histology (hĭstŏlˈəjē) [key], study of the groups of specialized cells called tissues that are found in most multicellular plants and animals. Histologists study the organization of tissues at all levels, from the whole organ down to the molecular components of cells. Animal tissues are classified as epithelium, with closely spaced cells and very little intercellular space; connective tissue, with large amounts of intercellular material; muscle, specialized for contraction; and nerve, specialized for conduction of electrical impulses. Blood is also sometimes considered a separate tissue type. These types are combined in different ways in the organism to form characteristic organs. Plants are composed of relatively undifferentiated tissue known as meristematic tissue; storage tissue, or parenchyma; vascular tissue; photosynthetic tissue, or chlorenchyma; and support tissue, or sclerenchyma and collenchyma. A variety of techniques are used for histological studies, including tissue culture, use of various fixatives and stains, the use of a microtome for preparing thin sections, light microscopy, electron microscopy, and X-ray diffraction. The field is divided into developmental histology, the study of tissue formation and specialization in growing embryos; histophysiology, the study of relations between morphological changes and physiological activity; and histochemistry, the study of the chemical composition of tissue structures. Genetic histological methodology utilizes in-situ hybridization of DNA probes that enable analysis of specific genetic sequences and polymerase chain reactions are used to identify single DNA molecules. Immunocyochemistry produces labeled antibodies that attach to specific parts of specified molecules, often used to quantify the available amount of substances (e.g., enzymes and receptor proteins). Histological investigation includes study of tissue death and regeneration and the reaction of tissue to injury or invading organisms. Because normal tissue has a characteristic appearance, histologic examination is often utilized to identify diseased tissue.