Page 75 - Concise Pathology for Exam Preparation ( PDFDrive )

60 SECTION I General Pathology

Fibrous Structural Proteins

1. Fibrillar and nonfibrillar collagen
(a) Collagen provides the extracellular framework of most tissues in the body. All col-
lagen types have a triple helical structure. Three separate polypeptide chains are
braided into a rope-like triple helix.
(b) Collagen proteins are rich in hydroxyproline and hydroxylysine.
(c) Thirty different types are known.
(d) Some types (I, II, III and V) form fibrils by virtue of lateral cross-linking of the
triple helices.
(e) Cross-linking is the result of covalent bonding catalysed by the enzyme lysyl
oxidase in the presence of vitamin C.
(f) Nonfibrillar collagens form the basement membrane (type IV collagen) or are com-
ponents of structures like intervertebral discs (type IX collagen) or dermoepidermal
junction (type VII collagen).
2. Elastin
(a) Gives elasticity to tissues, allowing them to stretch when needed and then return
to their original state.
(b) Especially important in walls of large vessels, lungs, uterus, skin and ligaments.
(c) Elastins are synthesized by fibroblasts and smooth muscle cells.
(d) Morphologically, consist of a central core of elastin surrounded by a meshwork of
a fibrillar glycoprotein.

Water Hydrated Gels
(a) Provide compressibility to the tissue and serve as reservoirs of growth factors.
(b) Proteoglycans are comprised of long polysaccharides called glycosaminoglycans
(heparan sulphate and dermatan sulphate) linked to a protein backbone.
(c) Hyaluronan is comprised of disaccharide repeats without any protein core.

Adhesive Glycoproteins and Adhesion Receptors
Structurally, diverse molecules that are involved in cell-to-cell adhesion, linkage between
cells and ECM and binding between ECM components include
(i) Fibronectin (main constituent of interstitial matrix) is a protein that connects cells with
collagen fibres in the ECM, allowing cells to move through the ECM. Fibronectins bind
collagen and cell surface integrins, facilitating cell movement.
(ii) Laminin (main component of basement membrane) assists in cell adhesion and binds
cells to other ECM components such as type IV collagen and heparin sulphate.
(iii) Adhesion receptors are also known as cell adhesion molecules or CAMs. CAMs
are grouped into four categories, namely, immunoglobulins, cadherins, selectins and
integrins.

Q. Write briefly on cell and tissue regeneration.
Ans. The ability to regenerate is dependent on the type of cell.
• Labile tissues undergo continuous renewal.
• In stable tissues, the tissue regeneration occurs but is a limited process (with the excep-
tion of liver). Pancreas, adrenals, kidneys, thyroid and lungs have limited regenerative
capacity; however, as much as 40–60% of the liver may regenerate subsequent to its
loss. Regeneration is dependent on many variables, eg, growth factors, inhibitors, signal
transduction pathways, transcription factors and ECM proteins.
• TNF and IL-6 stimulate transition of the cells from G 0 to G 1 phase of cell cycle and
HGF and EGF family of factors help in progression through the rest of the cell cycle.
FGF and TGF-a are mitogenic for hepatocytes. Most epithelial cells share a common
receptor (EGFR—epidermal growth factor receptor) with intrinsic tyrosine kinase
activity.

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