5.1.1. General findings

Well-formed and vital-looking capillaries with a diameter of 8 to 16 m and with tubular configurations within intraluminal lesions were a common finding in BOOP. The number of the capillaries within the lesions varied from a few capillaries to the histological picture of granulation tissue with abundant capillarization. The capillarization also seemed to be an early event and took place already when only a fibrin clot consisting of a few cells had been formed. In contrast to this, there were hardly any well-capillarized intraluminal connective tissue lesions in UIP. In a few cases of UIP, well-formed capillaries were seen in the middle of the connective tissue lesions, but usually there were only short capillaries peripherally in the stalk of an intraluminal lesion close to the interstitium. In UIP, there were also numerous intraluminal lesions without any capillaries. Instead, in UIP there was an increased number of capillary structures in the areas of mural incorporating fibrosis.

The capillaries of the intraluminal lesions were randomly scattered without a connecting BM, unlike in the alveolar walls, where the BMs of the capillaries and epithelial cells formed a continuous linear staining pattern well demonstrated by the laminin and type IV collagen staining. In most BOOP cases, it was clearly demonstrated that the capillaries did not only run along the stalk into the fibrous lesion, but there were many capillary protrusions from the surrounding alveolar walls into the lesion as well. Sometimes the capillaries heading toward the center of the lesion formed an asterisk-shaped figure.

In BOOP and in UIP, both plump and slender endothelial cells were seen inside the connective tissue lesions. The immunoreactivity of the endothelial cells was stronger for von Willebrand factor than for CD34. The BMs of the capillaries were of variable thickness. At light microscopic level, there were no differences in the size, configuration, endothelial cells, or BMs of the capillaries between BOOP and UIP.

5.1.2. Number of capillaries in newly formed connective tissue

For the statistical analysis, the number of the capillary cross sections per square millimeter was counted, and the results are shown in Table 5. For all the stainings, counts were higher in BOOP than in UIP, and statistically significant (p < 0.003 for laminin,  < 0.001 for von Willebrand factor, and < 0.001 for CD34). All the stainings used in the study (laminin vs. von Willebrand factor; laminin vs. CD34, CD34 vs. von Willebrand factor) correlated with each other (p < 0.001 for all three correlations).

The intraobserver (p < 0.001) and interobserver correlations (p < 0.002) were highly significant.