What characteristic primarily differentiates simple squamous epithelium?

Simple squamous epithelium is primarily differentiated by its flat cell shape. This unique morphology allows it to perform functions that are essential in various biological processes. The thin, flat structure of the cells minimizes the distance over which substances must diffuse, making this type of epithelium particularly effective for processes like filtration and diffusion. For example, in the lungs, the simple squamous epithelium facilitates the exchange of oxygen and carbon dioxide between the air in the alveoli and the blood in the capillaries. In addition, the flattened shape provides a smooth surface that reduces friction, which is beneficial in regions where movement occurs, such as pleural and peritoneal cavities.

The other characteristics mentioned in the choices do not accurately define simple squamous epithelium. A thick cellular structure is more indicative of stratified epithelia, which consist of multiple layers. The ability to secrete hormones is a feature of glandular epithelium rather than simple squamous epithelium. Lastly, while epithelial tissues are related to connective tissues, simple squamous epithelium primarily serves as a barrier or interface rather than playing a supportive role in connective tissue.