One of the unique characteristics of eukaryotic cells is the formation of tissues with division of labor, as different cells in a tissue may carry out different functions. For example, in the heart, some cells participate in the conduction pathways while others cause contraction; still others serve a supportive role, maintaining structural integrity of the organ. Epithelial tissues cover the body and line its cavities, providing a means for protection against pathogen invasion and desiccation.4 In certain organs, epithelial cells are involved in absorption, secretion, and sensation. Epithelial cells are tightly joined to each other to function as one cohesive unit as well as being joined to an underlying layer of connective tissue known as the basement membrane. Epithelial cells are highly diverse and serve numerous functions depending on the identity of the organ in which they are found. In most organs, epithelial cells constitute the parenchyma, or the functional parts of the organ.4 For example, nephrons in the kidney, hepatocytes in the liver, and acid-producing cells of the stomach are all composed of epithelial cells. Epithelial cells often have one side facing a lumen or the hollowed inside of an organ or tube, while the other side of the cell interacts with blood vessels and structural cells. For example, in the small intestine, one side of the cell will be involved in absorption of nutrients from the lumen, while the other side will be involved in releasing those nutrients into circulation for use in the rest of the body. We can classify different epithelia according to the number of layers they have and the shape of their cells. Simple epithelia have one layer of cells, stratified epithelia have multiple layers; and pseudostratified epithelia appear to have multiple layers due to differences in cell height, but are, in reality, only one layer.4 Turning to shape, cells may be classified as cuboidal, columnar, or squamous. As their names imply, cuboidal cells are cube-shaped and columnar cells are long and thin. Squamous cells are flat and scale-like. Connective tissue supports the body and provides a framework for the epithelial cells to carry out their functions.5 Connective tissues are the main contributors to the stroma or support structure whereas epithelial cells contribute to the parenchyma of an organ as mentioned earlier. Tendons, bone, blood, ligaments, adipose tissue, and cartilage are all examples of connective tissues. Most cells in connective tissues produce and secrete materials such as collagen and elastin to form the extracellular matrix.5
1) B Alberts, A. J. (2002). Molecular Biology of the Cell (4th ed.). New York: Garland Science.
2) O’Sullivan JM, P. D. (2013). The nucleolus: a raft adrift in the nuclear sea or the keystone in nuclear structure? Biomolecular Concepts, 277-86.
3) Hardin, J., Bertoni, G., & Kleinsmith, L. J. (2015). Becker’s World of the Cell (8th ed.). New York: Pearson.
4) Eurell, J. A., & al, e. (2006). Dellmann’s textbook of veterinary histology. Wiley-Blackwell.
5) Dorland, W. A. (2012). Dorland’s Illustrated Medical Dictionary (32 ed.). Elsevier.