Classification of Tissues

Overview of Tissues in Anatomy & Physiology

Tissues are groups of cells that work together to perform specific functions and maintain homeostasis in the body. The study of tissues is called histology, which is essential for understanding organ structure and function.

• Tissues: Groups of cells similar in structure and function.

• Organs: Composed of multiple tissue types working together.

• Four Primary Tissue Types:

  1. Epithelial Tissue

  2. Connective Tissue

  3. Nervous Tissue

  4. Muscle Tissue

Epithelial Tissue

Definition and Functions

Epithelial tissue (epithelium; plural: epithelia) consists of sheets of cells that cover body surfaces or line body cavities. It serves as a protective barrier and is involved in absorption, secretion, filtration, and sensory reception.

• Covering & Lining Epithelium: Forms the outer layer of skin and lines body cavities that open to the outside; covers walls and organs of closed ventral body cavities.

• Glandular Epithelium: Specialized for secretion.

Major Functions of Epithelial Tissue

• Protection: Shields underlying tissues from bacterial invasion and chemical damage.

• Absorption: Uptake of substances, e.g., nutrients in the stomach and small intestine.

• Filtration/Excretion/Secretion:

  • Filtration in kidney tubules

  • Excretion and secretion by glands

• Sensory Reception: Detects changes in the environment.

Special Characteristics of Epithelial Tissue

• Polarity: Membranes always have one free (apical) surface.

• Specialized Contacts: Cells fit closely together to form continuous sheets, bound by specialized junctions.

• Supported by Connective Tissue: Attached to and supported by a basement membrane (composed of basal lamina and reticular lamina).

• Avascular but Innervated: No blood vessels; nutrients diffuse from underlying connective tissue. Supplied by nerves.

• Regeneration: High capacity for renewal, especially when well-nourished and subjected to friction.

Classification of Epithelial Tissue

Arrangement and Number of Layers

Epithelial tissues are classified based on the number of cell layers and the shape of the cells at the apical surface.

• Simple Epithelium:

  • Single layer of cells attached to the basement membrane.

• Stratified Epithelium:

  • Two or more layers of cells.

  • Named according to the shape of cells at the apical surface.

Cell Shape Categories

• Squamous: Scale-like, flat cells.

• Cuboidal: Cube-shaped cells.

• Columnar: Column-shaped, tall cells.

Special Types of Epithelium

• Pseudostratified Epithelium:

  • Appears stratified but is actually a single layer of cells with varying heights and nuclei at different levels.

  • Often ciliated (e.g., lining of the trachea).

• Transitional Epithelium:

  • Stratified epithelium formed of rounded, plump cells.

  • Cells can slide over one another, allowing stretching (e.g., urinary bladder).

  • Cells are rounded when empty, flat when full.

Summary Table: Epithelial Tissue Classification

Example: Simple Squamous Epithelium

• Found in alveoli of lungs and lining of blood vessels.

• Allows for rapid diffusion of gases and nutrients.

Example: Transitional Epithelium

• Lines the urinary bladder.

• Permits expansion and recoil after stretching.

Additional info: The images provided in the slides illustrate the microscopic appearance and arrangement of epithelial cells, which is essential for histological identification in laboratory settings.