Page 508 - Textbook of Pathology, 6th Edition

492 Asbestos Disease
Asbestos as a mineral is known to mankind for more than
4000 years but its harmful effects have come to light during
the last few decades. Asbestos is a Greek word meaning
‘unquenchable’. In general, if coal is lot of dust and little fibrosis,
asbestos is little dust and a lot of fibrosis. Prolonged exposure
for a number of years to asbestos dust produces three types
of severe diseases: asbestosis of lungs, pleural disease and
tumours. In nature, asbestos exists as long thin fibrils which
are fire-resistant and can be spun into yarns and fabrics
suitable for thermal and electrical insulation and has many
applications in industries. Particularly at risk are workers
engaged in mining, fabrication and manufacture of a number
of products from asbestos such as asbestos pipes, tiles, roofs,
textiles, insulating boards, sewer and water conduits, brake Figure 17.31 Gross appearance of the lung in asbestosis, diagram-
matic appearance.
lining, clutch castings etc.
There are two major geometric forms of asbestos:
Serpentine consisting of curly and flexible fibres. It 6. Asbestos fibres are carcinogenic, the most carcinogenic
includes the most common chemical form chrysotile (white being crocidolite. There is high incidence of bronchogenic
asbestos) comprising more than 90% of commercially used carcinoma in asbestosis which is explained on the basis of
asbestos. the role of asbestos fibres as tumour promoters or by causing
Amphibole consists of straight, stiff and rigid fibres. It cell death of the airways so that it is exposed to the
includes the less common chemical forms crocidolite (blue carcinogenic effect of cigarette smoke. The development of
asbestos), amosite (brown asbestos), tremolite, anthophyllite and pleural mesothelioma in these cases is probably by carrying
actinolyte. However, the group of amphibole, though less of asbestos fibres via lymphatics to the pleura.
common, is more important since it is associated with MORPHOLOGIC FEATURES. As stated already, over-
induction of malignant pleural tumours, particularly in exposure to asbestos is associated with 3 types of lesions:
association with crocidolite. asbestosis, pleural disease and certain tumours.
However, in view of long term harmful effects of asbestos
SECTION III
exposure, it has been mostly replaced with synthetic mineral A. ASBESTOSIS. The gross pulmonary fibrosis caused
fibres such as fiberglass in developed countries since 1975 by asbestos exposure and histologic demonstration of
but it continues to be used in developing countries of the asbestos bodies on asbestos fibres is termed asbestosis.
world. Grossly, the affected lungs are small and firm with
cartilage-like thickening of the pleura. The sectioned
PATHOGENESIS. Overexposure to asbestos for more than surface shows variable degree of pulmonary fibrosis,
a decade may produce asbestosis of the lung, pleural lesions especially in the subpleural areas and in the bases of lungs
and certain tumours. How asbestos causes all these lesions (Fig. 17.31). The advanced cases may show cystic changes.
is not clearly understood but the following mechanisms have Histologically, the following changes are observed:
been suggested (Fig. 17.26,C): 1. There is non-specific interstitial fibrosis.
1. The inhaled asbestos fibres are phagocytosed by alveolar 2. There is presence of characteristic asbestos bodies in the
Systemic Pathology
macrophages from where they reach the interstitium. Some of involved areas (Fig. 17.32). These are asbestos fibres coated
the engulfed dust is transported via lymphatics to the pleura with glycoprotein and haemosiderin and appear beaded
and regional lymph nodes. or dumbbell-shaped. The coating stains positively for
2. The asbestos-laden macrophages release chemo-attractants Prussian blue reaction.
for neutrophils and for more macrophages, thus inciting
cellular reaction around them.
3. Asbestos fibres are coated with glycoprotein and
endogenous haemosiderin to produce characteristic beaded
or dumbbell-shaped asbestos bodies.
4. All types of asbestos are fibrogenic and result in interstitial
fibrosis. Fibroblastic proliferation may occur via macrophage-
derived growth factor such as interleukin-1. Alternatively,
fibrosis may occur as a reparative response to tissue injury
by lysosomal enzymes released from macrophages and
neutrophils or by toxic free radicals.
5. A few immunological abnormalities such as antinuclear
antibodies and rheumatoid factor have been found in cases Figure 17.32 Microscopic appearance of asbestos body. An
of asbestosis but their role in the genesis of disease is not asbestos body is an asbestos fibre coated with glycoprotein and
haemosiderin giving it beaded or dumbbell-shaped appearance with
clear. bulbous ends.