Epithelial tissue is a fundamental tissue type, forming widespread coverings and linings throughout the body. It creates boundaries between different environments, both external and internal. This tissue underlies many physiological processes and provides a protective interface for various organs and systems.
Defining Epithelial Tissue
Epithelial tissue is characterized by its tightly packed cells, forming continuous sheets with minimal intercellular space. These cells exhibit polarity, meaning they have distinct apical (free) and basal (attached) surfaces with differing structures and functions. The basal surface of epithelial cells is anchored to a non-cellular basement membrane, a thin extracellular layer composed of glycoproteins and collagen, which separates the epithelial tissue from underlying connective tissue. This tissue is avascular, meaning it lacks its own direct blood supply; instead, it receives nutrients through diffusion from blood vessels located in the underlying connective tissue. Despite being avascular, epithelial tissue is innervated, possessing a nerve supply that allows for sensory functions and responsiveness. Epithelial tissue has a high regenerative capacity, allowing for rapid replacement of damaged or lost cells, especially in areas prone to wear and tear like the skin or the lining of the gastrointestinal tract.
Key Locations in the Body
Epithelial tissue forms protective layers and specialized linings throughout the body. The outermost layer of the skin, known as the epidermis, is an example of epithelial tissue, providing a barrier against the external environment. Internally, epithelial tissue lines the entire length of the digestive tract, from the mouth to the anus, facilitating absorption and secretion. Similarly, the respiratory passages, including the airways, are lined by epithelial tissue, aiding filtration and protection. Blood vessels and lymphatic vessels are lined by a specialized form of epithelium called endothelium, which regulates substance passage. Furthermore, epithelial tissue forms the major component of glands, both exocrine and endocrine, producing and secreting various substances.
Vital Roles of Epithelial Tissue
Epithelial tissue performs several functions. Protection is a primary function, acting as a physical barrier against abrasion, pathogens, and dehydration, as seen in the skin. This protective role extends to internal organs, shielding them from damage and infection.
Epithelial tissue also functions in secretion, with glandular epithelium producing and releasing various substances like mucus, hormones, enzymes, and sweat. For instance, goblet cells within the digestive and respiratory tracts secrete mucus to trap particles and lubricate surfaces.
Absorption is another function, evident in the intestines, where the epithelial lining, often equipped with microvilli, increases surface area to efficiently take in nutrients from digested food.
Filtration is performed by epithelial tissues, such as those in the kidneys, which selectively allow substances to pass through while retaining others. This selective barrier function is also observed in simple squamous epithelia that facilitate diffusion in areas like the lung alveoli.
Finally, epithelial tissue contributes to sensory reception; specialized epithelial cells with sensory nerve endings detect external stimuli, as exemplified by taste buds in the tongue or olfactory receptor cells in the nasal cavity.
Varieties of Epithelial Tissue
Epithelial tissue is classified based on the shape of its cells and the number of cell layers.
Cell Shapes
Cell shapes include squamous, cuboidal, and columnar. Squamous cells are flat and scale-like, appearing wider than they are tall, with a flattened nucleus. Cuboidal cells are cube-shaped, with their height and width being roughly equal, and they typically have a spherical, centrally located nucleus. Columnar cells are taller than they are wide, resembling columns, and their nuclei are usually elongated and positioned near the base.
Number of Cell Layers
Based on the number of cell layers, epithelial tissue is categorized as simple, stratified, pseudostratified, or transitional.
Simple epithelium consists of a single layer of cells, where every cell contacts the basement membrane. This arrangement is common in areas where absorption, secretion, or filtration occur, such as the lining of blood vessels (simple squamous) or kidney tubules (simple cuboidal).
Stratified epithelium is composed of two or more cell layers, providing greater protection against mechanical or chemical stress. The outer layer of the skin (epidermis) is stratified squamous epithelium, offering defense against abrasion and water loss. Stratified cuboidal epithelium is found in the ducts of sweat and salivary glands, while stratified columnar epithelium lines parts of the male urethra.
Pseudostratified columnar epithelium appears to have multiple layers due to the varying heights of its cells and the irregular placement of their nuclei, but all cells are actually attached to the basement membrane. This type has cilia and goblet cells, lining the respiratory tract to move mucus and trapped particles.
Transitional epithelium, also known as urothelium, is a specialized stratified tissue found in organs that undergo stretching, like the urinary bladder. Its cells can change shape from cuboidal to squamous as the organ distends.