There are many characteristics common to all epithelial tissue making it easy to recognize. However, students sometimes still find a way to misidentify epithelial tissue. Therefore pay close attention to the following major elements that characterize this type of tissue.
Epithelial tissue covers surfaces and lines internal passage ways . As such epithelial tissue is found in 3 major places
–outer surface of the body
–surface of organs
–internal surface lining of tubules, vessels and hollow organs
The fact that Epithelial Tissue covers or lines means that most epithelial tissues have a free surface which does not contact other cells or extracellular material. This surface is called the apical surface. The opposite side is called the basal surface. On histological slides, this apical surface is the most easiest to recognize. It is at the interface where empty space (usually a vast area of white space) meets tissue.
Example of a typical epithelium layer
The picture on the right shows the epithelial layer sandwiched between the two green arrows. The obvious free empty space is located on top and the sharp point of the arrow thus points at the apical surface of the epithelium. The sharp point of the lower arrow meets up with the basal surface. The green line marks the boundary between the epithelium layer and the underlying tissue and is the site where the basement membrane can be located.
Cells of Epithelial Tissue fit closely together and form sheets of cells. There is hardly any extracellular material that separates the cells from each other.
The Basal surface of the cells are attached to a basement membrane which provides an anchor for attachment to underlying tissues such as connective tissue. NO BLOOD VESSELS PENETRATE THE BASEMENT MEMBRANE TO REACH THE EPITHELIAL LAYER.
And finally take note that most glands are composed primarily from epithelial cells.
General Characteristics Of Epithelial Tissue
Junctions
Adherens Junctions
Adherens junctions provide strong mechanical attachments between adjacent cells.
They hold cardiac muscle cells tightly together as the heart expands and contracts.
They hold epithelial cells together.
They seem to be responsible for contact inhibition.
Some adherens junctions are present in narrow bands connecting adjacent cells.
Others are present in discrete patches holding the cells together.
Adherens junctions are built from:
cadherins — transmembrane proteins (shown in red) whose
–extracellular segments bind to each other and
–whose intracellular segments bind to
catenins (yellow). Catenins are connected to actin filament
One of the oncogenes that is frequently found in colon cancer appears to be the mutated version of a protein that normally interacts with catenins. Loss of functioning adherens junctions may also lead to tumor metastasis.
What is Histology -2-
The plasma membranes of adjacent epithelial cells frequently contain transmembrane proteins called junctional complexes (also called intercellular junctions) that link cells together. Several types of junctional complexes exist:
Anchoring junctions (desmosomes, adherens junctions, hemidesmosomes) - Proteins embedded in, and spanning, the plasma membranes of adjacent cells allowing epithelia to adhere to one another forming structural units of cells - i.e., allowing individual cells to function as a solid sheet rather than as isolated cells. Fibers attached to anchoring junctions within cells help them to resist stretching forces. This is particularly important in those areas of the body that are subject to mechanical stresses (e.g., epidermis of the skin).
Tight junctions - These are junctional complexes that serve as barriers to the free diffusion of molecules across epithelial surfaces. By fusing the plasma membranes on their lateral surfaces, tight junctions prevent molecules from moving between the cells. This means that any substance absorbed across a layer of epithelial cells has to pass through the cells - not around them. This provides a mechanism by which epithelial tissue can control absorption (selective permeability), and keep tissue compartments with different chemical compositions separate from one another.
Gap junctions - These junctional complexes contain channels which permit ion flow and the passage of small molecules between contiguous cells. Cells with gap junctions are able to communicate and coordinate their activity.
Histology Definition
Epithelial tissues are physically separated from underlying connective tissues by a basement membrane (also called the basal lamina). The portion of an epithelial cell attached to the basement membrane is called its basal surface. The opposite side - facing the external environment, or lumen of a body cavity, is its apical surface. Basement membranes are composed of a special type of collagen and a substance called laminin (see below). The basement membrane helps epithelial cells orient themselves in relation to other tissues. After epithelial injury (e.g., an abrasion), the basement membrane serves as a scaffolding upon which new cells attach themselves during healing.
Epithelium: Surface and Glandular
Maximum cell-to-cell contact; minimum extracellular material; cell junctions; supported by basement membrane (basal lamella); not penetrated by blood vessels; derived from all embryonic germ layers, including mesoderm (endothelia and mesothelia)
Polarity: basal vs apical (luminal); lateral; specialized plasmalemma, appendages and junctional complexes
glycocalyx (apical) vs basement membrane (bm: basal lamina secreted by epithelium + lamina reticularis secreted by connective tissue)
Epithelia: thelia = nipple; epithelia = upon the nipple (convex [surface layer: epidermis, intestinal lining]); endothelia = within the nipple (concave [lining blood and lymph vessels and heart; cornea]); mesothelia = epithelia derived from splanchnic and somatic mesoderm lining body cavity (peripheral serosa) and covering organs (visceral serosa) within peritoneal, plural- and cardiac cavities.