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Histology is the study of the tissues of the body and how these tissues are arranged to constitute organs. Cells are the smallest units of life that do not operate independently of one another. In complex organisms, cells groupe, live and work together in cell communities called tissues.

Despite its complexity, the organs of the human body are composed of only four basic (primary) tissue types : epithelial, connective, muscular, and ner­vous tissues. The basic tissues, each containing extracellular matrix (ECM) as well as cells, associate with one another in the variable proportions and morphologies characteristic of each organ. Connective tissue is characterized by cells producing very abundant ECM, this tissue connects cells and other tissues of the body, offering structural and metabolic support. It also serves as a medium of conveying nutrients to and removing waste from tissues and body organs ; muscle tissue is composed of elongated cells specialized for contraction and movement; nervous tissue is composed of cells with long, fine processes specialized to receive, generate, and transmit nerve impulses and  epithelial tissue contains layers of closely packed cells that cover the external and internal surfaces, including cavities and tubes of the body.