Epithelium is a tissue that covers surfaces or forms glands. Most often these cells a characterized by having a free surface, that is one which is not attached to other cells. These surfaces include the skin and the lining within organs and vessels. Epithelial cells are attached to the underlying layer of cells by a Basement membrane, a substance made of carbohydrates and proteins secreted by the epithelium and the layer of cells below it. Although an epithelium may have nerves within it's layer(s), it does not have any blood vessels and thus materials must diffuse through the thin basement membrane to reach it.
Be sure to check out the electron micrographs
of these tissues by clicking on the thumbnails at the bottom of
Epithelial tissue is classified by the number
of cells as well as their shape. Tissue made of a single layer
of cells is called Simple epithelium while those made of
many cells (layered) is called Stratified epithelium. Epithelium
classification based on shape includes Squamous (flat),
Cuboidal (cube-like), and Columnar (tall and thin).
Usually epithelial tissue is named based on both number and shape,
such as Stratified Columnar. When tissue is stratified, it is
named based on shape along the free surface.
Classification of Epithelial Tissue
Simple Squamous: a single layer of flat cells which forms a thin boundry
of tissue that easily allows materials to diffuse through it,
such as is found in the alveolar sacs in the lungs, where the
exchange of CO2 and O2 occur. It can also be found covering organs
(serous membranes) in the thoracic and abdominal cavity where
it secretes a fluid that prevents friction from organs rubbing
against each other.
Simple Cuboidal: a single layer of cube-like cells which have a greater
volume than squamous cells and thus have more organelles. These
cells can produce lots of carrier proteins and ATP to help regulate
what materials may pass into or through the cells by means of
Avtive Transport and Facilitated Diffusion. For instance in the
kidney these cells excrete the waste products of cell metabolism
and reabsorb any needed materials from urine before it's excreted.
Simple Columnar: a single layer of tall, thin cells which are large
enough to contain many organelles, allowing for great variety
of function. For instance, this tissue lines the intestine, producing
digestive enzymes and absorbing digested nutrients by diffusion,
active transport, or facilitated diffusion.
Stratified Squamous: a multilayered epithelium with the deepest cells
being columnar and the outer layer squamous. The columnar continually
divide, forcing cells upward which flatten out. This tissue forms
the outer layer of the skin, forming protection against abrasion.
As outer cells get damaged, new cells below are produced, replacing
Pseudostratified Columnar: This type of epithelium appears to be stratified,
but in reality is not (hence the term "pseudo-"). This
is one layer of cells with all cells attached to the basement,
but not all cells have a free surface. Some are short and are
covered by surrounding tall cells, giving the appearence of two
layers. This tissue is found lining some of the respiratory passages.
Transitional: This tissue is a special type of stratified epithelium
capable of stretching and changing shape. The cells at rest resemble
cuboidal cells, but when the tissue is stretched, they flatten
out to cover more area, and in doing so resemble squamous cells.
Transitional epithelium covers cavities which are stretched greatly,
such as the urinary bladder.
Epithelial tissue is classified by the number of cells as well as their shape. Tissue made of a single layer of cells is called Simple epithelium while those made of many cells (layered) is called Stratified epithelium. Epithelium classification based on shape includes Squamous (flat), Cuboidal (cube-like), and Columnar (tall and thin). Usually epithelial tissue is named based on both number and shape, such as Stratified Columnar. When tissue is stratified, it is named based on shape along the free surface.
The number of cell layers and the shape of the cells is a tip off to the function of the epithelial tissue. The two basic functions of epithelium is the control of materials moving through the epithelium and protection of underlying tissue layers. Simple epithelium, with it's single layer is found primarily in organs whose function is diffusion, filtration, secretion, or absoption. The alveoli in the lungs (diffusion), the tubules of the kidney (filtration), the many glands of the body (secretion), and the lining of the intestine (absorption) are all excellant examples of the movement of materials being controlled by a layer of epithelium. Stratified tissue would hinder such movement by the nature of it's many layers, thus it's use as a protective layer. As damage is done to outer layers, the lower layers replace those cells which were damaged. The skin, anus and vagina are all examples of organs with stratified tissue.
The shape of the cell also reflects it's function. Cells which are involved in diffusion are typically squamous in shape, while those which absorb or secrete typically are cuboidal or columnar. This allows these cells to have a greater number of organelles which may be involved in these processes. The cells of the stomach, for example, have many vesicles which are responsible for producing mucus, necessary for protecting the stomach from the digestive enzymes and acids.
Most epithelia have a free surface, one which is not in contact with other cells but faces towards the cavity of an organ or vessel. The nature of this surface is also important. Smooth surfaces are for material to flow over it readily, with no friction to slow these materials (fluids primarily). The lining of a blood vessel is an example (simple squamous). Some have small, finger-like projections called microvilli, which increase the surface area of the cell to allow greater absorption (intestinal epithelium). Still others, such as the pseudostratified of the nasal cavity, have cilia attached to this surface which is used, in conjunction with mucus from these cells, to keep the passageway free of dust and gunk. The beating cilia will move trapped materials out of the cavity.
Epithelial cells are connected to each other in several ways. Tight junctions bind adjacent cells together and form permeability barriers. Because tight junctions completely surround each cell, they prevent the passage of materials between cells. Tight junctions force materials to pass though the cells, which can regulate what materials cross the epithelial layer. Tight junctions are found in the lining of the intestines and most other simple epithelial, particularly those which control the passage of materials through the tissue. Desmosomes are mechanical links that function to bind cells together. Many desmosomes are found in epithelium subject to stress, such as the stratified squamous epithelium of the skin. Gap junctions are small channels that allow materials to pass from one epithelial cell to an adjacent epithelial cell. Most epithelial cells are connected to each other by gap junctions, and it is believed the gap junction provides a means of intercellular communication.
A gland is a single cell or a multicellular structure that secretes substances onto a surface, into a cavity, or into the blood. Most glands are composed primarily of epithelium. Glands with ducts are called Exocrine glands. They can be simple, with ducts that have no branches, or compound, with ducts that have many branches. The end of a duct can be tubular or expanded into a saclike structure called an acinus or alveolus. Secretions from the glands pass through the ducts onto a surface or into an organ. For example, sweat (from sweat glands) and oil (from sebaceous glands) flow onto the skin surface.
Endocrine glands have no ducts and empty their secretions directly into the blood. These secretions, called hormones, are carried by the blood to other parts of the body.