The nodules are connective tissue arranged in a whorled pattern, similar to that seen if an onion
is being cut in half. The macrophages that contain the silica quartz crystals are present at the
periphery of the nodule with an accumulation of collagen fibres in the center. With time, the collagen
fibres arranged themselves into concentric configurations and can become hyalinised over time.
The center of the nodule can as a result of this, become deprived of blood supply and therefore
also oxygen delivery and can result into necrosis of the tissue in the center of the nodule (11).Figure
4 represents a photo of a silicotic nodule. The number of nodules increases with the degree and
length of exposure respectively to silica. A typical silicotic nodule have the following characteristics:
Central zone = whorls of dense, hyalinised. (glassy or transparent) fibrous tissue, midzone =
concentrically arranged collagen fibres similar to onion skinning feature, periphery or outer zone =
randomly orientated collagen fibres, mixed with dust loaded macrophages and lymphoid cells.
Under polarized light microscopy, crystalline material can be observed in the center of the lesion
Ref.(11). The degree of restrictive lung disease is also related to the degree and length of
crystalline silica exposure. The number of nodules increases with the degree and length of
exposure.
Other pathological features of silicosis are that the pleura of the lungs can be adherent and
thickened. This is therefore the primary reason why workers diagnosed with silicosis will present a
lung function test outcome that indicate towards a pulmonary lung restrictive disease. Under
autopsy, the lungs may have a gritty feeling.
Theories related to the pathogenesis of silicosis
There are currently three theories relating to the pathogenesis of silicosis namely:
● Piezoelectric theory
● Solubility theory
● Theory regarding damage to the alveolar macrophage.
Attention will only be given to the last theory i.e, damage to the alveolar macrophage as this is
the theory that is currently the mostly accepted theory.
The influence of silica on the lung can be summarised in the following flow diagram (Figure 5).
Figure 5: The influences of silica exposure on alveolar macrophage in the Lung