Epithelium, also known as epithelial tissue, is a fundamental type of tissue found throughout the body. It forms continuous layers that cover all external and internal surfaces, lining body cavities, hollow organs, and ducts. This tissue is a primary component of glands, specialized structures involved in secretion. Epithelial cells are tightly packed, forming a cohesive barrier with minimal space between them. Its widespread distribution and diverse forms contribute to the body’s overall function and integrity, as every substance entering or leaving an organ must first pass through it.
Epithelium as a Protective Barrier
Epithelial tissue functions as a primary protective barrier throughout the body. The outermost layer of the skin, the epidermis, safeguards against physical damage, invading microorganisms, UV radiation, and excessive water loss. This barrier is composed of multiple layers of tightly packed epithelial cells, defending against external threats.
Internal body surfaces also rely on epithelial protection. The lining of the respiratory tract, for instance, is covered by ciliated pseudostratified columnar epithelium. These cells have cilia that move mucus and trapped particles, such as dust, pollen, and pathogens, out of the airways. This “mucociliary escalator” system prevents harmful substances from reaching the lungs, maintaining respiratory health.
The epithelial lining of the digestive tract forms a barrier against ingested toxins and microorganisms, while selectively allowing nutrient absorption. Tight junctions contribute to this barrier function across various epithelial tissues. These protein complexes seal adjacent epithelial cells, preventing substances from passing between them. Instead, substances must pass through the cells, where their passage is regulated. This control ensures only beneficial substances cross into the body while harmful ones are excluded.
Epithelium in Secretion and Absorption
Epithelial tissue plays a central role in both secretion and absorption of various substances throughout the body. Many epithelial cells are specialized to form glands that produce and release specific compounds. These glands can be exocrine, secreting substances like sweat, saliva, or digestive enzymes onto a surface or into a duct, or endocrine, releasing hormones directly into the bloodstream to regulate distant bodily functions.
For absorption, epithelial cells are adapted to absorb nutrients and other molecules. The lining of the small intestine, for example, has specialized epithelial cells that absorb digested food particles into the bloodstream. These cells have microvilli, which increase the surface area for absorption, sometimes by as much as 40 times.
Beyond the digestive system, epithelial tissue is active in absorption within the kidneys and lungs. In the kidneys, epithelial cells lining the renal tubules reabsorb essential substances like water, glucose, and electrolytes back into the blood, preventing their loss in urine. In the lungs, the thin epithelial lining of the alveoli facilitates oxygen absorption and carbon dioxide release.
Epithelium in Filtration and Sensation
Epithelial tissue performs specialized functions in filtration and sensation. Filtration occurs in the kidneys, specifically within the glomeruli. Here, specialized epithelial cells called podocytes form a filtration barrier alongside the glomerular basement membrane and endothelial cells.
Podocytes have foot processes that create narrow filtration slits. These slits allow water, ions, and small molecules from the blood to pass through, forming a preliminary filtrate. They block larger molecules like proteins and blood cells from entering the urine. This selective filtration maintains the body’s fluid balance and removes waste products.
Beyond filtration, epithelial cells are integral to sensory perception. In the tongue, taste buds contain specialized epithelial cells that detect chemical molecules in food, allowing us to perceive sweet, sour, salty, bitter, and umami tastes. These cells translate chemical stimuli into signals the nervous system can interpret.
The inner ear relies on specialized epithelial cells, hair cells, for hearing and balance. These hair cells convert mechanical vibrations from sound waves or head movements into electrical signals transmitted to the brain. In the eye, photoreceptor cells in the retina are specialized cells that detect light, converting it into visual signals. The olfactory epithelium in the nasal cavity contains receptor cells with cilia that bind to odor molecules, allowing for the sense of smell.
The Role of Epithelium in Repair and Regeneration
Epithelial tissue has a strong capacity for repair and regeneration, important for maintaining the body’s integrity. This regenerative ability is due to stem cells within epithelial layers that continuously divide and differentiate to replace damaged or worn-out cells.
Skin wound healing demonstrates this regenerative power. When the skin is injured, epithelial cells at the wound edges, along with stem cells in the epidermis and hair follicles, become activated. They proliferate and migrate to cover the exposed area, re-establishing the protective barrier. This process, known as re-epithelialization, restores the skin’s surface.
The lining of the digestive tract also demonstrates rapid and continuous renewal. The intestinal epithelium, for instance, replaces its entire lining approximately every five to seven days. This constant turnover allows the digestive system to withstand the abrasive environment of food breakdown and nutrient absorption, maintaining its functionality.
The epithelial cells lining the respiratory passages undergo continuous replacement. Despite constant exposure to inhaled irritants and pathogens, the respiratory epithelium can repair itself by producing new cells from basal stem cells. This ongoing regeneration across various epithelial tissues preserves barrier function, facilitates physiological processes, and maintains bodily health.