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 main tissue types:

  • Epithelial

  • Connective

  • Muscle

  • Nervous

• Example: Nervous tissue forms the brain, spinal cord, and nerves; epithelial tissue lines organs and skin surfaces.

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 surface (exposed to the exterior or cavity) and a basal surface (attached to underlying connective tissue).

• Innervation: Epithelial tissues are innervated (supplied by nerves).

• Microvilli: Fingerlike extensions of the cell membrane that increase surface area for absorption.

• Basement membrane: A thin layer that anchors epithelial cells to underlying connective tissue.

• Example: The lining of the digestive tract and skin surface (epidermis).

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 is a single layer of flat cells; stratified cuboidal epithelium has multiple layers of cube-shaped cells.

Simple Squamous Epithelium

Structure and Function

Simple squamous epithelium consists of a single layer of flattened cells. Its thinness allows for efficient exchange of materials by diffusion and filtration.

• Key Points:

  • Facilitates rapid diffusion of gases and small molecules.

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

• Example: Gas exchange in the alveoli of the lungs.

Simple Cuboidal Epithelium

Structure and Function

Simple cuboidal epithelium is composed of a single layer of cube-shaped cells. It is specialized for secretion and absorption.

• Key Points:

  • Provides a protective barrier and facilitates absorption and secretion.

  • Located in kidney tubules, ducts and secretory portions of small glands, and ovary surface.

• Example: Kidney tubules filter blood and reabsorb water and nutrients using simple cuboidal epithelium.

Summary Table: Epithelial Tissue Types

Additional info: The above table includes inferred locations and functions for completeness.