, and growth factors.
Blood Cell Formation (Hematopoiesis): Occurs in red marrow cavities of certain bones.
Triglyceride (Fat) Storage: Stored in bone cavities as an energy source.
Hormone Production: Osteocalcin regulates insulin secretion, glucose homeostasis, and energy metabolism.

Organization of the Human Skeleton

Axial and Appendicular Skeleton

Axial Skeleton: Forms the long axis of the body; includes the skull, vertebral column, and rib cage.
Appendicular Skeleton: Comprises bones of the upper and lower limbs and girdles attaching limbs to the axial skeleton.

Types of Bone Shapes

Classification by Shape

Long Bones: Longer than they are wide (e.g., femur, humerus).
Short Bones: Cube-shaped (e.g., wrist and ankle bones).
Flat Bones: Thin, flattened, and usually curved (e.g., sternum, ribs, skull bones).
Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).
Sesamoid Bones: Form within tendons (e.g., patella).

Major Structures of Long Bone

Gross Anatomy

Diaphysis: Tubular shaft forming the long axis; consists of compact bone surrounding the medullary cavity (contains 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; contains osteogenic cells, nerve fibers, and blood vessels.
- Endosteum: Connective tissue membrane lining internal bone surfaces.
Bone Marrow:
- Red Marrow: Found in trabecular cavities of spongy bone and flat bones; site of hematopoiesis.
- Yellow Marrow: Fat storage; can convert to red marrow if needed.

Bone Markings

Types and Functions

Projections: Sites of muscle, ligament, and tendon attachment (e.g., tuberosity, crest, trochanter, epicondyle, spine).
Depressions and Openings: Allow passage of blood vessels and nerves.

Types of Bone Cells and Their Functions

Five Major Types

Osteogenic (Osteoprogenitor) Cells: Stem cells that differentiate into osteoblasts.
Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix (osteoid), rich in collagen and calcium-binding proteins.
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: Bone-resorbing cells derived from hematopoietic stem cells; break down bone matrix for remodeling and calcium release.

Bone Development and Ossification

Overview of Ossification

Ossification (Osteogenesis): Process of bone tissue formation.
Types:
- Endochondral Ossification: Bone forms by replacing hyaline cartilage (most bones below the skull).
- Intramembranous Ossification: Bone develops from fibrous membranes (flat bones of skull, clavicle).
Bone Growth:
- Interstitial (Longitudinal) Growth: Lengthwise growth at the epiphyseal plate.
- Appositional Growth: Growth in thickness; occurs throughout life.

Homeostatic Functions of Bone

Calcium Homeostasis

Bones act as a reservoir for calcium, which is vital for nerve transmission, muscle contraction, blood coagulation, and cell division.
Calcium absorption requires Vitamin D.
Regulated by a negative feedback loop involving parathyroid hormone (PTH):
- Low blood calcium stimulates PTH release, which activates osteoclasts to resorb bone and release calcium into the blood.
- High blood calcium inhibits PTH and may trigger calcitonin release from the thyroid gland.
Imbalances can cause hyperexcitability (low calcium) or nonresponsiveness (high calcium), and may lead to kidney stones.

Hormonal Regulation of Bone Growth

Growth Hormone: Stimulates epiphyseal plate activity during infancy and childhood.
Thyroid Hormone: Modulates growth hormone activity for proper bone proportions.
Sex Hormones: Promote growth spurts and induce epiphyseal plate closure at puberty.
Deficiencies or excesses of these hormones cause abnormal skeletal growth.

Bone Remodeling and Repair

Remodeling Process

Bone remodeling is continuous and regulated by mechanical stress and hormonal signals.
Compact bone is replaced every 10 years; spongy bone every 3-4 years.
Wolff's Law: Bones grow or remodel in response to demands placed on them.

Bone Repair Stages

Hematoma Formation: Blood vessels hemorrhage, forming a clot.
Fibrocartilaginous Callus Formation: New trabeculae form in the callus.
Bony Callus Formation: Spongy bone replaces the callus.
Bone Remodeling: Compact bone reconstructs shaft walls, restoring original structure.

Bone Diseases

Common Disorders

Osteoporosis: Bone resorption exceeds deposit, leading to fragile bones and increased fracture risk.
Osteomalacia/Rickets: Bones are inadequately mineralized, causing softness and deformity (often due to vitamin D or calcium deficiency).
Paget's Disease: Excessive and haphazard bone deposit and resorption, resulting in abnormal bone structure.

Summary Table: Types of Cartilage

Type	Main Features	Locations
Hyaline	Support, flexibility, resilience; collagen fibers	Joints, ribs, nose, respiratory tract
Elastic	Contains elastic fibers; flexible	External ear, epiglottis
Fibrocartilage	Thick collagen fibers; tensile strength	Intervertebral discs, menisci of knee

Summary Table: Bone Cell Types

Cell Type	Function
Osteogenic	Stem cells; differentiate into osteoblasts
Osteoblasts	Form new bone matrix
Osteocytes	Maintain bone matrix; sense mechanical stress
Bone-lining cells	Maintain bone matrix on surfaces
Osteoclasts	Resorb (break down) bone matrix

Key Equations

Calcium Homeostasis (Negative Feedback):

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for exam preparation.