Micropapillomatosis of the bronchial basement membrane: pathogenesis and types.

Fingerlike excrescences of the bronchial basement membrane may occur as a consequence of alterations in human bronchial mucosa. Such lesions contain capillary vessels and subepithelial connective tissue in their interior (capillar body phenomenon, micropapillomatosis). The papillae develop passively as a result of capillary vessels pushing towards the epithelium. The precondition for the formation of papillae is an alteration of the epithelial layer from pseudostratified to stratified epithelium. The broad lower layer of the basement membrane is thinned due to the piling-up of wide-lumened capillary vessels at the basement membrane. The capillary vessel increases in length and forms loops which push the basement membrane up into the epithelial layer. In this process the epithelial cells are pushed apart. Four different types of papillae can be distinguished, the forms of which may be correlated with particular disturbances in the texture of the epithelial layer. Papillomatosis of the bronchial basement membrane is not pathological as such. It can contribute to the diagnosis of bronchus biopsy material. In the case of scaled-off epithelium its presence is evidence for an alteration of the mucosa into stratified epithelium (basal cell hyperplasia, squamous metaplasia, textureless epithelial layer).