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Histology of connective tissue
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The connective tissue maintains the form of the organs throughout the body
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g
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Transported to site of
action by blood
Hematopoietic stem cells
in bone marrow
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Transported to site of
action by blood
Hematopoietic stem cells
in bone marrow
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Cells of connective tissue
Fibroblasts
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Most common cell in connective tissue
Secrete collagen and elastin, GAGs multiadhesive glycoproteins and
proteoglycans for the ground substance
Fibroblasts can be divided into 2 categories:
- Fibroblasts – Actively transcribing fibroblasts with large, transparent
nucleus, abundant RER, irregular shape
- Fibrocyte – Quiescent, smaller, darker nucleus, little RER, spindle shape
Fibroblasts are involved in wound repair
- In adults they rarely undergo division, but can be sensitive to growth
factors if damage to organ is detected
- These fibroblasts are known as myofibroblasts

Medical application
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In tissues that divide poorly – e
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the heart – fibroblasts fill the spaces
caused by damages such as ischemia, forming dense, irregular scar tissue
Myofibroblasts have contractile function similar to smooth muscle
(because they have abundant actin) which helps wounds close up

Adipocytes
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Functions include cytoplasmic storage of lipids and neutral fats, production of
heat (especially in infants), and cushioning and insulating the skin and other
organs
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With alternating dark and light patches

Medical application
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Plasma cells produce specific antibodies that only bind to the antigen that
stimulated them, neutralising the antibody
Antibody-antigen complexes are removed by phagocytosis

Leukocytes
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White blood cells which cross into connective tissues in venules in a process
called diapedesis
Leukocytes increase at sites of inflammation
o This is because inflammation causes the release of chemicals e
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histamine from mast cells, which increase blood flow and vascular
permeability producing local swelling (oedema)
o This allows leukocytes to cross over from blood to site of inflammation
very easily

Connective tissue fibres
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Three main types:
o Collagen (formed from collagen protein)
o Reticular (formed from collagen protein)
o Elastic (composed mainly of elastin)

Collagen
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Family of proteins for structural functions – vary in rigidity, elasticity and
strength
Most abundant protein in human body (30% of dry weight)
More than 20 different types categorised into 4 categories
o Collagens forming long fibrils
o Fibril-associated collagens
o Collagens forming anchoring fibrils
o Collagens forming networks

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Main role
is structural maintenance in expandable organs

Type I collagen
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C – collagen bundles
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Taken from Mescher, Junquiera’s Basic Histology: Text and Atlas, 12th Ed
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Collagens that form anchoring fibrils
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Type VII collagen; main function is binding basal lamina of epithelial cells to
reticular fibres in underlying connective tissue

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Every third amino acid is glycine (needed for triple helix formation)
Lysines and pralines are hydroxylated to hydroxyproline and hydroxylysine by
prolyl and lysyl hydroxylases, plus co-factors ascorbic acid (Vit A), O2 and Fe2+
Some hydroxylysine residues are glycosylated
Three molecules of procollagen a chain assemble to form a triple helix
(procollagen); extension propeptides are found at either end of the three
chains (do not form part of the a-helix)
Procollagen transported via Golgi and exocytosed
Procollagen peptidases remove the extension propeptides
Self-assembly of procollagen into fibrils
Some collagen fibrils assemble into fibres, which are stabilised by FACIT
collagens and covalent crosslinks between collagen molecules (catalysed
by lysyl oxidase)
Other collagens synthesised in a similar manner

Collagen turnover and renewal
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Generally a slow process e
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stable in tendons and ligaments, but high in
periodontal ligament surrounding teeth
Firstly broken down by collagenases, then further degraded by proteases

Taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, Twelfth Edition

Taken from Proto-col, What is Collagen? Available at http://www
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com/blog/2014/07/collagen/

Reticular fibres

Medical application

Osteogenesis imperfecta
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Mutations in type I collagen genes; often result in glycine amino acid
substitutions (glycine required for triple helix formation)
Spontaneous fractures, cardiac insufficiency

Progressive systemic sclerosis
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Majority of organs show excessive accumulation of collagen (fibrosis),
esp
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e
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Mescher
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Twelfth Edition

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