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Histology is that branch of anatomy that studies tissues of animals and plants. This textbook, however, discusses only animal, and more specifically human, tissues. In its broader aspect, the word histology is used as if it were a synonym for microscopic anatomy, because its subject matter encompasses not only the microscopic structure of tissues but also that of the cell, organs, and organ systems.

The body is composed of cells, intercellular matrix, and a fluid substance, extracellular fluid (tissue fluid), which bathes these components. Extracellular fluid, which is derived from plasma of blood, carries nutrients, oxygen, and signaling molecules to cells of the body. Conversely, signaling molecules, waste products, and carbon dioxide released by cells of the body reach blood and lymph vessels by way of the extracellular fluid. Extracellular fluid and much of the intercellular matrix are not visible in routine histological preparations, yet their invisible presence must be appreciated by the student of histology.

The subject of histology no longer merely deals with the structure of the body; it also concerns itself with the body’s function. In fact, histology has a direct relationship to other disciplines and is essential for their understanding. This textbook, therefore, intertwines the disciplines of cell biology, biochemistry, physiology, embryology, gross anatomy, and, as appropriate, pathology. Students will recognize the importance of this subject as they refer to the text later in their careers. An excellent example of this relationship will be evident when the reader learns about the histology of the kidney and realizes it is the intricate and almost sublime structure of that organ (down to the molecular level) that is responsible for the kidney’s ability to perform its function. Alterations of the kidney’s structure are responsible for a great number of life-threatening conditions.