Lec3- Introduction to Skeletal Tissue: Structure, Function, and Development of Bone

Chapter 5: The Skeletal System – Introduction to Skeletal Tissue

Learning Objectives

This chapter introduces the skeletal system, focusing on the structure, function, and development of bone tissue. Students will learn to:

• Compare and contrast the functions of different cell types found in bone tissue.

• Describe the processes involved in bone growth and healing.

• Understand how bone is maintained, remodeled, and repaired.

• Identify and classify bones according to shape, location, and function.

Introduction to Skeletal Tissue

Overview of the Skeletal System

The skeletal system is a vital organ system that provides structure, protection, and support for the human body.

• Divisions: Axial skeleton (skull, vertebral column, rib cage) and appendicular skeleton (limbs and girdles).

• Functions: Support, protection, movement, mineral storage, and blood cell production.

• Bone tissue: A dynamic, living tissue that constantly undergoes remodeling.

Functions of the Skeletal System

Major Functions

• Support: Provides a framework for the body.

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

• Movement: Acts as levers for muscles to produce movement.

• Mineral Storage: Reservoir for calcium and phosphorus.

• Blood Cell Production: Hematopoiesis occurs in red bone marrow.

• Energy Storage: Yellow bone marrow stores fat.

Characteristics & Properties of Bone

Bone as a Composite Material

• Composition: Bone is made of organic (collagen fibers) and inorganic (hydroxyapatite crystals) components.

• Strength: Bone is stronger in compression than in tension or shear.

• Dynamic Nature: Bone remodels under the influence of forces (Wolff's Law).

Table: Comparison of Bone Properties

Property	Organic Component	Inorganic Component
Flexibility	High (collagen)	Low
Strength	Moderate	High (hydroxyapatite)
Resistance to Compression	Low	High

The Cells of Mature Bone

Major Bone Cell Types

• Osteoblasts: Bone-forming cells responsible for synthesizing new bone matrix.

• Osteocytes: Mature bone cells that maintain bone tissue and communicate with other cells via canaliculi.

• Osteoclasts: Large, multinucleated cells that resorb bone matrix, aiding in bone remodeling.

• Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts, especially active during bone repair.

Table: Functions of Bone Cells

Cell Type	Function
Osteoblast	Bone formation
Osteocyte	Maintenance of bone tissue
Osteoclast	Bone resorption
Osteoprogenitor	Stem cell for osteoblasts

Structure of Bone

Gross Anatomy of Long Bone

• Diaphysis: Shaft of the bone, composed mainly of compact bone.

• Epiphysis: Ends of the bone, containing spongy bone and red marrow.

• Metaphysis: Region between diaphysis and epiphysis, site of growth plate in children.

• Medullary Cavity: Central cavity containing yellow marrow.

Bone Membranes

• Periosteum: Outer fibrous membrane covering bone, contains blood vessels and nerves, essential for bone growth and repair.

• Endosteum: Inner membrane lining the medullary cavity, involved in bone remodeling.

Organization of Compact and Spongy Bone

Compact Bone

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

• Perforating (Volkmann's) Canals: Connect osteons and allow passage of blood vessels and nerves.

Spongy Bone

• Trabeculae: Lattice-like network of bone tissue, provides strength with minimal weight.

• Medullary Cavity: Contains bone marrow (red or yellow).

Table: Structural Differences Between Compact and Spongy Bone

Feature	Compact Bone	Spongy Bone
Location	Diaphysis, outer layer	Epiphysis, inner layer
Structure	Osteons	Trabeculae
Marrow	Yellow	Red

Bone Development and Growth

Ossification

Ossification is the process of bone tissue formation. There are two main types:

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue (e.g., flat bones of the skull).

• Endochondral Ossification: Bone develops by replacing hyaline cartilage (e.g., long bones).

Key Steps in Ossification

• Osteoprogenitor cells differentiate into osteoblasts.

• Osteoblasts secrete bone matrix.

• Matrix calcifies, trapping osteoblasts, which become osteocytes.

• Osteoclasts resorb bone to shape and remodel.

Formula: Bone Growth Rate

The rate of bone growth can be described by:

$\text{Rate} = k[\text{Osteoblast Activity}] - k'[\text{Osteoclast Activity}]$

where $k$ and $k'$ are proportionality constants.

Summary Table: Types of Ossification

Type	Origin	Example Bones
Intramembranous	Mesenchymal tissue	Flat bones (skull, clavicle)
Endochondral	Hyaline cartilage	Long bones (femur, humerus)

Additional info:

• Bone remodeling is a lifelong process involving the coordinated activity of osteoblasts and osteoclasts.

• Hormones such as parathyroid hormone (PTH) and calcitonin regulate bone metabolism.

• Diseases such as osteoporosis result from imbalances in bone remodeling.