In multicellular organisms cells of similar structure and function come together to create tissues
. Complex organisms have many different cell types that fulfill different structural and physiological needs of the organism. There are four basic tissue types: epithelial
types share several common features:
- Cells are typically arranged into continous sheets (with the exception of glandular epithelium)
- The basal surface of the cells is attached to a basement membrane between the epithelium and underlying connective tissue
- The apical surface is free (not attached to other tissues)
Epithelial tissue is classified according to the number of cell layers
and the cell shape
of each tissue type. Epithelial cells can also be ciliated
(with small hairs) or unciliated
Classification of epithelium: Number of cell layers
Epithelium can be simple
(a single layer of cells) or stratified
(more than one layer of cells). In general, simple epithelium is much more delicate than stratified epithelium and occurs in areas of the body that are not exposed to much physical or mechanical stress. Stratified epithelium is much more durable and is found in places where the epithelium may contact and rub against other objects or body parts.
epithelium is simple epithelium that appears to be stratified because of the cell shape.
Classification of epithelium: Cell shape
The shape of epithelial cells also determines the type of epithelium.
cells are flattened and have an irregular shape, much like a pillow. They are most often found in layers (stratified) and when layered provide a lot of protection from mechanical stress.
Ex. The skin is made mostly of stratified squamous epithelium
cells are taller than squamous cells and are roughly cube-shaped.
Ex. The surface of the ovaries is simple cuboidal epithelium
cells are elongated and are attached to neighboring cells on their longest sides. Columnar cells are often ciliated and specialize in absorption. Becuase of their elongated shape, they do not withstand mechanical stress very well.
Ex. The walls of the trachea are lined with ciliated simple columnar epithelium