What are Tissues?The word tissue is derived from a Latin word meaning to "weave." Cells that make up tissues are sometimes "woven" together with extracellular fibers.
Likewise, a tissue can sometimes be held together by a sticky substance that coats its cells.
There are four main categories of tissues: epithelial, connective, muscle and nervous. Let's take a look at epithelial tissue.
Epithelial TissueEpithelial tissue covers the outside of the body and lines organs and cavities. The cells in this type of tissue are very closely packed together and joined with little space between them.
With a tightly packed structure we would expect epithelial tissue to perhaps serve some type of barrier and protective function and that is certainly the case.
Epithelial tissue helps to protect organisms from microorganisms, injury, and fluid loss.
In an epithelium, the free surface is usually exposed to fluid or the air while the bottom surface is attached to a basement membrane.
ClassifyingEpithelia are commonly classified based on the shape of the cells on the free surface, as well as the number of cell layers. Sample types include:
Simple Epithelium: A simple epithelium has a single layer of cells.
Stratified Epithelium: A stratified epithelium has multiple layers of cells.
Likewise, the shape of the cells on the free surface can be:
Analogous to the shape of dice.
Analogous to the shape of bricks on an end.
Analogous to the shape of flat tiles on a floor.
By combining the terms for shape and layers, we can derive epithelial types such as stratified squamous epithelium or simple columnar epithelium.