Tissues

Definition and Overview

Tissues are groups of cells that are similar in structure and perform a common or related function. In the human body, tissues are fundamental organizational units that contribute to the formation of organs and organ systems.

• Four Basic Types of Tissues:

  • Epithelial

  • Connective

  • Muscle

  • Nervous

• Example: Nervous tissue is found in the brain, spinal cord, and nerves; epithelial tissue lines the surface of organs and the skin.

Epithelial Tissue

Characteristics and Functions

Epithelial tissue consists of closely packed cells that line body surfaces and cavities. It serves as a protective barrier and is involved in absorption, secretion, and sensation.

• Polarity: Epithelial cells have an apical (top) surface exposed to the outside or to the cavity of an internal organ, and a basal (bottom) surface attached to underlying connective tissue.

• Innervation: Epithelial tissues are supplied by nerves.

• Attachment: The basal surface is attached to a basement membrane, which supports and anchors the epithelium.

• Specializations: Microvilli are fingerlike extensions of the cell membrane that increase surface area for absorption.

• Example: The lining of the small intestine has microvilli to maximize nutrient absorption.

Classification of Epithelial Tissue

Criteria for Classification

Epithelial tissues are classified based on two criteria: the number of cell layers and the shape of the cells.

• Number of Layers:

  • Simple epithelium: Single layer of cells

  • Stratified epithelium: Two or more layers of cells

• Cell Shape:

  • Squamous: Flattened, scale-like cells

  • Cuboidal: Box-like cells

  • Columnar: Tall, column-shaped cells

• Example: Simple squamous epithelium consists of a single layer of flat cells, while stratified squamous epithelium has multiple layers of flat cells.

Simple Squamous Epithelium

Structure and Function

Simple squamous epithelium is composed of a single layer of flattened cells. This structure allows for efficient diffusion and filtration of materials.

• Main Function: Permits the passage of materials by diffusion and filtration in sites where protection is not important.

• Location Example: Found in the air sacs (alveoli) of the lungs, where gas exchange occurs.

Simple Cuboidal Epithelium

Structure and Function

Simple cuboidal epithelium consists of a single layer of cube-shaped cells. These cells are specialized for secretion and absorption.

• Main Function: Secretion and absorption of substances.

• Location Example: Found in the kidney tubules and in the ducts and secretory portions of small glands.

Summary Table: Epithelial Tissue Classification

Additional info: Epithelial tissues are avascular (lack blood vessels) but are innervated (supplied by nerve fibers). They regenerate rapidly, which is important for tissues exposed to friction and environmental damage.