Chapter 2 - Epithelium
Epithelia are sheets of cells that cover the external surface and line the internal surfaces of the body. They are classified by:
- Shape of the surface cells
- Whether it has a single or multiple layers of cells
All epithelia make a basement membrane that forms a boundary between it and the underlying connective tissue.
Epithelial cells are polarized:
- Apical surface- parts of cells or those cells next to the free surface or lumen
- Basal surface - parts of cells or those cells next to the basement membrane
- Lateral surfaces - parts of cells adherent to the sides of adjacent epithelial cells
It is not necessary to learn the names of specific tissues for this chapter, but rather learn to recognize variations in epithelia.
Simple Squamous Epithelium
A simple squamous epithelium consists of a single layer of flattened cells. It often occurs at sites of metabolite, fluid or gas exchange across or between cells.
Simple Cuboidal Epithelium
A simple cuboidal epithelium consists of a single layer of cells of similar height and width. It may simply serve as linings in ducts or be structurally adapted for secretion or absorption.
Simple Columnar Epithelium
A single columnar epithelium consists of a single layer of cells that are taller than they are wide. It is primarily associated with secretion and absorption.
Cilia and Microvilli
Lateral specializations between individual epithelial cells of an epithelium. Those near the apical surface are Known as the junctional complex by electron microscopy and terminal bar by light microscopy.
Examples of intercellular junctions in the simple columnar epithelium of the small intestine.
Pseudostratified Epithelium Epithelium
A pseudostratified columnar epithelium can appear stratified because: (1) not all cells reach the surface, (2) the nuclei appear at different levels and (3) the cells appear closely packed due to varying cellular shapes. However, it is a form of a simple columnar epithelium since all the cells rest on the basement membrane.
Stratified Squamous Epithelium
A stratified squamous epithelium has two or more layers of cells. Only the basal layer of cells rests on the basement membrane. Its name arises from the squamous appearance of the surface layer of cells.