Tight junctions inhibit the passage of substances between cells, maintaining the integrity of the tissue barrier.
The structure and properties of a specific tissue are influenced by the nature of the extracellular material that surrounds the tissue cells and the connections between the cells that compose the tissue.
Tissues may be hard, semisolid, or even liquid in their consistency, as exemplified by bone, fat, and blood.
Simple squamous epithelium commonly lines the cardiovascular and lymphatic systems (heart, blood vessels, lymphatic vessels) where it is known as endothelium, and forms the epithelial layer of serous membranes (peritoneum, pleura, pericardium) where it is called mesothelium. It is also found in air sacs of lungs, glomerular (Bowman’s) capsule of kidneys, and the inner surface of the tympanic membrane (eardrum).
The functions of stratified cuboidal epithelium are protection, and limited secretion and absorption.
The basal surface of an epithelial cell is opposite the apical surface and adheres to extracellular materials such as the basement membrane. Hemidesmosomes in the basal surfaces anchor the epithelium to the basement membrane.
Transitional epithelium allows urinary organs to stretch and maintain a protective lining while holding variable amounts of fluid without rupturing.
An exocrine gland is a gland whose secretory products are released into ducts that empty onto the surface of a covering and lining epithelium, such as the skin surface or the lumen of a hollow organ.
Ciliated simple columnar epithelium is a single layer of ciliated columnlike cells with oval nuclei near the base of cells. Goblet cells are usually interspersed.
Exocrine glands secrete their products into ducts that empty onto the surface of a covering and lining epithelium, such as the skin surface or the lumen of a hollow organ. Their secretions have limited effects and some can be harmful if they enter the bloodstream.
Epithelial tissue is avascular and relies on the blood vessels of the adjacent connective tissue to bring nutrients and remove wastes through diffusion.
Cuboidal cells are as tall as they are wide and are shaped like cubes or hexagons. They may have microvilli at their apical surface and function in either secretion or absorption.
Epithelial tissue consists of cells arranged in continuous sheets, in either single or multiple layers, with cells closely packed and held tightly together by many cell junctions, resulting in little intercellular space between adjacent plasma membranes.
In untreated diabetes mellitus, the basement membrane of small blood vessels (capillaries) thickens due to increased production of collagen and laminin, leading to improper blood vessel function, which can result in blindness and kidney failure.
A compound gland is a gland whose duct branches.
Tight junctions consist of weblike strands of transmembrane proteins that fuse together the outer surfaces of adjacent plasma membranes to seal off passageways between adjacent cells.
Simple multicellular exocrine glands have a single unbranched duct, while compound multicellular exocrine glands have a branched duct.
Stratified cuboidal epithelium has two or more layers of cells; cells in the apical layer are cube-shaped. It is a fairly rare type of epithelium.
Nonciliated pseudostratified columnar epithelium lines the epididymis, larger ducts of many glands, and parts of the male urethra.
Nonkeratinized stratified squamous epithelium lines wet surfaces such as the lining of the mouth, esophagus, part of the epiglottis, part of the pharynx, and vagina, and covers the tongue.
Stratified epithelium consists of two or more layers of cells that protect underlying tissues in locations where there is considerable wear and tear.
Epithelial tissues lack blood vessels because they form surface layers and are not covered by another tissue, except in the case of the epithelial lining of blood vessels.
Cell junctions are contact points between the plasma membranes of tissue cells.
Exocrine glands are located in sweat, oil, and earwax glands of the skin; digestive glands such as salivary glands (secrete into mouth cavity) and pancreas (secretes into small intestine).
Ciliated simple columnar epithelium lines some bronchioles of the respiratory tract, uterine tubes, uterus, some paranasal sinuses, central canal of the spinal cord, and ventricles of the brain.
The function of glandular epithelium is secretion, which is accomplished by glandular cells that often lie in clusters deep to the covering and lining epithelium. A gland consists of epithelium that secretes substances into ducts, onto a surface, or eventually into the blood.
The two types of epithelial tissue are covering and lining epithelium (surface epithelium) and glandular epithelium.
Columnar cells are much taller than they are wide, like columns, and protect underlying tissues. Their apical surfaces may have cilia or microvilli, and they often are specialized for secretion and absorption.
Epithelial tissue is arranged in two general patterns: (1) covering and lining various surfaces and (2) forming the secreting portions of glands.
Acinar glands, also called alveolar glands, have rounded secretory portions.
A compound tubular gland has a tubular secretory portion that attaches to a branched duct. Example: bulbourethral (Cowper’s) glands.
Tight junctions are commonly found in the cells of epithelial tissue that lines the stomach, intestines, and urinary bladder.
Transitional epithelium lines the urinary bladder and portions of the ureters and urethra.
Tissues contribute to homeostasis by providing diverse functions including protection, support, communication among cells, and resistance to disease.
Simple squamous epithelium is a single layer of flat cells that resembles a tiled floor when viewed from the apical surface; it has a centrally located nucleus that is flattened and oval or spherical in shape.
Endothelium is the simple squamous epithelium that lines the cardiovascular and lymphatic systems (heart, blood vessels, lymphatic vessels).
The function of nonciliated pseudostratified columnar epithelium is absorption and secretion.
Multicellular glands are composed of many cells that form a distinctive microscopic structure or macroscopic organ. Examples include sudoriferous (sweat), sebaceous (oil), and salivary glands. They are categorized according to whether their ducts are branched or unbranched and the shape of their secretory portions.
Nonciliated simple columnar epithelium is a single layer of nonciliated columnlike cells with oval nuclei near the base of the cells. It contains columnar epithelial cells with microvilli at the apical surface and goblet cells.
Histology is the science that deals with the study of tissues.
Nervous tissue detects changes in a variety of conditions inside and outside the body and responds by generating electrical signals called nerve action potentials (nerve impulses) that activate muscular contractions and glandular secretions.
Ciliated pseudostratified columnar epithelium lines the airways of most of the upper respiratory tract.
The five most important types of cell junctions are tight junctions, adherens junctions, desmosomes, hemidesmosomes, and gap junctions.
Desmosomes contain plaque and have transmembrane glycoproteins (cadherins) that extend into the intercellular space between adjacent cell membranes and attach cells to one another. The plaque of desmosomes attaches to intermediate filaments, which consist of the protein keratin. This structural arrangement contributes to the stability of the cells and tissue.
The main structural difference is the ratio of cells to extracellular matrix. Epithelial tissue has many cells tightly packed together with little to no extracellular matrix, while connective tissue has a few scattered cells surrounded by large amounts of extracellular matrix.
The functions of stratified columnar epithelium are protection and secretion.
The functions of simple cuboidal epithelium are secretion and absorption.
A tissue is a group of cells that usually have a common origin in an embryo and function together to carry out specialized activities.
Muscular tissue is composed of cells specialized for contraction and generation of force. In the process, muscular tissue generates heat that warms the body.
A simple acinar gland has a rounded secretory portion that attaches to a single unbranched duct. Example: glands of penile urethra.
Apocrine glands accumulate their secretory product at the apical surface of the secreting cell, which then pinches off by exocytosis to release the secretion. Example: mammary glands.
Multicellular exocrine glands are structurally classified based on the branching pattern of the duct and the shape of the secreting portion.
Stratified columnar epithelium lines part of the urethra, large excretory ducts of some glands such as esophageal glands, small areas in the anal mucous membrane, and part of the conjunctiva of the eye.
Keratinized stratified squamous epithelium forms the superficial layer of the skin.
Simple cuboidal epithelium covers the surface of the ovary, lines the anterior surface of the capsule of the lens of the eye, forms the pigmented epithelium at the posterior surface of the retina of the eye, lines kidney tubules and smaller ducts of many glands, and makes up the secreting portion of some glands such as the thyroid gland and ducts of some glands such as the pancreas.
The functions of nonciliated simple columnar epithelium are secretion and absorption. Larger columnar cells contain more organelles and thus are capable of a higher level of secretion and absorption than cuboidal cells. Secreted mucus lubricates the linings of the digestive, respiratory, and reproductive tracts, and most of the urinary tract; it also helps prevent the destruction of the stomach lining by acidic gastric juice secreted by the stomach.
Connective tissue protects and supports the body and its organs. Various types of connective tissues bind organs together, store energy reserves as fat, and help provide the body with immunity to disease-causing organisms.
A simple tubular gland has a straight tubular secretory part that attaches to a single unbranched duct. Example: glands in the large intestine.
Adherens junctions contain plaque, a dense layer of proteins on the inside of the plasma membrane that attaches both to membrane proteins and to microfilaments of the cytoskeleton. Transmembrane glycoproteins called cadherins join the cells. In epithelial cells, adherens junctions often form extensive zones called adhesion belts.
Gap junctions are formed by membrane proteins called connexins that create tiny fluid-filled tunnels called connexons, connecting neighboring cells. These junctions allow ions and small molecules to diffuse between cells, facilitating communication and the spread of nerve or muscle impulses.
The cilia in ciliated simple columnar epithelium beat in unison, moving mucus and foreign particles toward the throat, where they can be coughed up and swallowed or spit out. Cilia also help move oocytes expelled from ovaries through uterine tubes into the uterus.
A Papanicolaou test, also called a Pap test or Pap smear, involves the collection and microscopic examination of epithelial cells that have been scraped off the apical layer of a tissue. It is mainly performed to detect early changes in the cells of the female reproductive system that may indicate a precancerous condition or cancer.
Simple epithelium is a single layer of cells that functions in diffusion, osmosis, filtration, secretion, or absorption.
Transitional cells change shape from squamous to cuboidal and back, as organs such as the urinary bladder stretch to a larger size and then collapse to a smaller size.
Epithelial tissue protects, secretes (mucus, hormones, and enzymes), absorbs (nutrients in the gastrointestinal tract), and excretes (various substances in the urinary tract).
Tubuloacinar glands have both tubular and rounded secretory parts.
A compound acinar gland has a rounded secretory portion that attaches to a branched duct. Example: mammary glands.
Transitional epithelium (urothelium) has a variable appearance. In a relaxed or unstretched state, it looks like stratified cuboidal epithelium, except the apical layer cells tend to be large and rounded. As the tissue is stretched, the cells become flatter, giving the appearance of stratified squamous epithelium. Multiple layers and elasticity make it ideal for lining hollow structures subject to expansion from within.
A tissue is a group of cells that work together to perform specific functions in the body.
The four basic types of tissues in the human body are epithelial tissue, connective tissue, muscular tissue, and nervous tissue.
Exocrine glands produce substances such as sweat to help lower body temperature, oil, earwax, saliva, or digestive enzymes.
Mesothelium is the simple squamous epithelium that forms the epithelial layer of serous membranes such as the peritoneum, pleura, and pericardium.
Stratified columnar epithelium has basal layers that usually consist of shortened, irregularly shaped cells; only the apical layer has columnar cells. It is uncommon.
Stratified squamous epithelium has two or more layers of cells; cells in the apical layer and several layers deep to it are squamous, while cells in deeper layers vary from cuboidal to columnar. As basal cells divide, daughter cells arising from cell divisions push upward toward the apical layer, becoming dehydrated and less metabolically active as they move away from the blood supply.
Nonkeratinized stratified squamous epithelium does not contain large amounts of keratin in the apical layer and several layers deep. It is constantly moistened by mucus from salivary and mucous glands, and organelles are not replaced.
The basement membrane anchors the epithelium to its underlying connective tissue, forms a surface for epithelial cell migration during growth or wound healing, restricts the passage of larger molecules between epithelium and connective tissue, and participates in the filtration of blood in the kidneys.
Simple cuboidal epithelium is a single layer of cube-shaped cells with a round, centrally located nucleus. The cuboidal cell shape is obvious when the tissue is sectioned and viewed from the side.
Microvilli are fingerlike cytoplasmic projections that increase the surface area of the plasma membrane, thus increasing the cell’s rate of absorption.
Nonciliated simple columnar epithelium lines the gastrointestinal tract (from stomach to anus), ducts of many glands, and the gallbladder.
A pathologist is a physician who examines cells and tissues to help other physicians make accurate diagnoses, particularly by examining tissues for any changes that might indicate disease.
Epithelial tissue covers body surfaces and lines hollow organs, body cavities, and ducts; it also forms glands. This tissue allows the body to interact with both its internal and external environments.
A biopsy is the removal of a sample of living tissue for microscopic examination. This procedure is used to help diagnose many disorders, especially cancer, and to discover the cause of unexplained infections and inflammations.
A simple gland is a gland whose duct does not branch.
A simple branched tubular gland has a branched tubular secretory part that attaches to a single unbranched duct. Example: gastric glands.
A compound tubuloacinar gland has both tubular and rounded secretory portions that attach to a branched duct. Example: acinar glands of the pancreas.
Hemidesmosomes resemble desmosomes but do not link adjacent cells. The transmembrane glycoproteins in hemidesmosomes are integrins rather than cadherins. Integrins attach to intermediate filaments made of keratin on the inside and to laminin in the basement membrane on the outside, anchoring cells to the basement membrane.
Simple squamous epithelium is present at sites of filtration (such as blood filtration in kidneys) or diffusion (such as diffusion of oxygen into blood vessels of lungs) and at sites of secretion in serous membranes. It is not found in body areas subject to mechanical stress (wear and tear).
Stratified cuboidal epithelium is located in the ducts of adult sweat glands and esophageal glands, and part of the male urethra.
Nonciliated pseudostratified columnar epithelium appears to have several layers because the nuclei of the cells are at various levels. All cells are attached to the basement membrane in a single layer, but some cells do not extend to the apical surface. It contains cells without cilia and lacks goblet cells.
Endocrine glands are glands whose secretions (hormones) enter the interstitial fluid and then diffuse into the bloodstream without flowing through a duct. These secretions have far-reaching effects because they are distributed throughout the body by the bloodstream.
Unicellular glands are single-celled exocrine glands. Goblet cells are important unicellular exocrine glands that secrete mucus directly onto the apical surface of a lining epithelium.
Stratified squamous epithelium provides protection against abrasion, water loss, ultraviolet radiation, and foreign invasion. Both keratinized and nonkeratinized types form the first line of defense against microbes.
Pseudostratified epithelium appears to have multiple layers of cells because the cell nuclei lie at different levels and not all cells reach the apical surface, but it is actually a simple epithelium because all its cells rest on the basement membrane.
Squamous cells are thin and flat, allowing for the rapid passage of substances through them.
In epithelial tissue, many cells are tightly packed together with little or no extracellular matrix, whereas in connective tissue, a large amount of extracellular material separates cells that are usually widely scattered.
Epithelial tissue lacks blood vessels and forms surfaces, so it is always found immediately adjacent to blood vessel-rich connective tissue, which enables it to make the exchanges with blood necessary for the delivery of oxygen and nutrients and the removal of wastes.
The apical (free) surface of an epithelial cell faces the body surface, a body cavity, the lumen (interior space) of an internal organ, or a tubular duct that receives cell secretions. Apical surfaces may contain cilia or microvilli.
The basement membrane is a thin extracellular layer that commonly consists of two layers: the basal lamina and reticular lamina. The basal lamina is closer to the epithelial cells and contains proteins such as laminin and collagen, while the reticular lamina is closer to the underlying connective tissue and contains proteins such as collagen produced by fibroblasts.
A simple coiled tubular gland has a coiled tubular secretory part that attaches to a single unbranched duct. Example: sweat glands.
Merocrine glands synthesize secretions on ribosomes attached to rough ER, process, sort, and package them by the Golgi complex, and release them via exocytosis. Example: salivary glands and pancreas.
The function of ciliated pseudostratified columnar epithelium is to secrete mucus that traps foreign particles, and the cilia sweep away the mucus for elimination from the body.
Keratinized stratified squamous epithelium develops a tough layer of keratin in the apical layer of cells and several layers deep to it. Keratin is a tough, fibrous intracellular protein that helps protect skin and underlying tissues from heat, microbes, and chemicals. The relative amount of keratin increases in cells as they move away from the nutritive blood supply and organelles die.
The main roles of epithelial tissue are protection, filtration, secretion, absorption, and excretion. It also combines with nervous tissue to form special organs for smell, hearing, vision, and touch.
Goblet cells are modified columnar epithelial cells that secrete mucus, a slightly sticky fluid, at their apical surfaces. The mucus accumulates in the upper portion of the cell, causing it to bulge and making the whole cell resemble a goblet or wine glass.
The four basic types of tissues are epithelial tissue, connective tissue, muscular tissue, and nervous tissue.
Tubular glands are glands with tubular secretory parts.
A simple branched acinar gland has a branched rounded secretory part that attaches to a single unbranched duct. Example: sebaceous glands.
Holocrine glands accumulate a secretory product in their cytosol, and as the secretory cell matures, it ruptures and becomes the secretory product. Example: sebaceous glands.
Ciliated pseudostratified columnar epithelium appears to have several layers because cell nuclei are at various levels. All cells are attached to the basement membrane in a single layer, but some cells do not extend to the apical surface. It contains cells that extend to the surface and secrete mucus (goblet cells) or bear cilia.
