Tissues and Epithelial Tissue: Structure, Classification, and Function

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 units that organize cells to carry out specialized activities.

• 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 body exterior or the cavity of an internal organ, and a basal (bottom) surface attached to underlying connective tissue.

• Innervation: Epithelial tissues are supplied by nerves.

• Microvilli: Fingerlike extensions of the cell membrane that increase surface area for absorption (e.g., in the small intestine).

• Basement Membrane: A thin, fibrous layer that anchors the epithelium to underlying connective tissue.

• Example: The lining of the small intestine contains 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: Single layer of cells

  • Stratified: 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 exchange of materials by diffusion and filtration.

• Main Function: Permits rapid diffusion or filtration of substances.

• Location: Found in the air sacs (alveoli) of the lungs, lining of blood vessels (endothelium), and serous membranes.

• Example: Oxygen and carbon dioxide diffuse across the simple squamous epithelium in the alveoli during gas exchange.

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: Found in kidney tubules, ducts and secretory portions of small glands, and on the surface of the ovary.

• Example: The walls of kidney tubules are lined with simple cuboidal epithelium to facilitate filtration and reabsorption.

Summary Table: Epithelial Tissue Types

Type	Cell Layers	Cell Shape	Main Function	Location
Simple Squamous	1	Flat	Diffusion, filtration	Alveoli, blood vessels
Simple Cuboidal	1	Cube-like	Secretion, absorption	Kidney tubules, glands
Simple Columnar	1	Tall, column	Absorption, secretion	Digestive tract lining
Stratified Squamous	2 or more	Flat (surface)	Protection	Skin, mouth, esophagus
Stratified Cuboidal	2 or more	Cube-like (surface)	Protection	Large ducts of glands
Stratified Columnar	2 or more	Column (surface)	Protection, secretion	Male urethra, some ducts