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histology, 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.

Histology (from the Greek ἱστός = 'tissue') is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining a thin slice of tissue under a light microscope. The ability to visualize or differentially identify microscopic structures is frequently enhanced through the use of histological stains. Histology is an essential tool of biology and medicine.

Histopathology, the microscopic study of diseased tissue, is an important tool in anatomical pathology, since accurate diagnosis of cancer and other diseases usually requires histopathological examination of samples. Trained medical doctors, frequently board certified as Pathologists, are the personnel who perform histopathological examination and provide diagnostic information based on their observations.

The trained scientists who perform the preparation of histological sections are histotechnicians, histology technicians (HT), histology technologists (HTL), medical scientists, medical laboratory technicians, or biomedical scientists. Their field of study is called histotechnology.

Technical procedure


Fixatives are used to preserve tissue from degradation, and to maintain the structure of the cells inclusive of sub-cellular components such as cell organelles (e.g., nucleus, endoplasmic reticulum, mitochondria). The most common fixative is 10% neutral buffered formalin (4% formaldehyde in phosphate buffered saline). Formaldehyde-based fixatives preserve tissues or cells by irreversibly cross-linking primary amino groups in proteins with other nearby nitrogen atoms in protein through a CH2 (methylene) linkage, thereby "fixing" proteins in the shape that they're in. This process destroys the biological functionality of proteins, and can also denature them to a certain extent. This can be detrimental to certain histological techniques.


Tissues must be impregnated with the material that will be used for embedding so that the tissue samples adhere to the embedding material, therefore the type of processing depends on the kind of embedding material that will be used. The most common histological technique is paraffin wax embedding, and many larger facilities utilize automated tissue processing machines. During this process samples are immersed in multiple baths of progressively more concentrated ethanol to remove the water from (dehydrate) the tissues, followed by a clearing agent, such as xylene, to remove the alcohol, and finally hot molten paraffin wax which replaces the xylene.


After the tissues have been dehydrated and impregnated with the embedding material they are ready for embedding. During this process the tissue samples are placed into moulds along with liquid embedding material which is then allowed to harden. The hardened blocks containing the tissue samples are then ready to be sectioned.

Embedding can also be accomplished using frozen, non-fixed tissue in a water-based medium. Pre-frozen tissues are placed into moulds with the liquid embedding material, usually a water-based glycol or resin, which is then frozen to form hardened blocks.


The tissue is then sliced into very thin (0.5–10 microns; 1000 microns = 1 mm) sections using a machine called a microtome. These slices, usually thinner than the average cell, are then placed on a glass slide for staining.

Frozen tissue embedded in a freezing medium is cut on a microtome in a cooled machine called a cryostat.


Routine staining: This is done to give contrast to the tissue being examined, as without staining it is very difficult to see differences in cell morphology. Hematoxylin and eosin (H&E) are the most commonly used stains in histology and histopathology. Hematoxylin colors nuclei blue; eosin colors the cytoplasm pink. To see the tissue under a microscope, the sections are stained with one or more pigments.

Special staining: There are hundreds of various other techniques that have been used to selectively stain cells and cellular components. Other compounds used to color tissue sections include safranin, oil red o, Congo red, fast green FCF, silver salts, and numerous natural and artificial dyes that were usually originated from the development dyes for the textile industry.

Histochemistry refers to the science of using chemical reactions between laboratory chemicals and components within tissue. A commonly performed histochemical technique is the Perls Prussian blue reaction, used to demonstrate iron deposits in diseases like hemochromatosis.

Histology samples have often been examined by radioactive techniques. In historadiography a slide (sometimes stained histochemically) is X-rayed. More commonly, autoradiography is used to visualize the locations to which a radioactive substance has been transported within the body, such as cells in S phase (undergoing DNA replication) which incorporate tritiated thymidine, or sites to which radiolabeled nucleic acid probes bind in in situ hybridization. For autoradiography on a microscopic level, the slide is typically dipped into liquid nuclear tract emulsion, which dries to form the exposure film. Individual silver grains in the film are visualized with dark field microscopy.

Recently, antibodies are used to specifically visualize proteins, carbohydrates, and lipids: this is called immunohistochemistry, or when the stain is a fluorescent molecule, immunofluorescence. This technique has greatly increased the ability to identify categories of cells under a microscope. Other advanced techniques, such as nonradioactive in situ hybridization, can be combined with immunochemistry to identify specific DNA or RNA molecules with fluorescent probes or tags that can be used for immunofluorescence and enzyme-linked fluorescence amplification (especially alkaline phosphatase and tyramide signal amplification). Fluorescence microscopy and confocal microscopy are used to detect fluorescent signals with good intracellular detail. Digital cameras are increasingly used to capture histological and histopathological image

Common laboratory stains

Stain Common use Nucleus Cytoplasm Red blood cell (RBC) Collagen fibers Specifically stains
Haematoxylin General staining when paired with eosin Blue N/A N/A N/A Nucleic acids—blue Blue eER (ergastoplasm)—blue
Eosin General staining when paired with haematoxylin N/A Pink Orange/red Pink Elastic fibers—pink Reticular fibers—pink
Toluidine blue General staining Blue Blue Blue Blue Mast cells granules—purple
Masson's trichrome stain Connective tissue Black Red/pink Red Blue/green Cartilage—blue/green Muscle fibers—red
Mallory's trichrome stain Connective tissue Red Pale red Orange Deep blue Keratin—orange Cartilage—blue Bone matrix—deep blue Muscle fibers—red
Weigert's elastic stain Elastic fibers Blue/black N/A N/A N/A Elastic fibers—blue/black
Heidenhains'azan trichrome stain Distinguishing cells from extracellular components Red/purple Pink Red Blue Muscle fibers—red Cartilage—blue Bone matrix—blue
Silver stain Reticular fibers, nerve fibers, fungi N/A N/A N/A Reticular fibers—brown/black Nerve fibers—brown/black
Wright's stain Blood cells Bluish/purple Bluish/gray Red/pink N/A Neutrophil granules—purple/pink Eosinophil granules—bright red/orange Basophil granules—deep purple/violet Platelet granules—red/purple
Orcein stain Elastic fibres Deep blue [or crazy red] N/A Bright red Pink Elastic fibres—dark brown Mast cells granules—purple Smooth muscle—light blue
Periodic acid-Schiff stain (PAS) Basement membrane, localizing carbohydrates Blue N/A N/A Pink Glycogen and other carbohydrates—magenta

Table sourced fromMichael H. Ross, Wojciech Pawlina, (2006). Histology: A Text and Atlas. Hagerstown, MD: Lippincott Williams & Wilkins.

The Nissl method and Golgi's method are useful in identifying neurons.

Alternative techniques

Alternative techniques include cryosection. The tissue is frozen and cut using a cryostat. Tissue staining methods are similar to those of wax sections. Plastic embedding is commonly used in the preparation of material for electron microscopy. Tissues are embedded in epoxy resin. Very thin sections (less than 0.1 micrometer) are cut using diamond or glass knives. The sections are stained with electron dense stains (uranium and lead) so that they can be seen with the electron microscope.


In the 19th century, histology was an academic discipline in its own right. The 1906 Nobel Prize in Physiology or Medicine was awarded to histologists Camillo Golgi and Santiago Ramon y Cajal. They had dueling interpretations of the neural structure of the brain based in differing interpretations of the same images. Cajal won the prize for his correct theory and Golgi for the staining technique he invented to make it possible.

Histological classification of animal tissues

There are four basic types of tissues: muscle tissue, nervous tissue, connective tissue, and epithelial tissue. All tissue types are subtypes of these four basic tissue types (for example, blood cells are classified as connective tissue, since they generally originate inside bone marrow).

  • Epithelium: the lining of glands, bowel, skin and some organs like the liver, lung, kidney
  • Endothelium: the lining of blood and lymphatic vessels
  • Mesothelium: the lining of pleural and pericardial spaces
  • Mesenchyme: the cells filling the spaces between the organs, including fat, muscle, bone, cartilage, and tendon cells
  • Blood cells: the red and white blood cells, including those found in lymph nodes and spleen
  • Neurons: any of the conducting cells of the nervous system
  • Germ cells: reproductive cells (spermatozoa in men, oocytes in women)
  • Placenta: an organ characteristic of true mammals during pregnancy, joining mother and offspring, providing endocrine secretion and selective exchange of soluble, but not particulate, blood-borne substances through an apposition of uterine and trophoblastic vascularised parts
  • Stem cells: cells able to turn into one or several of the above types

Note that tissues from plants, fungi, and microorganisms can also be examined histologically. Their structure is very different from animal tissues.

Related sciences

  • Cell biology is the study of living cells, their DNA and RNA and the proteins they express.
  • Anatomy is the study of organs visible by the naked eye.
  • Morphology studies entire organisms.


Artifacts are structures or features in tissue that interfere with normal histological examination. These are not always present in normal tissue and can come from outside sources. Artifacts interfere with histology by changing the tissues appearance and hiding structures. These can be divided into two categories:


These are features and structures that have being introduced prior to the collection of the tissues. A common example of these include: ink from tattoos and freckles (melanin) in skin samples.


Artifacts can result from tissue processing. Processing commonly lead to changes like shrinkage, color changes in different tissues types and alterations of the structures in the tissue. Because these are caused in a laboratory the majority of post histology artifacts can be avoided or removed after being discovered. A common example is mercury pigment left behind after using Bouin's fixative to fix a section.


1. Merck Source (2002). Dorland's Medical Dictionary. Retrieved 2005-01-26.

2. Stedman's Medical Dictionaries (2005). Stedman's Online Medical Dictionary Retrieved 2005-01-26.

3. 4,000 online histology images (2007). (

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