Epithelial tissue is one of the four basic tissue types, consisting of cells arranged in continuous sheets that cover exterior surfaces or line internal cavities and tubes. This arrangement forms the boundary between the body and the external environment, or between different organ systems. Epithelial sheets are defined by a high density of closely bound cells with very little extracellular material. The cells rest upon a basement membrane, a thin layer that anchors the epithelium to the underlying connective tissue, allowing it to serve as a functional interface.
Mechanical and Biological Protection
The epithelium’s most apparent function is its role as a physical barrier against external threats and mechanical stress. The skin, covered by stratified squamous epithelium, provides robust defense against abrasion and physical trauma. Multiple layers of cells are stacked, ensuring that damage to the superficial layer does not compromise deeper tissues.
The epithelium also provides biological protection, acting as the first line of defense against invading pathogens. This barrier is achieved through specialized tight junctions, which are protein complexes that seal the space between adjacent cells. These junctions prevent the movement of bacteria and toxins between cells, forcing substances to pass through the cells.
The epithelial barrier also regulates internal body hydration. The epidermis contains keratin, a tough, fibrous protein that makes the outer layer relatively waterproof. This keratinized layer prevents excessive water loss through evaporation, maintaining the body’s internal fluid balance. The integrity of this protective sheet is maintained by constant cell turnover in areas subject to high wear, such as the skin and mouth lining.
Selective Absorption and Filtration
Many epithelial tissues perform the metabolically active function of controlled substance movement. This process includes absorption, which takes materials into the body, and filtration, which selectively removes materials from the blood plasma. This complex physiological role often requires significant energy expenditure.
In the small intestine, simple columnar epithelium is specialized for absorbing digested nutrients. The apical surface of these cells is covered with microvilli, forming a brush border that dramatically increases surface area. This structure allows for the efficient uptake of molecules like glucose and amino acids into the bloodstream.
The movement of these substances often requires energy, utilizing mechanisms like active transport. Specific carrier proteins use adenosine triphosphate (ATP) to move molecules, sometimes against their concentration gradient.
The epithelium lining the kidney tubules performs filtration and reabsorption of blood plasma. The initial epithelial barrier filters fluid, water, and small solutes into the tubule space, creating filtrate. This filtrate contains both waste products and valuable substances that must be recovered.
Subsequent epithelial segments, such as the proximal convoluted tubule, are responsible for recovering filtered water, glucose, and beneficial ions. The cells here possess a dense concentration of mitochondria to power the pumps and channels required for this massive reabsorption effort. This precise cellular work ensures that waste products like urea are concentrated and excreted, while essential molecules are retained to maintain physiological homeostasis.
Secretion of Specialized Compounds
Epithelial cells are specialized to synthesize and release specific chemical compounds, a function known as secretion. This involves the internal manufacturing and packaging of the product within the cell’s organelles, distinguishing it from simple material transport.
Glandular epithelium forms the basis of all glands, categorized as exocrine or endocrine. Exocrine glands, such as salivary and sweat glands, secrete products onto an external surface or into a duct system. These cells often use merocrine secretion, releasing contents via exocytosis.
A common example is the secretion of mucus by goblet cells lining the respiratory and digestive tracts. Mucus production lubricates surfaces and traps particulate matter, providing a protective layer that is continuously moved and replaced.
Endocrine glands, such as the thyroid and pituitary glands, lack ducts and release hormones directly into the bloodstream. These hormones act as chemical messengers, traveling to distant target cells to regulate physiological processes like metabolism and growth.
Specialized Sensory Reception
In specific locations, epithelial cells are modified to detect changes in the external or internal environment. These sensory cells are often called neuroepithelium because they transduce a physical or chemical stimulus into an electrical signal for nervous system interpretation.
This specialized function is seen in several areas. Hair cells in the inner ear detect sound vibrations and gravity. Taste receptor cells within the taste buds respond to dissolved chemicals. Sensory neurons in the olfactory epithelium detect airborne chemical compounds.