Fibroblast foci are not discrete sites of lung injury or repair: the fibroblast reticulum

Abstract

Background: Usual interstitial pneumonia (UIP), the pathologic correlate of idiopathic pulmonary fibrosis, contains characteristic discrete areas of fibroblasts, myofibroblasts, and newly formed collagen, termed "fibroblast foci." These lesions are argued to represent isolated sites of recurrent acute lung injury and suggested to be the mechanism of disease progression. We hypothesized that, rather than isolated, these lesions are part of an organized neoplasm.

Methods: Morphometric analysis of pentachrome-stained histologic sections of UIP was performed. Using point-counting technique on serial sections, fibroblast foci, arteries, and macrophage clusters were identified and we determined their individual "connectiveness" by estimating the Euler number. Two-dimensional micrographs were collated into a three-dimensional array from which a visual three-dimensional reconstruction could be constructed. Clonality analysis was performed using human androgen receptor gene methylation assay.

Results: Blood vessels show significant connectivity with a Euler number of 2, whereas macrophage clusters exhibited no connectivity. The fibroblast foci showed a high level of interconnection with Euler numbers ranging from 19 to 39. The computer generated three-dimensional models provide a visual confirmation of this connectiveness. Human androgen receptor gene methylation assay analysis of the foci showed balanced methylation consistent with polyclonality.

Conclusions: The fibroblast foci of UIP are the leading edge of a complex reticulum that is highly interconnected and extends from the pleura into the underlying parenchyma. It is a reactive, rather than a malignant, process.

Figure 1.

Pentachrome-stained sections of usual interstitial pneumonia (UIP) are shown at 100× (A), with a high power (400×) view of a fibroblast foci (B). There is heterogeneous deposition of mature collagen (yellow) throughout the parenchyma with associated architectural distortion and focal deposits of young, immature collagen (blue-green) within the subepithelial interstitium. Pentachrome-stained sections of cryptogenic organizing pneumonia (COP) are shown at 100× (C), with a high power (400×) view of a Masson body (D). In contrast to the sections of UIP, there is minimal mature collagen deposition (yellow), and only young immature collagen (blue-green) is present within airways.

Figure 1.

Pentachrome-stained sections of usual interstitial pneumonia (UIP) are shown at 100× (A), with a high power (400×) view of a fibroblast foci (B). There is heterogeneous deposition of mature collagen (yellow) throughout the parenchyma with associated architectural distortion and focal deposits of young, immature collagen (blue-green) within the subepithelial interstitium. Pentachrome-stained sections of cryptogenic organizing pneumonia (COP) are shown at 100× (C), with a high power (400×) view of a Masson body (D). In contrast to the sections of UIP, there is minimal mature collagen deposition (yellow), and only young immature collagen (blue-green) is present within airways.

Figure 1.

Pentachrome-stained sections of usual interstitial pneumonia (UIP) are shown at 100× (A), with a high power (400×) view of a fibroblast foci (B). There is heterogeneous deposition of mature collagen (yellow) throughout the parenchyma with associated architectural distortion and focal deposits of young, immature collagen (blue-green) within the subepithelial interstitium. Pentachrome-stained sections of cryptogenic organizing pneumonia (COP) are shown at 100× (C), with a high power (400×) view of a Masson body (D). In contrast to the sections of UIP, there is minimal mature collagen deposition (yellow), and only young immature collagen (blue-green) is present within airways.

Figure 1.

Pentachrome-stained sections of usual interstitial pneumonia (UIP) are shown at 100× (A), with a high power (400×) view of a fibroblast foci (B). There is heterogeneous deposition of mature collagen (yellow) throughout the parenchyma with associated architectural distortion and focal deposits of young, immature collagen (blue-green) within the subepithelial interstitium. Pentachrome-stained sections of cryptogenic organizing pneumonia (COP) are shown at 100× (C), with a high power (400×) view of a Masson body (D). In contrast to the sections of UIP, there is minimal mature collagen deposition (yellow), and only young immature collagen (blue-green) is present within airways.

Figure 2.

Computer-generated three-dimensional reconstructions of (A) pleural surface (yellow) and blood vessels (red) and (B) with the fibroblast reticulum added (green).

Figure 2.

Computer-generated three-dimensional reconstructions of (A) pleural surface (yellow) and blood vessels (red) and (B) with the fibroblast reticulum added (green).

Figure 3.

Computer-generated three-dimensional reconstructions of UIP (A) and COP (B). In this rendering, fibroblastic tissue is white, blood vessels are red, and the pleural surface is brown.

Figure 3.

Computer-generated three-dimensional reconstructions of UIP (A) and COP (B). In this rendering, fibroblastic tissue is white, blood vessels are red, and the pleural surface is brown.

Figure 4.

Clonality analysis based on human androgen receptor gene methylation assay polymerase chain reaction of DNA from HEC1A; endometrial adenocarcinoma cells are presented with and without digestion by HhaI. A fibroblast focus of UIP, uninvolved (normal) lung, and a Masson body of COP, are presented after digestion by HhaI. In UIP and COP, two parental alleles are present after HhaI digestion representing the balanced methylation pattern and a polyclonal population of cells. In the uninvolved (normal) lung, both alleles are amplified equally after HhaI digestion, excluding the possibility of unequal lyonization of normal tissue.