Histology

Histology is the branch of biology and medicine that studies the microscopic structure of tissues. It analyzes how cells are organized, how they interact, and how the extracellular matrix forms tissues in both health and disease. In animals, tissues are commonly categorized into epithelial, connective, muscle, and nervous tissue, and the study of tissue architecture underpins both anatomy and pathology. Clinically, the term histology is often used interchangeably with histopathology, the study of tissue changes caused by disease.

The field originated with early microscopy in the 17th century and advanced in the 19th century with the development of staining techniques, especially hematoxylin and eosin, which provide contrasting colors for nuclei and cytoplasm. Modern histology combines light microscopy with a variety of specialized methods.

Typical histology workflow includes fixation to preserve tissue structure, dehydration and clearing, embedding in paraffin, thin sectioning with a microtome, and staining. In addition to H&E, tissues may be examined with special stains (e.g., PAS, Masson's trichrome) or subjected to immunohistochemistry to detect specific proteins. Electron microscopy reveals ultrastructural detail beyond light microscopy, and fluorescence or confocal microscopy enables labeled molecules to be visualized in three dimensions.

A tissue’s functional unit is its parenchyma, formed by the specialized cells, supported by the stroma, which includes connective tissue, vasculature, and ducts. Histology provides essential information for education, basic research, and clinical diagnosis, where microscopic examination of biopsies guides treatment decisions.