Tissues

Overview of Tissues

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.

• Definition: A tissue is a group of cells with similar structure and function.

• Main Types of Tissues:

  • Epithelial

  • Connective

  • Muscle

  • Nervous

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

Epithelial Tissue

Characteristics of Epithelial Tissue

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.

• Functions: Protection, absorption, filtration, excretion, secretion, and sensory reception.

• Polarity: Has an apical surface (exposed to outside or cavity) and a basal surface (attached to underlying connective tissue).

• Specialized Structures:

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

  • Basement Membrane: Thin layer anchoring epithelium to connective tissue.

• Innervation: Epithelial tissues are innervated (have nerve supply).

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.

Types of Epithelial Tissue

Simple Squamous Epithelium

This tissue consists of a single layer of flattened cells. It is specialized for rapid passage of materials by diffusion and filtration.

• Key Features:

  • Single layer of flat cells

  • Allows materials to pass through by diffusion

  • 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

This tissue is composed of a single layer of cube-shaped cells. It is primarily involved in secretion and absorption.

• Key Features:

  • Single layer of cube-shaped cells

  • Functions in secretion and absorption

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

• Example: Absorption and secretion in kidney tubules.

Additional info:

• Other epithelial types include simple columnar, pseudostratified columnar, stratified squamous, stratified cuboidal, and transitional epithelium, each with specialized functions and locations in the body.