Tissues

Definition and Major Types

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 Major Types of Tissues:

  • Epithelial

  • Connective

  • Muscle

  • Nervous

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

Epithelial Tissue

Structure and Characteristics

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.

• Key Features:

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

  • Innervated: Contains nerve fibers.

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

  • Basement Membrane: Thin layer that anchors epithelial cells to connective tissue.

• 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 is found in the air sacs of the lungs; stratified squamous epithelium forms the outer layer of skin.

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 (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 respiration.

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:

  • Cells have large, central nuclei.

  • Commonly found in kidney tubules, ducts of glands, and ovary surface.

  • Functions in secretion (e.g., hormones, enzymes) and absorption (e.g., water, ions).

• Example: Kidney tubules are lined with simple cuboidal epithelium to facilitate filtration and reabsorption.

Summary Table: Epithelial Tissue Types

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