Skeletal System I: Bone Tissue – Structure, Function, and Classification

Skeletal System I: Bone Tissue

Introduction

The skeletal system is a fundamental organ system in the human body, providing structure, protection, and support. It is composed of bones and cartilage, each with distinct properties and functions. This guide covers the differences between bone and cartilage, the types and features of bone, and the classification of bones.

Learning Objectives

• Understand the differences between bone and cartilage.

• Identify the various features, types, and cells of bone.

• Describe how bone forms and develops.

• Explain how bone repairs itself.

Level of Organization

Hierarchical Structure

The skeletal system is organized from the molecular level (minerals and proteins) up to the organ system level (bones and joints). This hierarchical organization allows for complex functions such as movement, support, and mineral storage.

The Skeletal System: Overview

General Features

• Bones have a long evolutionary history and are commonly found as fossils due to their mineralized nature.

• The skeletal system consists of both bones and cartilage.

Functions of the Skeletal System

• Support: Provides structural framework for the body.

• Protection: Shields vital organs (e.g., brain, heart).

• Movement: Facilitates movement by serving as attachment points for muscles.

• Mineral Storage and Release: Stores minerals such as calcium and phosphorus, releasing them as needed.

• Blood Cell Production: Produces blood cells in bone marrow.

Cartilage

Definition and Properties

• Cartilage is a semi-rigid connective tissue composed of cells (chondrocytes) and an extracellular matrix of fibers and ground substance.

• It provides support, flexibility, and cushioning in joints.

Chondrocytes

• Chondrocytes are cells embedded within the cartilage matrix, residing in spaces called lacunae.

• They synthesize and maintain the cartilage matrix.

• Chondrocytes originate from chondroblasts, which produce cartilage.

Types of Cartilage

There are three main types of cartilage, each with unique properties and locations in the body.

• Hyaline Cartilage: Smooth, glassy cartilage that is both strong and flexible. Found in the nose, articular surfaces of joints, and ends of ribs. Forms the temporary skeleton of the embryo.

• Fibrocartilage: Contains bundles of collagen fibers, providing high tensile strength. Found in intervertebral discs and pubic symphysis.

• Elastic Cartilage: Contains elastic fibers and collagen, allowing for flexibility and support. Found in the external ear, epiglottis, and parts of the larynx.

Bone

Definition and Functions

• Bones are rigid organs that form the skeleton, providing support, protection, and enabling movement.

• Bones also serve as a reservoir for minerals and as a site for blood cell production.

Mineral Storage

• Bones store minerals such as calcium and phosphorus, which can be released to maintain blood levels and physiological processes.

• Calcium is essential for muscle contraction, nerve impulse transmission, and blood clotting.

Bone Marrow

• The soft connective tissue inside bones is called bone marrow.

• There are two types: yellow marrow (stores fat) and red marrow (produces blood cells).

• Red marrow is the site of hematopoiesis—the production of red blood cells (RBCs), white blood cells (WBCs), and platelets.

Classifications of Bones

Overview

There are over 206 bones in the adult skeleton, classified based on their shapes and functions.

Types of Bones

• Long Bones: Longer than they are wide; found in arms and legs (e.g., femur, humerus). Important for movement and support.

• Short Bones: Cube-like in shape; found in wrists and ankles (e.g., carpals, tarsals). Provide stability and support.

• Flat Bones: Thin and broad; found in the skull, ribs, and sternum. Protect internal organs and provide surfaces for muscle attachment.

• Irregular Bones: Complex shapes that do not fit into other categories; found in vertebrae and facial bones.

• Sesamoid Bones: Small, round bones embedded within tendons; the patella is the largest example in humans.

Classification Table

The following table summarizes the main bone classifications, their shapes, and examples.

Bone Type	Shape	Examples	Function
Long	Longer than wide	Femur, humerus, tibia	Movement, support
Short	Cuboid	Carpals, tarsals	Stability, support
Flat	Thin, broad	Skull, ribs, sternum	Protection, muscle attachment
Irregular	Complex	Vertebrae, facial bones	Protection, support
Sesamoid	Small, round	Patella	Reduce friction, modify pressure

Key Terms and Concepts

• Chondrocyte: Cartilage cell that maintains the matrix.

• Lacuna: Small cavity within cartilage or bone where cells reside.

• Osteocyte: Mature bone cell.

• Hematopoiesis: Production of blood cells in bone marrow.

• Articular Surface: Joint surface covered by cartilage.

Summary Table: Cartilage Types

Type	Main Features	Location	Function
Hyaline	Strong, flexible, smooth matrix	Nose, joints, ends of ribs, embryonic skeleton	Support, flexibility, smooth movement
Fibrocartilage	Dense collagen bundles, tough	Intervertebral discs, pubic symphysis	Shock absorption, tensile strength
Elastic	Elastic fibers, flexible	External ear, epiglottis, larynx	Flexibility, support

Important Equations

• Calcium Homeostasis Equation:

$Ca^{2+}_{blood} = Ca^{2+}_{released~from~bone} - Ca^{2+}_{deposited~in~bone}$

*Additional info: The above equation represents the balance of calcium in the blood, regulated by bone resorption and deposition processes.*

Examples and Applications

• Example: The patella (kneecap) is a sesamoid bone that improves the leverage of the quadriceps muscle during knee extension.

• Application: Understanding bone classification helps in diagnosing fractures and bone diseases.