histology

 

(hĭstŏl'əjē) , 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) is the study of tissue sectioned as a thin slice, using a microtome. It can be described as microscopic anatomy. The photographing of stained cells is called histography. Histology is an essential tool of biology.

Histopathology, the microscopic study of diseased tissue, is an important tool of anatomical pathology since accurate diagnosis of cancer and other diseases usually requires histopathological examination of samples.

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

Fixation

Main article: Fixation (histology)

Fixatives are used to preserve the tissue, the structures of the cell, and the cell organelles found in the individual cells (eg. nucleus, rough endoplasmic reticulum, mitochondria, and a lot more). The tissues are mechanically and biochemically stabilized in a fixative. The most common fixative is neutral buffered formalin (10% formaldehyde in Phosphate buffered saline (PBS)). It is important to consider that a fixative should not be too toxic to its handler, and it should not damage the tissue being preserved.

Processing

The most common technique is wax processing. The samples are immersed in multiple baths of progressively more concentrated ethanol to dehydrate the tissue, followed by a clearing agent such as, xylene or Histoclear, and finally hot molten paraffin wax (impregnation). During this 12 to 16 hour process, paraffin wax will replace the xylene:

Embedding

Soft, moist tissues are turned into a hard paraffin block, which is then placed in a mold containing more molten wax (embedded) and allowed to cool and harden.

Embedding can also be accomplished using frozen, non-fixed tissue in a freezing medium. This freezing medium is liquid at room temperature but when cooled will solidify. Non-fixed tissue allows for procedures such as in-situ hybridizations for specific mRNA that would have been destroyed during the fixing process. It also allows for very short turnaround where that is needed, as with a patient currently undergoing surgery.

Sectioning

The tissue is then sectioned into very thin (2 - 8 micrometer) sections using 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.

Staining

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 (abbreviated 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 which 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 can be combined with this, such as nonradioactive in situ hybridization 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 images.

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	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	N/A	Bright Red	Pink	Elastic fibres - Dark Brown Mast cells granules - purple Smooth Muscle - Light Blue
Periodic acid-Schiff stain (PAS)	Basement Membrane, Localising carbohydrates	Blue	N/A	N/A	Pink	Glycogen and other Carbohydrates - Magenta

Table sourced from Michael H. Ross, Wojciech Pawlina, (2006). Histology: A Text and Atlas. Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 0-7817-5056-3. 

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 micrometers) 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.

History

In the 19th century, histology was an academic discipline in its own right. The 1906 Nobel Prize in Physiology or Medicine was awarded to the 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.

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, and
• Stem cells: cells able to turn into one or several of the above types.

Note that tissues from plant, fungus and microorganisms can also be examined histologically. Their structure is very different from animal tissue.

Related sciences

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

Artifacts

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:

Pre-histology

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.

Post-histology

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.

References

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). (http://histology-online.com)

See also

• Pathology
• Anatomical pathology
• Histopathology
• Biological staining
• Important publications in histology
• Geoffrey Bourne

External links

• Histology Protocols
• Immunohistochemistry - In Situ Hybridization
• Histoweb
• SIU SOM Histology
• Visual Histology Atlas
• Histology Glossary
• Histology Group of Victoria Incorporated
• Histology Photomicrographs

Histology: epithelial tissue
Types	Columnar (simple, stratified) - Cuboidal (simple, stratified) - Pseudostratified/Respiratory - Squamous (simple, stratified) - Transitional - Olfactory
Features	Lateral/cell-cell: Tight junction - Adherens junction - Desmosome - Gap junction Basal/cell-matrix: Basal lamina - Hemidesmosome - Focal adhesion Apical: Cilia - Microvilli - Stereocilia

Histology: connective tissue
Classification	proper (loose/areolar, dense, adipose brown and white, reticular) embryonic (mucous, mesenchymal) specialized (cartilage, bone, blood)
Extracellular matrix	ground substance (tissue fluid) fibers (collagen, reticular fiber, elastic fibers)
Cells	resident (fibroblast, adipocyte, chondroblast, osteoblast), wandering cell

Histology: muscle tissue
skeletal muscle/general	epimysium, fascicle, perimysium, endomysium, muscle fiber (intrafusal, extrafusal), myofibril sarcomere (a, i, and h bands; z and m lines), myofilaments (thin filament/actin, thick filament/myosin, elastic filament/titin, nebulin), tropomyosin, troponin (T, C, I) costamere (dystrophin, α,β-dystrobrevin, syncoilin, synemin/desmuslin, dysbindin, sarcoglycan, dystroglycan, sarcospan), desmin neuromuscular junction, motor unit, muscle spindle, excitation-contraction coupling, sliding filament mechanism myoblast, satellite cell, sarcoplasm, sarcolemma, sarcoplasmic reticulum, T-tubule
cardiac muscle	myocardium, intercalated disc, nebulette
smooth muscle	calmodulin, vascular smooth muscle

Histology: nervous tissue
Neurons (gray matter)	soma, axon (axon hillock, axoplasm, axolemma, neurofibril/neurofilament), dendrite (Nissl body, dendritic spine) types (bipolar, pseudounipolar, multipolar, pyramidal, Purkinje, granule)
Afferent nerve/Sensory nerve/Sensory neuron	GSA, GVA, SSA, SVA, fibers Ia, Ib or Golgi, II or Aβ, III or Aδ or fast pain, IV or C or slow pain
Efferent nerve/Motor nerve/Motor neuron	GSE, GVE, SVE, Upper motor neuron, Lower motor neuron (Aα motorneuron, Aγ motorneuron) - Gamma motoneuron
Synapses	neuropil, synaptic vesicle, neuromuscular junction, electrical synapse - Interneuron (Renshaw)
Sensory receptors	Free nerve ending, Meissner's corpuscle, Merkel nerve ending, Muscle spindle, Pacinian corpuscle, Ruffini ending, Olfactory receptor neuron, Photoreceptor cell, Hair cell, Taste bud
Glial cells	astrocyte, oligodendrocyte, ependymal cells, microglia, radial glia
Myelination (white matter)	Schwann cell, oligodendrocyte, nodes of Ranvier, internode, Schmidt-Lanterman incisures, neurolemma
Related connective tissues	epineurium, perineurium, endoneurium, nerve fascicle, meninges

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