FIGURE 2.

Plasma exudation at sites of epithelial loss‐regeneration: In vivo in guinea‐pig airways, with human airway‐like pseudostratified epithelium and rich subepithelial microcirculation, it has been demonstrated that the mere loss of epithelial cells, without bleeding and without injury to the basement membrane, causes prompt exudation of bulk plasma. The exudation creates and maintains a biologically active, physical fibrin‐fibronectin barrier gel restricted to the area of denudation—the plasma‐derived molecular milieu also promotes speedy epithelial regeneration in which all types of pseudostratified epithelial cells participate as stem cells. The epithelial regeneration milieu is continuously supplied with exuded plasma until the basement membrane has received a new cellular cover of poorly differentiated epithelial regeneration cells derived from ciliated, secretory and basal cells bordering the desquamated site. Hence, whether viral infection is the cause of sloughing of pseudostratified epithelium or not, vulnerable sites of epithelial regeneration would be well protected physically and biologically by exuded plasma. Yellow colour represents plasma proteins. Abbreviations: DCC = Dedifferentiating ciliated cell internalizing or shedding its cilia, flattening, and migrating; DRC = Dedifferentiated mesenchymal‐like regeneration cells migrating speedily; DSC = Dedifferentiating secretory cell releasing its secretory granules, flattening, and migrating; IEG = Venular inter‐endothelial gaps through which non‐sieved plasma proteins extravasate. Modified from references 11, 12 and 13