I Feel Your Pain
Epithelial tissue, one of the four basic types of human tissue, covers a surface or forms the lining of a hollow organ. The surface of your skin is made of epithelial tissue, as is the inside lining of your stomach and intestines. In fact, the inside of your body cavity is covered with a thin layer of epithelial cells. The function of these covering cells is to protect and to provide a watertight barrier to keep material out (skin preventing pathogens from entering) or keeping material in (lining of the stomach keeping hydrochloric acid from damaging other areas of the body).
Types of Epithelial Cells
Epithelial cells are classified, in part, based on their shape. Cells can be very flat, much like a fried egg, with the nucleus of the cells bulging upward like the yolk of the egg. These flat cells are called squamous epithelium. Epithelial cells can also be cube-shaped, where the width of the side is the same as the height of the cell. These are known as cuboidal epithelium. Lastly, cells that are taller than they are wide, thus looking like columns, are classified as columnar epithelium.
Number of Layers
Epithelial tissue composed of a single layer of cells is called a simple epithelium. In simple epithelium, all of the cells in the tissue are in contact with the structure that underlies the tissue (called a basement membrane). When epithelial tissue has multiple layers of epithelial cells, it is referred to as stratified. In this stratified layer, only the cells at the bottom of the layer are in contact with the underlying tissue. In some areas of the body, epithelial tissue appears to be made up of multiple layers of cells, but closer observation shows that all of the cells are in contact with the basement membrane and it just happens that the cells are tall or have a variety of heights. This type of epithelium is classified as pseudostratified, and is easy to confuse with the truly stratified layers.
When describing the epithelium, both their shape and the number of layers are considered. A single layer of flattened cells would be called a simple squamous epithelium. Likewise, an epithelium consisting of multiple layers and having a surface layer of cube-shaped cells would be called a stratified cuboidal epithelium.
How is epithelial tissue classified when multiple layers are present?
When multiple layers are present, the shape of the cells at the surface of the epithelial tissue will be used in the classification regardless of the underlying cells.
Transitional epithelial tissue is found along the urinary tract and in the lining of the bladder. While this is a stratified epithelium, the surface cells are large and either dome-shaped (when the bladder is empty) or flattened (when the bladder is full). Often, the cells of this tissue will contain 2 nuclei, making for easy identification.
Apical Modification
The top of the epithelial cells that are adjacent to the lumen, or the hollow space of an organ, is referred to as the apical surface; these cells have membrane specializations that affect the physiological function of the tissue. One such modification on the epithelial cells lining the respiratory tract is cilia. These hairlike structures extend upward from the apical cell surface and can bend back and forth to move materials.
On cells in the intestines, the apical modifications are called microvilli. These fingerlike projections increase the surface area of the cell for greater absorption of nutrients and water.
Basement Membrane
Beneath every epithelial layer is a zone of molecules that aids in anchoring the cells to the underlying tissues in much the same way as a foundation or a basement secures a home to the ground. The basement membrane (BM) is also a transition zone where cells anchor to molecules such as laminin (a cell adhesion molecule) and other BM molecules interconnect with the underlying connective tissue, firmly anchoring the epithelial layers to it.
The BM consists of three zones, each referred to as a lamina, which is another term for layer:
· The lamina lucida is a clear layer directly beneath and in contact with the bottommost epithelial cell, containing cell adhesion molecules.
· The next layer is called lamina densa, so named because it’s dark, a result of its highly compact network of type IV collagen fibers that resemble a net. This provides another anchorage point for the cells.
· The deepest layer is the lamina reticularis. In this layer, fibers from the underlying connective tissue extend upward and interconnect with the molecules of the lamina densa.
Anatomy of a Word
basal lamina
Together the lamina lucida and lamina densa compose the basal lamina. Some people confuse this with the basement membrane, but they are not interchangeable terms.