Of particular note, epithelial tissue that lines vessels in the lymphatic and cardiovascular systems is called endothelium whereas epithelial tissue that forms the serous membranes lining the true cavities is called mesothelium.
Regardless of its location and function, all epithelial tissue shares important structural features. First, epithelial tissue is highly cellular, with little or no extracellular material present between cells. Second, adjoining cells form specialized intercellular connections called cell junctions. Third, epithelial cells exhibit polarity with differences in structure and function between the exposed, or apical , facing cell surface and the basal surface closest to the underlying tissue.
Fourth, epithelial tissues are avascular; nutrients must enter the tissue by diffusion or absorption from underlying tissues or the surface. Last, epithelial tissue is capable of rapidly replacing damaged and dead cells, necessary with respect to the harsh environment this tissue encounters. The cells of an epithelium act as gatekeepers of the body, controlling permeability by allowing selective transfer of materials across its surface.
All substances that enter the body must cross an epithelium. Many epithelial cells are capable of secreting mucous and other specific chemical compounds onto their apical surfaces. For example, the epithelium of the small intestine releases digestive enzymes and cells lining the respiratory tract secrete mucous that traps incoming microorganisms and particles. Epithelial cells are typically characterized by unequal distribution of organelles and membrane-bound proteins between their apical and basal surfaces.
Structures found on some epithelial cells are an adaptation to specific functions. For example, cilia are extensions of the apical cell membrane that are supported by microtubules.
These extensions beat in unison, allowing for the movement of fluids and particles along the surface. Such ciliated epithelia line the ventricles of the brain where it helps circulate cerebrospinal fluid and line the respirtatory system where it helps sweep particles of dust and pathogens up and out of the respiratory tract. Epithelial cells in close contact with underlying connective tissues secrete glycoproteins and collagen from their basal surface which forms the basal lamina.
The basal lamina interacts with the reticular lamina secreted by the underlying connective tissue, forming a basement membrane that helps anchor the layers together. Cells of epithelia are closely connected with limited extracellular material present. Three basic types of connections may be present: tight junctions, anchoring junctions, and gap junctions Figure 4. Epithelial cells are held close together by cell junctions. The three basic types of cell-to-cell junctions are tight junctions, gap junctions, and anchoring junctions.
A Tight junction restricts the movement of fluids between adjacent cells due to the presence of integral proteins that fuse together to form a firm seal. Tight junctions are observed in the epithelium of the urinary bladder, preventing the escape of fluids comprising the urine. An anchoring junction provides a strong yet flexible connection between epithelial cells. There are three types of anchoring junctions: desmosomes, hemidesmosomes, and adherens. Desmosomes hold neighboring cells together by way of cadherin molecules which are embedded in protein plates in the cell membranes and link together between the adjacent cells.
Hemidesmosomes , which look like half a desmosome, link cells to components in the extracellular matrix, such as the basal lamina. While similar in appearance to desmosomes, hemidesmosomes use adhesion proteins called integrins rather than cadherins. Adherens use either cadherins or integrins depending on whether they are linking to other cells or matrix. These junctions are characterized by the presence of the contractile protein actin located on the cytoplasmic surface of the cell membrane.
These junctions influence the shape and folding of the epithelial tissue. In contrast with the tight and anchoring junctions, a gap junction forms an intercellular passageway between the membranes of adjacent cells to facilitate the movement of small molecules and ions between cells.
These junctions thus allow electrical and metabolic coupling of adjacent cells. Epithelial tissues are classified according to the shape of the cells composing the tissue and by the number of cell layers present in the tissue. Figure 4. Stratified epithelia are more durable and protection is one their major functions. Since stratified epithelium consists of two or more layers, the basal cells divide and push towards the apex, and in the process flatten the apical cells.
Stratified epithelia can be columnar, cuboidal, or squamous type. However, it can also have the following specializations:. In keratinized epithelia, the most apical layers exterior of cells are dead and lose their nucleus and cytoplasm.
They contain a tough, resistant protein called keratin. This specialization makes the epithelium waterproof, and it is abundant in mammalian skin. The lining of the esophagus is an example of a non-keratinized or moist stratified epithelium. Transitional epithelia are found in tissues that stretch and it can appear to be stratified cuboidal when the tissue is not stretched, or stratified squamous when the organ is distended and the tissue stretches. It is sometimes called the urothelium since it is almost exclusively found in the bladder, ureters, and urethra.
Privacy Policy. Skip to main content. Organization at the Tissue Level. Search for:. Epithelial Tissue. Characteristics of Epithelial Tissue The human body consists of four types of tissue: epithelial, connective, muscular, and nervous.
Learning Objectives Describe the primary functions and characteristics of epithelial tissue. Key Takeaways Key Points Epithelial tissue is composed of cells laid together in sheets with the cells tightly connected to one another.
Epithelial layers are avascular, but innervated. Epithelial cells have two surfaces that differ in both structure and function. Glands, such as exocrine and endocrine, are composed of epithelial tissue and classified based on how their secretions are released. Key Terms epithelium : A membranous tissue composed of one or more layers of cells that form the covering of most internal and external surfaces of the body and its organs.
Types of Epithelial Tissue Epithelial tissue is classified by cell shape and the number of cell layers. Apical is derived from the term apex which means peak. Basal, on the other hand, is derived from the base. In the medical nomenclature for hollow organs, the term luminal is often used to refer to the pole which faces the lumen. The pole oriented away from the lumen is called abluminal. In other parts, epithelial tissues form the inner lining and external lining of body parts.
To summarize, the apical pole faces the surface, while the basal pole is attached to the connective tissue located below the epithelium. There are 3 different types of epithelial tissue: squamous, cuboidal, and columnar. The differentiation and classification of the types of epithelial tissue are based on the shape of the cell e. Image: Simple classification of epithelium. By Lecturio. Image: Stratified classification of epithelium.
Image: Pseudo-stratified and transitional classification of epithelium. General functions of the epithelial tissue include a barrier protection of tissues from radiation, desiccation, and invasion by pathogens and toxins , secretion substance release of hormones, sweat, mucus, and enzymes and absorption substance intake.
The epithelial tissue has a general characteristic that their cells are closely bound to each other through specialized structures such as tight junctions, desmosomes, interdigitations, gap junctions, and intercellular bridges.
These gap junctions help in communication with 2 cells together by cementing each other. They constitute the linings of organs and body parts.
They have 1 surface that is not attached to any cell, either exposed to the external environment or the lumen of an organ. They do not possess blood vessels and their nutrients requirement is fulfilled by connective tissue forming the basement membrane.
Image: The 3 characteristics of any epithelium. Epithelial cellular organizations are located on the basal membrane. The basal membrane is a thin extracellular matrix. It consists of the basal lamina, which is itself divided into the laminae rara externa and interna, the lamina densa, and the lamina fibroreticularis except in renal corpuscles. The function of the basal membrane is to stabilize the epithelial and endothelial cell layers.
The attachment of cells is achieved with the help of syndecan or laminin and integrin. Microfilaments attach the lamina fibrorecticularis to the perlecan in the lamina densa. Epithelial cells regenerate on the basal membrane. Image: Electron microscopic image of the basal lamina. By Robert M. Hunt, License: Public domain. The surface epithelium primarily has barrier and transport functions. It is a mechanical and chemical barrier that prevents the uncontrolled substance exchange.
It takes substances in absorption or releases them into the tissue secretion. Image : Schematic representation of the endothelium with astrocytes in the blood-brain barrier. The simple epithelial tissue is a closed network of flat epithelial cells. These are located on the basal membrane. It is composed of a single layer of cells that are specialized in diffusion, osmosis, filtration, secretion, and absorption.
The simple epithelial tissue is found in the alveolar epithelium pulmonary alveolus , the endothelium lining of blood vessels and lymph vessels , and the mesothelium lining of the body cavities. Medically, the simple epithelium is further divided into 2 types: simple cuboidal epithelium and simple columnar epithelium. The cells of the cuboidal epithelium are equally tall as they are wide and have a central round nucleus. The simple cuboidal epithelium is found, e.
Epithelial tissues are widespread throughout the body. They form the covering of all body surfaces, line body cavities and hollow organs, and are the major tissue in glands. They perform a variety of functions that include protection, secretion, absorption , excretion, filtration, diffusion , and sensory reception.
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