Fortunately, although there are many differentiated cell types, there are only four main categories of animal tissue. Each tissue type can be easily recognized in the light microscope, and looks surprisingly the same across the animal phyla. The term histology refers to the study of tissues. The general characteristics of tissues can be seen using the light microscope, but the electron microscope is needed to study how cells contact other cells. Even the electron microscope is not powerful enough to show us the individual proteins that form specialized cell junctions.
Epithelial tissues form linings and coverings.
The cells are attached to each other, they have a free surface and an attached surface, and they sit on an acellular basement membrane, about which I will say more later. If the cells are flat, the epithelium is referred to as squamous . This is typical of epithelia through which gases need to move, such as the capillaries that bring blood to the tissues or the alveoli of the lungs, which take up oxygen and release carbon dioxide. These squamous epithelia are specialized for diffusion. Some squamous epithelia have multiple layers, and are referred to as stratified .