Histology & Its Methods of
Study
2015/16
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Introduction…
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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:
Epithelial tissue,
Connective tissue,
Muscular tissue, and
Nervous tissue.
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Epithelial tissue
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Introduction…
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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
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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
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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)…
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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)…
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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 )…
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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)…
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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)…
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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
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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
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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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