The Tissue Level of Organization: Understanding Epithelial Tissue

Polarity and Physiological Properties

Epithelial cells are not symmetrical; they exhibit polarity. This means they have two distinct sides:

• Apical Surface: The "top" side that faces the exterior of the body or the hollow space (lumen) of an internal organ.
• Basal Surface: The "bottom" side that is attached to the underlying connective tissue.

Furthermore, epithelial tissue is avascular, meaning it contains no blood vessels of its own. Because it lacks its own blood supply, it must rely on diffusion from the nutrient-rich connective tissues located beneath its basal surface. However, it is innervated, meaning it is supplied directly by nerve fibers to allow for sensation. Finally, because these tissues are often exposed to friction or environmental damage, they are highly regenerative, possessing a high capacity for repair through rapid cell division via mitosis.

Classification of Epithelia

Epithelial tissue functions in two primary ways: as a covering/lining for surfaces or as glandular tissue for secretion. To identify a specific type of epithelium, scientists look at two main criteria: the number of cell layers and the shape of the cells.

Simple Epithelia (Single Layer)

Simple epithelia consist of only one layer of cells, making them ideal for processes like absorption or filtration where substances need to pass through quickly.

• Simple Squamous: Extremely thin, flat cells. These are typically found in open spaces where rapid diffusion is required.
• Simple Cuboidal: Cube-shaped cells often found lining the inner spaces of tubules.
• Simple Columnar: Tall, column-like cells that line various inner open spaces.

Stratified and Specialized Epithelia

When a tissue needs more protection, it utilizes multiple layers.

• Stratified Squamous: Multiple layers of cells that provide a robust protective barrier.
• Pseudostratified Columnar: This is a "trick" tissue. It appears to be layered because the nuclei are at different levels, but it is actually a single layer of cells of varying heights.
• Transitional: A specialized form designed for flexibility, allowing tissues to stretch and distend (such as in the bladder).

Glandular Epithelium

Not all epithelium serves as a barrier; some specialize in making and secreting fluids. These are known as glands. Glands are classified into two categories:

1. Endocrine Glands: These are ductless glands that secrete hormones (such as amino acids, proteins, and steroids) directly into the bloodstream.
2. Exocrine Glands: These are more numerous and secrete their products onto body surfaces (like sweat) or into body tubes/cavities (like saliva or mucus) via specialized ducts.

Exam Alert: If a question asks about a gland that uses a duct to release oil onto the skin, it is referring to an exocrine gland. If it mentions a hormone entering the blood without a duct, it is endocrine.

Summary Table of Epithelial Types