Epithelial tissue, or epithelium, is one of the four primary tissue types, alongside connective, muscle, and nervous tissue. This tissue functions as a continuous sheet of cells, creating a selective boundary between different environments within the body. It forms the outer layer of the skin, lines internal pathways like the digestive and respiratory tracts, and covers the surface of organs. Epithelium is also the primary component of most glandular tissue. All substances that enter or leave the body must first cross a layer of epithelial tissue.
Defining Structural Characteristics
Epithelial tissue is defined by several structural characteristics. It exhibits high cellularity, meaning it is composed almost entirely of tightly packed cells with very little extracellular material between them. These cells are securely joined by specialized intercellular connections, such as tight junctions and anchoring junctions, which establish a robust, continuous barrier.
Every epithelial cell exhibits polarity, having two distinctly different surfaces: an apical surface and a basal surface. The apical surface is the exposed side, facing the exterior environment or an internal body cavity. The basal surface is attached to the underlying connective tissue via a non-cellular layer called the basement membrane.
Epithelium is avascular, meaning it contains no blood vessels of its own. Cells receive oxygen and nutrients through diffusion from blood vessels located in the underlying connective tissue. The basement membrane facilitates this exchange. This structure allows the tissue to perform its barrier function, but necessitates a high capacity for cell regeneration to replace damaged cells rapidly.
Classification Based on Shape and Layers
Epithelium is classified based on two main criteria: the number of cell layers and the shape of the cells on the apical surface. Simple epithelium has a single layer of cells, typically found where absorption, secretion, or filtration occur. Stratified epithelium consists of multiple layers stacked upon one another, providing greater durability.
Cell shape describes the appearance of the cells closest to the free surface.
Cell Shapes
- Squamous cells are thin, flattened, and scale-like, facilitating the rapid passage of substances. Simple squamous epithelium lines the air sacs of the lungs, where quick gas exchange occurs.
- Cuboidal cells are boxy, with roughly equal height and width, and are frequently involved in secretion and absorption. Simple cuboidal epithelium is common in glandular tissues and kidney tubules.
- Columnar cells are tall and column-shaped. These cells line the stomach and intestines, often featuring surface modifications like microvilli to increase the absorptive area.
Specialized variations also exist. Pseudostratified columnar epithelium appears layered because the nuclei are at different heights, but every cell contacts the basement membrane, making it structurally simple. Transitional epithelium, unique to the urinary system, can change shape from cuboidal to squamous as the bladder or ureters stretch.
Primary Physiological Roles
The arrangement and location of epithelial tissue support several physiological tasks.
Protection
Epithelium forms a physical barrier against abrasion, chemicals, and microbial invasion. The stratified squamous epithelium of the epidermis, the outer layer of the skin, provides a robust, multi-layered shield.
Absorption
This process involves taking up substances from the external environment into the body. The simple columnar epithelium lining the small intestine is specialized for this, using apical microvilli to increase the surface area for nutrient uptake into the bloodstream. This selective barrier regulates which materials pass into deeper tissues.
Secretion
Epithelial cells are the foundation of all glands. Glandular epithelium forms exocrine glands, which release products like sweat or digestive enzymes onto a surface via ducts. It also forms endocrine glands, which secrete hormones directly into the bloodstream.
Filtration and Excretion
Epithelium plays a significant role in filtration and excretion, particularly within the urinary system. Simple squamous epithelium forms the thin walls of kidney glomeruli, allowing for the passive filtration of blood plasma to begin urine formation. Specialized cuboidal cells lining the kidney tubules actively participate in the excretion of waste products and the reabsorption of necessary substances.