Histology & Its Methods of Study

2015/16

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Introduction… Histology (Gr. histo, tissue, + logos,

study) is the study of the tissues of the body and of how these tissues are arranged to constitute organs.

Four fundamental tissues are recognized: 1. Epithelial tissue, 2. Connective tissue, 3. Muscular tissue, and 4. Nervous tissue.

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Epithelial tissue

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Introduction… Tissues are made of cells and extracellular

matrix, two components that were formerly considered separate entities.

The main functions formerly attributed to the extracellular matrix were to furnish mechanical support for the cells, to transport nutrients to the cells, and to carry away catabolites and secretory products. In addition to these functions, it is now known that cells not only produce extracellular matrix components but are also influenced by them

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Methods of Study The small size of cells and matrix

components makes histology dependent on the use of microscopes.

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Preparation of Tissues for Microscopic Examination

Sectioning Because tissues and organs are usually too

thick for light to pass through them, they must be sectioned to obtain thin, translucent sections.

The most common procedure used in the study of tissues is the preparation of histological sections that can be studied with the aid of the light microscope

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Preparation of Tissues (fixation)…

If a permanent section is desired, tissues must be fixed. To avoid tissue digestion by enzymes present within the cells (autolysis) or by bacteria and to preserve the structure and molecular composition, pieces of organs should be promptly and adequately treated before or as soon as possible after removal from the animal's body.

This treatment—fixation—can be done by chemical or, less frequently, physical methods. In chemical fixation, the tissues are usually immersed in solutions of stabilizing or cross-linking agents called fixatives (a buffered isotonic solution of 4% formaldehyde).

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Preparation of Tissues (dehydration and clearing)…

Water is first extracted from the fragments to be embedded by bathing them successively in a graded series of mixtures of ethanol and water (usually from 70% to 100% ethanol).

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Preparation of Tissues (Embedding )…

Tissues are usually embedded in a solid medium to facilitate sectioning. Embedding materials include paraffin and plastic resins. Paraffin is used routinely for light microscopy; resins are used for both light and electron microscopy.

The hard blocks containing the tissues are then taken to a microtome and are sectioned by the microtome's steel or glass blade to a thickness of 1–10 µm.

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Preparation of Tissues (staining)…

To be studied microscopically most sections must be stained. To be studied microscopically most sections must be stained. With few exceptions, most tissues are colorless, so observing them unstained in the light microscope is useless.

Tissue components that stain more readily with basic dyes are termed basophilic and those with an affinity for acid dyes are termed acidophilic.

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Preparation of Tissues (staining)…

Of all dyes, the combination of hematoxylin and eosin (H&E) is the most commonly used. Hematoxylin stains the cell nucleus and other acidic structures (such as RNA-rich portions of the cytoplasm and the matrix of hyaline cartilage) blue

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Resolution The critical factor in obtaining a crisp, detailed image

with the microscope is its resolving power, that is, the smallest distance between two particles at which they can be seen as separate objects. The maximal resolving power of the light microscope is approximately 0.2 µm; this power permits good images magnified 1000–1500 times. Objects smaller than 0.2 µm (such as a membrane or a filament of actin) cannot be distinguished with this instrument

The whole procedure, from fixation to observing a tissue in a light microscope, may take from 12 h to 21days, depending on the size of the tissue, the fixative, and the embedding medium.

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Things to be covered Epithelial tissue Connective tissue Nerve tissue Muscle tissue Circulatory system Lymphoid system Digestive system Respiratory system Integumentary system Urinary system Endocrine system Reproductive system

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