Classification of Epithelia

The epithelial tissue is classified based on the number of layers that they possess and their shape.

For the number of layers; If the number of layers, n=1 then it is simple. If the number of layers, n>1, then it is striated. For pseudostratified(pseudo means false), it is actually a single layer, that 'appears' like as if it has more than one layer.

For the shape; Squamos(from the Latin word squama which means scale) describes flat, thin epithelial cells. Cuboidal describes epithelial cells that have a cube-like shape. Columnar on the other hand describes tall, elongated cells with a stretched nucleus.

Characteristics of Epithelial Tissue

The cells of the epithelial tissue have what we call 'polarity'. This means that the cells have both an apical and basal surface. The basal surface of the cell is attached to the basal lamina of the basement membrane. Note that the apical surface may bear microvilli(microscopic cellular membrane protrusions that increase the area of a cell) and cilia(latin for 'eyelashes' which are hair-like structures that help moves the cell around or tiny substances around the cell).


The cells of the epithelial tissue are also closely packed. These cells are held together by intercellular connections. If you notice the diagram above, you find that the epithelial tissue is supported by the reticular lamina(a thin extracellular layer that lies below the basal lamina which comprises mainly of collageneous fibers).


The epithelial tissue is avascular. It also rapidly divides via mitosis, thus, damaged or dead cells are quickly(not so fast but fast enough) replaced by new ones.

Basic Tissues

A tissue is a level of organization that is the intermediate of a cell and organ. A group of cells that are not necessarily identical but work together as an assemblage are organized into tissues. Tissues are grouped into four basic types and they are the epithelial, connective, nervous and muscle(note that for muscle we have the skeletal, smooth and cardiac muscle tissue) tissue. These tissues associate and interact with each other to form organs that carry out specialized functions.

The following is a diagram illustrating the various tissue types in the body: