Types and Functions of Epithelial Tissue

These cells often feature specialized structures to enhance their function:

• Microvilli: Small projections that increase surface area for better absorption.
• Goblet cells: Specialized cells that secrete mucus to lubricate and protect the lining.

Depending on their location, these cells may be ciliated (possessing hair-like projections to move substances, such as in the uterine tubes) or non-ciliated (found in the stomach and intestines).

Pseudostratified Columnar Epithelium

The term "pseudostratified" means "falsely layered". While these cells appear to be in multiple layers because their nuclei sit at different levels, they are actually a single layer because every cell makes contact with the underlying basement membrane. This tissue is most commonly found in the respiratory tract, where it often contains both cilia and goblet cells to trap and move mucus.

Study Tip: If a question asks about a tissue that looks layered but is actually a single layer, the answer is almost always pseudostratified epithelium.

Stratified Epithelia: Multi-Layered Protection

When a tissue consists of two or more layers of cells, it is called "stratified". The primary function of stratified epithelium is protection against mechanical or chemical injury.

Stratified Squamous Epithelium

This is one of the most common stratified tissues. It is named based on the shape of the cells at the outermost (apical) surface. It exists in two forms:

1. Keratinized: Found in the epidermis (the outer layer of skin), where the surface cells are filled with a tough protein called keratin to prevent water loss and provide protection.
2. Non-keratinized: Found in moist areas that require protection but do not need waterproofing, such as the oral cavity, esophagus, vagina, and anal canal.

Stratified Cuboidal and Columnar Epithelia

These are much rarer types of tissue:

• Stratified cuboidal epithelium is typically only two to three layers thick and is found in the lining of the lumens of certain glands, such as sweat, mammary, and salivary glands.
• Stratified columnar epithelium has a limited distribution in the body, appearing in small amounts in the male urethra and some glandular ducts. It features columnar cells on the surface and cuboidal cells in the basal (bottom) layers.

Transitional Epithelium

Transitional epithelium is a unique type of stratified tissue composed of irregular, multi-layered cells. Its defining characteristic is that it is highly expandable; the cells can change shape to allow the tissue to stretch and then recoil to its original size. This makes it essential for lining the urinary bladder, ureters, and urethra.

Glandular Epithelium and Secretion

Glands are organs composed of epithelial tissue that specialize in the secretion of aqueous fluids. Glands are categorized in two ways: by where they release their products and by how they release them.

Classification by Release Site

• Endocrine Glands: These glands secrete their products (hormones) directly into the bloodstream or surrounding interstitial fluid.
• Exocrine Glands: These glands release their secretions through ducts onto an epithelial surface (such as the skin or the lining of the digestive tract).

Classification by Secretory Method

The method by which a cell releases its product is vital for understanding tissue health and function:

1. Merocrine: The most common method, where the cell "merely secretes" its product via exocytosis without any damage to the cell itself.
2. Apocrine: A method where a portion of the cell's apical surface is pinched off along with the secretory product.
3. Holocrine: A more drastic method where the cell "dies for its cause." The entire cell ruptures and dies to release its contents (e.g., sebaceous glands).

Summary Table of Epithelial Types