Connective
Tissue Cells
Connective tissue cells are usually divided into
two groups based on their ability to move within the connective tissue.
Fibrocytes (or fibroblasts) and fat cells are fixed
cells. Macrophages, monocytes, lymphocytes, plasma cells, eosinophils
and mast cells are wandering cells.
Fibrocytes
Fibrocytes are the most common cell type in
connective tissues. They are the "true" connective tissue cells.
Usually only their oval, sometimes flattened nuclei are visible in LM sections.
The cytoplasm of a resting (i.e. inactive) fibrocyte does not contain many
organelles. This situation changes if the fibrocytes are stimulated, for
example, by damage to the surrounding tissue. In this case the fibrocyte is
transformed into a fibroblast, which contains large amounts of the organelles
which are necessary for the synthesis and excretion of proteins needed to
repair the tissue damage. Fibrocytes do not usually leave the connective
tissue. They are, however, able to perform amoeboid movement.
The terms fibrocyte and fibroblast
refer here to the inactive and active cells respectively - at times you will
see the two terms used as synonyms without regard for the state of activity of
the cell.
Reticular cells
Reticular cells are usually larger than an average
fibrocyte. They are the "fibrocytes" of reticular connective tissue
and form a network of reticular fibres, for example, in the lymphoid organs.
Their nuclei are typically large and lightly stained (H&E) and the cytoplasm
may be visible amongst the cells which are housed within the network of
reticular fibres.
Adipocytes
Fat cells or adipocytes
are fixed cells in loose connective tissue. Their main function is (what surprise!) the storage of lipids. If "well
fed" the cytoplasm only forms a very narrow rim around a large central
lipid droplet. The flattened nucleus may be found in a slightly thickened part
of this cytoplasmic rim - if it is present in the section, which may not be the
case since the diameter of an adipocyte (up to 100 µm)
is considerable larger than the thickness of typical histological sections. A
"starving" adipocyte may contain multiple small lipid droplets and
gradually comes to resemble a fibrocyte.
Lipid storage/mobilisation is under nervous (sympathetic) and hormonal (insulin)
control. Adipocytes also have an endocrine function - they secrete the protein leptin which provides brain centers which regulate
appetite with feedback about the bodies fat reserves.
Leptin deficiency in experimental animals results in
obesity.
Adipocytes are very long-lived cells. Their number
is determined by the number of preadipocytes (or
lipoblast) generated during foetal and early postnatal development.
Macrophages
Macrophages arise from precursor cells called monocytes. Monocytes originate in the bone marrow from
where they are released into the blood stream. They are actively mobile and
leave the blood stream to enter connective tissues, where they differentiate
into macrophages. Macrophages change their appearance depending on the demand
for phagocytotic activity. Resting macrophages may be as numerous as
fibrocytes.
Resting macrophages are difficult to distinguish from
fibrocytes in H&E stained sections.
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sections of liver or lymph nodes - carbon injected animal - trichrome or H&E
sections of lung - H&E
In the lung, macrophages "patrol" the
respiratory surfaces and ingest airborne particles which settle there. They
"retire" to the connective tissue of the lung were they can be
identified by the accumulations of fine particles in their cytoplasm.
sections containing irregular connective connective
tissues - Alcian blue & van Gieson
In Alcian blue & van Gieson stained sections
macrophages may appear as intensely green, roundish cells with a dark nucleus.
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Liver, rabbit - ink injected, trichrome
Macrophages are usually difficult to distinguish from other cell types in
connective tissues. One way to visualize them is to inject an experimental
animal with very fine carbon particles. Macrophages which come into contact
with the circulating particles will phagocytose some of them. In sections the
particles will be visible as dark, black-brown accumulations in the cytoplasm
of the macrophages.
Draw a few macrophages in situ.
Macrophages found in the liver are also called Kupffer cells. They adhere to the epithelial lining
of the liver sinusoids, i.e. blood filled spaces between the liver cells.
Blood will not be visible in some types of preparations and the sinusoids
appear empty.
Once you have identified macrophages, go hunting for
some good collagen - in this trichrome stains the collagen fibres will appear
green(ish). Typically you will see them only in the connective tissue
surrounding larger blood vessels. Improve your knowledge on epithelia and
look out for ducts lined by a simple cuboidal or columnar epithelium.
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Title:
Connective Tissue Cells
by:
om
at
2013-02-15T02:52:00+07:00
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Connective Tissue Cells