Multiple Basaloid Follicular Hamartomas in Blaschko`S Lines with and without Extracutaneous Malformations: Towards a Unifying Concept

Background: Basaloid follicular hamartomas (BFHs) may occur as single isolated tumors, as localized tumors or as multiple tumors in a disseminated or in a patterned distribution. Non-hereditary multiple and mostly unilateral basaloid follicular hamartomas arranged according to Blaschko`s lines with associated extracutaneous malformations have been designated as a genetic mosaicism disease and one entity. The transition of BFH into basal cell carcinoma as well as the formation of concurrent extracutaneous malignancies has been published. Methods: We present a case of multiple and unilateral basaloid follicular hamartomas arranged according to Blaschko`s lines in multiple and anatomically separated skin areas but lacking extracutaneous malformations. Its clinical features are compared to published cases and discussed on the basis of recent findings in embryology. Results: We would propose the hypothesis that in multiple and unilateral BFHs with or without extracutaneous symptoms, the affected cell clone arises shortly before or during gastrulation and comprises only a small fraction of cells of the epiblast. The mutated cells are then displaced and mixed with normal cells by a collective whorl-like migration of epiblast cells as it has been observed by life-microscopy in chicken embryogenesis. This could explain the predominantly unilateral distribution of BFHs in Blaschko`s lines as well as the dispersal of mutated cells along the anterior-posterior axis. Conclusions: The proposed mechanism leading to multiple and mostly unilateral BFHs in Blaschko`s lines with or without extracutaneous symptoms might serve as a blueprint for other mosaicizm diseases with cutaneous symptoms and facultative extracutaneous malformations.