Epithelial Tissue — Core Principles

Core Principles

Epithelial tissue is a fundamental animal tissue that covers body surfaces, lines internal cavities and organs, and forms glands. Key characteristics include high cellularity with minimal extracellular matrix, distinct polarity (apical, lateral, basal surfaces), avascularity (nutrients via diffusion from underlying connective tissue), and a high regenerative capacity.

All epithelial tissues rest on a basement membrane, which provides support and acts as a selective barrier. Classification is based on the number of cell layers (simple for one layer, stratified for multiple layers) and cell shape (squamous, cuboidal, columnar, transitional).

Simple epithelia are specialized for absorption, secretion, and filtration, found in areas like the lungs and intestines. Stratified epithelia provide protection against abrasion, such as in the skin.

Specialized forms include glandular epithelium for secretion (exocrine and endocrine glands) and neurosensory epithelium for sensory reception. Intercellular junctions like tight junctions, adhering junctions, and gap junctions ensure cell-to-cell adhesion and communication, vital for epithelial integrity and function.

Important Differences

vs Connective Tissue

Aspect	This Topic	Connective Tissue
Cellularity	High cellularity, cells tightly packed.	Low cellularity, cells widely dispersed.
Extracellular Matrix (ECM)	Minimal ECM.	Abundant ECM (fibers and ground substance).
Vascularity	Avascular (no direct blood supply).	Highly vascular (rich blood supply, except cartilage).
Basement Membrane	Always present, anchoring epithelium.	Absent, cells embedded in ECM.
Polarity	Exhibits distinct apical-basal polarity.	Generally lacks polarity.
Primary Functions	Protection, secretion, absorption, filtration, sensory reception.	Support, bind, protect, insulate, store reserve fuel, transport substances.
Regenerative Capacity	High regenerative capacity.	Variable, from high (e.g., loose CT) to low (e.g., cartilage).

Epithelial tissue and connective tissue represent two distinct fundamental tissue types with contrasting structures and functions. Epithelium is characterized by densely packed cells, minimal extracellular matrix, avascularity, and distinct polarity, primarily serving as a covering, lining, and glandular tissue. In contrast, connective tissue features widely spaced cells embedded in an abundant extracellular matrix, is typically vascular, lacks polarity, and primarily functions in support, binding, protection, and transport. The presence of a basement membrane is a defining feature of epithelium, anchoring it to the underlying connective tissue, which itself lacks such a membrane.