GAP Ch4: Tissues

Introduction to Tissues

The term tissue refers to a group of cells that share a similar structure and work collectively as a unit. Tissues are part of the biological hierarchy of organization, which begins with atoms and molecules, leading to cells, and finally to tissues.

Types of Tissues

In the human body, there are four primary types of tissues: epithelial, connective, muscle, and nervous. The focus of this chapter will be on epithelial and connective tissues, while muscle and nervous tissues will be examined in more detail in future chapters.

Epithelial Tissue

Epithelial tissue, or epithelium, consists of sheets of cells that cover exterior surfaces, line internal cavities and passageways, and form glands. Examples include the epidermis (skin) and the lining of organs such as the digestive tract. Epithelial tissues can be characterized by four major traits:

• Protection: Serve as the first line of defense against physical and chemical wear.

• Cellularity: Composed primarily of closely packed cells with minimal extracellular material.

• Glandular Capacity: Many epithelial cells can secrete substances like mucus through specialized structures called glands.

• Avascularity: Epithelial tissues lack blood vessels; nutrients diffuse from deeper tissues.

Cell Shapes and Layers

Epithelial cells are classified by shape:

• Squamous: Flat, like a fried egg.

• Cuboidal: Cube-shaped.

• Columnar: Tall and column-like.

The number of layers is another classification criteria:

• Simple: One layer—ideal for diffusion and absorption functions, such as in lung alveoli.

• Stratified: Multiple layers—designed for protection, found in skin epidermis.

• Pseudostratified: Appears multilayered but is a single layer attached to the basement membrane.

• Transitional: Can change shape (e.g., bladder cells).

Glandular Epithelial Tissue

Glands are specialized epithelial cells that synthesize and secrete substances.

• Exocrine glands: Release substances through ducts to external surfaces (e.g., goblet cells producing mucus).

• Endocrine glands: Release hormones into surrounding fluids and tissues for circulation.

Connective Tissue

Connective tissue is diverse, characterized by its scattered cells in an extensive extracellular matrix, and serves various functions:

1. Support and Connection: Connects organs and tissues, crucial for structural integrity.

2. Protection: Forms a fibrous capsule around organs.

3. Transport: Blood acts as a transport medium for nutrients and waste.

4. Energy Storage: Adipose tissue stores energy and provides insulation.

Types of Connective Tissue

Connective tissue can be categorized into three main groups based on their matrices:

• Connective Tissue Proper: Contains loose and dense tissues. Loose connective tissue provides flexibility and support, while dense connective tissue offers strength.

• Supportive Connective Tissue: Includes cartilage and bone, which offer structural support.

• Fluid Connective Tissue: Blood and lymph circulate within a fluid matrix.

Connective Tissue Proper

Fibroblasts produce three fiber types in the extracellular matrix:

• Collagen fibers: Provide tensile strength.

• Elastic fibers: Allow stretch and recoil.

• Reticular fibers: Form a supportive mesh framework.

Adipose tissue stores fat, while loose connective tissue connects various structures. Dense connective tissue is stronger, with dense regular types (e.g., ligaments) and dense irregular types (e.g., dermis of the skin).

Supportive Connective Tissue

Cartilage and bone provide rigidity and strength. Each type of cartilage (hyaline, fibrocartilage, elastic) has specific functions and locations in the body. For instance:

• Hyaline cartilage: Found in joints and respiratory structures.

• Fibrocartilage: Provides support in intervertebral discs.

• Elastic cartilage: Offers flexibility in structures like the external ear.

Bone is the rigid connective tissue characterized by a matrix rich in calcium phosphate, offering strength and serving the skeletal system's functions.

Muscle Tissue

Muscle tissue is responsible for movement and is categorized into three types:

• Skeletal Muscle: Voluntary and striated, controlled by conscious effort.

• Cardiac Muscle: Involuntary and striated, found only in the heart.

• Smooth Muscle: Involuntary and non-striated, located in walls of organs and vessels.

Nervous Tissue

Nervous tissue transmits impulses throughout the body and consists of:

• Neurons: Excitable cells that conduct impulses.

• Neuroglial Cells: Support neurons by maintaining homeostasis, forming myelin, and providing protection.

Conclusion

Understanding these different tissues' structure and function is fundamental in the study of anatomy and physiology, forming the foundations for more complex systems and interactions within the body.