Connective Tissue
Connective tissue consists of cells separated by varying amounts of
extracellular substance. In connective tissues cells typically account for only
a small fraction of the tissue volume. The extracellular substance consists of
fibres which are embedded in ground substance containing tissue fluid. Fibres
in connective tissue can be divided into three types:
collagen
fibres, reticular fibres and elastic fibres.
Extracellular Substance
Collagen fibres
Collagen fibres are the dominant fibre type in most
connective tissues. The primary function of collagen fibres is to add strength
to the connective tissue.
The thickness of the fibres varies from ~ 1 to 10 µm. Longitudinal
striations may be visible in thicker fibres. These striations reveal that the
fibres are composed of thinner collagen fibrils
(0.2 to
0.5 µm in diameter). Each of these fibrils is composed of microfibrils,
which are only visible using electron microscopy.
Microfibrils are assemblies of
tropocollagen,
which, in turn, is an spiral-like assembly of three collagen molecules (triple
helix). The organisation of the tropocollagen within the microfibrils is highly
regular. A small gap
(60 nm wide) is found between
the subsequent tropocollagens which form the microfibrils.
Staining solutions used in electron microscopy tend to fill in
these gaps, and the alignment of the gaps gives the microfibrils a
cross-striated appearance
(with 68 nm intervals)
in EM images.
Coarse collagen fibres are formed by type I
tropocollagen.
There are many different tropocollagen types around
(currently named type I to XXI). These types differ in their content of the
amino acids hydroxyproline and hydroxylysine. They also differ in the amount of
carbohydrates attached to the collagen molecules. The different types of
tropocollagen give the fibres the structural and functional features which are
appropriate for the organ in which the fibres are found. Types I, II and III
are the major fibre-forming tropocollagens. Tropocollagen type IV is an
important structural component of the basal lamina.
A tensile force of several hundred kg/cm2 is
necessary to tear human collagen fibres. The fibres stretch by only 15-20%.
Reticular fibres
Reticular fibres are very delicate and form fine networks instead of thick
bundles. They are usually not visible in histological sections but can be
demonstrated by using special stains. For example, in silver stained sections
reticular fibres look like fine, black threads - coarse collagen fibres appear
reddish brown in the same type of preparation.
Because of their different staining characteristics, reticular fibres were
initially thought to be completely different from collagen fibres.
Cross-striations with the same periodicity as in coarse collagen fibres are
however visible using electron microscopy. We now know that
reticular fibres consist of collagen - although the main type
of tropocollagen found in reticular fibres, type III, is different from that of
the coarse collagen fibres.
Reticular fibres give support to individual cells,
for example, in muscle and adipose tissue.
sections of
liver, spleen or lymph
nodes - reticulin
Liver - Reticulin Stain
The liver is one of the organs in which the cells are supported by a network
of reticular fibres. They appear as fine black lines in this silver stained
preparation. The fibres surround the individual sheets of liver cells
(hepatocytes) and are the only fibrous connective tissue component supporting
the cells. While providing support, the fine, open meshwork of reticular
fibres facilitates the exchange of substances between the hepatocytes and the
blood, which circulates in the irregularly shaped blood vessels (sinusoids)
between the hepatocytes. Reticular fibres are also present in the connective
tissue surrounding the larger vessels, which penetrate the parenchyma of the
liver.
Draw reticular fibres as they surround a nice piece of a row
of liver cells at high magnification - include a suitable scale and label
your drawing.
Blood will not be visible in some types of preparations
and the sinusoids appear empty.
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Elastic fibers
Elastic fibres are coloured in fresh tissues - they are light yellow - but
this colouration is only visible if large amounts of elastic fibres are present
in the tissue, for example, in the elastic ligaments of the vertebral column.
Special stains are necessary to show elastic fibres in tissue sections.
Resorcin fuchsin is one of these stains, which gives the
elastic fibres a dark violet colour.
Light microscopy does not reveal any substructure in the elastic fibres.
Electron microscopy shows that
elastic fibres consist of
individual microfibrils, which are embedded in an amorphous matrix. The
matrix accounts for about 90% of the fibre and is composed of the protein
elastin. Neither the elastin nor the microfibrils are
collagens.
Elastic fibres can be stretched to about 150% of
their original length. They resume their original length if the tensile forces
applied to the elastic fibres are relaxed.
Elastin is a somewhat odd protein in that its amino acid
sequence does not determine a specific three-dimensional structure of the
molecule. Instead, elastin remains unfolded as a "random coil".
Elastin molecules are cross-linked to each other by desmosin and isodesmosin
links, which are only found between elastin molecules. Tensile forces
straighten the cross-linked mesh of elastin coils.
sections of
blood vessels or skin - elastin
Skin, human - elastin & van Gieson or
Artery, human - elastin & eosin
Like reticular fibres, elastic fibres require special stains to be visualized.
Typically elastic fibres will appear as fine, dark violet and gently undulating
fibres in the tissue. Elastic fibres can form membranes - not unlike the
collagen membrane in the basal lamina of epithelia. This is the case at some
levels in the walls of blood vessels.
Collagen and elastic fibres intermingle in the dermis, i.e. the connective
tissue beneath the epithelium of the skin. Immediately beneath the epithelium
both fibre types are relatively fine - they appear much thicker in the deeper
parts of the dermis. At least the internal elastic lamina should be visible in
the smaller arteries which course through the dermis.
A combination with a second stain is necessary to visualize
other tissue components. Colours visible in the sections will depend on the
method used in combination with the elastin stain. Eosin gives an even pink or
red colour to many tissue components. Nuclei of cells remain unstained without
the inclusion of the haematoxylin in the staining solutions.
Identify the artery and the vein in the section. Their
walls contain large amounts of elastic fibres.
Blood vessels: draw a small section of the wall of a vessel,
preferably an artery, at high magnification. Identify elastic laminae, fine and
coarse elastic fibres in your drawing.
Skin: draw a small
section of the dermis - preferablyof a part of the dermis where both the very
fine and the coarse fibres are visible.
![thumbnail]( "Connective Tissue")
Title:
Connective Tissue
by:
om
at
2013-02-15T02:50:00+07:00
Rating: 4.8
of 5 Reviews
Connective Tissue