Squamous cells have the appearance of thin, flat plates. The shape of the nucleus usually corresponds to the cell form and help to identify the type of epithelium. Squamous cells, for example, tend to have horizontall flattened, elliptical nuclei because of the thin flattened form of the cell. They form the lining of cavities such as the mouth, blood vessels, heart and lungs and make up the outer layers of the skin.
Priority of this kind of tissue is to allow rapid exchange by diffusion or filtration
Simple Cuboidal Epithelium:
As their name implies, cuboidal cells are roughly square or cuboidal in shape. Each cell has a spherical nucleus in the centre. Cuboidal epithelium is found in glands and in the lining of the kidney tubules as well as in the ducts of the glands. They also constitute the germinal epithelium which produces the egg cells in the female ovary and the sperm cells in the male testes.
Most cuboidal cells in this kind of tissue are involved in secretion, filtration and absorption
Simple Columnar Epithelium:
Columnar epithelial cells occur in one or more layers. The cells are elongated and column-shaped. The nuclei are elongated and are usually located near the base of the cells. Columnar epithelium forms the lining of the stomach and intestines. Some columnar cells are specialised for sensory reception such as in the nose, ears and the taste buds of the tongue. Goblet cells (unicellular glands) are found between the columnar epithelial cells of the duodenum. They secrete mucus or slime, a lubricating substance which keeps the surface smooth.
Many columnar cells in this kind of tissue are involved in secretion and absorption
Those that are ciliated are involved in movement of particles
Squamous (pavement) epithelium
Simple epithelium
Simple epithelium can be subdivided according to the shape and function of its cells.
1. Shape of the cells “Squamous”
Cells are thin and flat
Often display many angular outlines when viewed from above
“Cuboidal”
Cells are approximately as tall as wide
“Columnar”
Cells are definitely more taller than wide
2. The number of cell layers present “Simple”
Only one layer of cells
All cells touch the basement membrane
“Stratified”
There is more than one layer of cells
Only the lowest cells rest on the basement membrane
Other cells are stacked upon each other in various configuration making up the "layers"
All cells arise from cell division by the lowest cells
Depending on the number of layers, displacement of the cells upwards into the different layers causes them to change shape
Types of Epithelial Tissue
Epithelial tissue can be divided into two groups depending on the number of layers of which it is composes. Epithelial tissue which is only one cell thick is known as simple epithelium. If it is two or more cells thick such as the skin, it is known as stratified epithelium.
Germ Cell Origin: Epithelial tissues are derived from all three primary germ cell layers.
Ectoderm: The epithelial cells of the skin and oral cavity (epidermis) are derived from ectoderm. Epithelial cells covering the cornea and lens, as well as sensory receptors of the eyes, ears, and nose, are also ectodermal in origin.
Mesoderm: The epithelial lining of blood vessels (endothelium) is derived from mesoderm. The epithelial lining of the pleural and peritoneal cavities (mesothelium) also originate from mesodermal cells.
Endoderm: The epithelial lining of the respiratory system and digestive tracts - as well as the functional cells (parenchyma) of the liver, pancreas, gallbladder, thyroid, and parathyroid, are derived from endoderm.
General Characteristics Of Epithelial Tissue
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.
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.