Bones and Skeletal Tissue

Introduction

The human skeleton is initially composed of cartilage, which is gradually replaced by bone during development. The skeletal system provides support, protection, movement, mineral storage, and blood cell formation. Understanding the structure and function of cartilage and bone is essential for studying anatomy and physiology.

Cartilage Structure and Function

Types of Cartilage

Cartilage is a resilient, flexible connective tissue found in various parts of the body. It is avascular (lacks blood vessels) and consists of chondrocytes within lacunae, surrounded by an extracellular matrix.

• Hyaline Cartilage: Most abundant type; provides support, flexibility, and resilience. Contains fine collagen fibers only. Found in articular surfaces (joints), costal cartilage (ribs), larynx, and nasal cartilage (nose tip).

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

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

Perichondrium is a layer of dense connective tissue that surrounds cartilage, providing nutrients and resisting outward expansion.

Growth of Cartilage

Cartilage grows by two mechanisms:

• Appositional Growth: Cartilage-forming cells in the perichondrium secrete new matrix on the external surface of existing cartilage.

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

Cartilage growth is most active during childhood and adolescence but can occur in adulthood during repair.

Bone Structure and Development

Functions of Bone

Bones serve several vital functions:

• Support: Framework for the body and soft organs.

• Protection: Shields vital organs (e.g., skull protects the brain).

• Movement: Muscles attach to bones, enabling movement.

• Mineral Storage: Reservoir for calcium and phosphorus.

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

• Triglyceride Storage: Fat stored in yellow marrow.

Classification of Bones

Bones are classified by shape:

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

• Short Bones: Cube-shaped (e.g., carpals, tarsals).

• Flat Bones: Thin, flat, slightly curved (e.g., sternum, skull bones).

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

• Sesamoid Bones: Form within tendons (e.g., patella).

Bone Tissues

Bones are organs containing several tissue types:

• Osseous (bone) tissue predominates.

• Also contains nervous tissue, cartilage, muscle cells, and epithelial cells in blood vessels.

Compact vs. Spongy Bone

• Compact Bone: Dense outer layer; appears smooth and solid.

• Spongy Bone (Cancellous Bone): Honeycomb of small, needle-like or flat pieces called trabeculae; spaces filled with red or yellow bone marrow.

Structure of Long Bones

• Diaphysis: Shaft; composed of compact bone surrounding the medullary cavity (contains yellow marrow in adults).

• Epiphyses: Ends of long bones; compact bone externally, spongy bone internally. Articular cartilage covers joint surfaces.

• Epiphyseal Plate: Region of growing bone in children; site of lengthwise growth.

• Membranes: Periosteum (outer covering except joints) and Endosteum (lines internal bone surfaces).

Bone Cells

Types of Bone Cells

• Osteoprogenitor Cells: Stem cells in periosteum and endosteum; differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells; secrete unmineralized bone matrix (osteoid) made of collagen and calcium-binding proteins.

• Osteocytes: Mature bone cells in lacunae; maintain bone matrix and act as stress sensors.

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

• Osteoclasts: Multinucleated cells; break down bone matrix during bone resorption.

Microscopic Anatomy of Bone

Compact Bone Structure

• Osteon (Haversian System): Structural unit of compact bone; consists of concentric lamellae around a central canal.

• Central (Haversian) Canal: Contains blood vessels and nerve fibers.

• Perforating (Volkmann's) Canals: Connect blood vessels and nerves of periosteum to central canal.

• Lacunae: Small cavities containing osteocytes.

• Canaliculi: Tiny canals connecting lacunae; allow communication and nutrient exchange.

Spongy Bone Structure

• Trabeculae align along lines of stress; contain lamellae and osteocytes connected by canaliculi.

• Spaces between trabeculae are filled with bone marrow.

Bone Development (Ossification)

Overview

Ossification (osteogenesis) is the process of bone tissue formation. Begins in the second month of embryonic development and continues into early adulthood.

• Up to 8 weeks, the skeleton is fibrous membranes and hyaline cartilage.

• Two types of ossification:

  • Endochondral Ossification: Bone forms by replacing hyaline cartilage; forms most bones of the skeleton.

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

Endochondral Ossification Steps

1. Bone collar forms around hyaline cartilage model.

2. Cartilage in the center calcifies and develops cavities.

3. Primary ossification center forms in the diaphysis.

4. Secondary ossification centers form in the epiphyses.

5. Cartilage remains at articular surfaces and epiphyseal plates.

Intramembranous Ossification Steps

1. Ossification centers appear in fibrous connective tissue membrane.

2. Osteoid is secreted and calcifies.

3. Woven bone and periosteum form.

4. Compact bone replaces woven bone; red marrow appears.

Growth and Remodeling of Bones

Longitudinal Bone Growth

Occurs at the epiphyseal plate and involves five zones:

• Resting (quiescent) zone

• Proliferation (growth) zone

• Hypertrophic zone

• Calcification zone

• Ossification (osteogenic) zone

Growth continues until the epiphyseal plate closes (around age 18-21).

Bone Remodeling

• Bone is continuously remodeled by resorption (osteoclasts) and deposition (osteoblasts).

• Maintains bone strength and mineral homeostasis.

Summary Table: Types of Cartilage

Key Terms and Definitions

• Chondrocyte: Cartilage cell found in lacunae.

• Osteoblast: Bone-forming cell.

• Osteocyte: Mature bone cell maintaining bone matrix.

• Osteoclast: Bone-resorbing cell.

• Perichondrium: Dense connective tissue surrounding cartilage.

• Epiphyseal Plate: Growth plate in long bones.

• Ossification: Process of bone formation.

Relevant Equations

• Bone growth rate (simplified):

• Density of bone:

Example Application

Damage to the epiphyseal plate in children can result in stunted bone growth, highlighting the importance of cartilage and bone development in overall skeletal health.

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