In the epithelium, the cell nuclei are clearly visible. The acidophilic (pink) areas surrounding each nucleus correspond to the cytoplasm of the epithelial cells. There is very little space between neighboring cells because they are closely packed together.
In the connective tissue, the nuclei are more widely separated from one another. The cytoplasm surrounding each nucleus is often thin and may be difficult to see. Much of the space between the cells is occupied by extracellular matrix, which contains numerous pink collagen fibers of different sizes and thicknesses. Some apparently empty spaces are also present. These spaces are most likely technical artifacts produced during tissue preparation, when components of the extracellular matrix shrink or retract.
Blood vessels: There are no blood vessels within the epithelium, whereas blood vessels can be found in the underlying connective tissue. The vessels supply the epithelium indirectly by allowing oxygen and nutrients to diffuse from the connective tissue to the epithelial cells.
The connective tissue layer that lies beneath the epithelia lining the internal cavities of the digestive, respiratory, and urinary systems is called lamina propria.
The combination of an epithelium and its underlying lamina propria that covers the moist surfaces of internal body cavities is called mucous membrane, commonly shortened to mucosa. Examples include the oral mucosa and gastric mucosa.
The epithelial layer that covers the outer surfaces of organs located within the major body cavities, such as the thoracic and abdominal cavities, is called mesothelium.
The epithelium that lines the inner surface of blood vessels is called endothelium.
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