Ectomesenchymal Developmental Field Disorders: A Conceptual Framework for Combined Cutaneous Malformations

Composite cutaneous malformations may comprise histologically distinct components that occur in an organized association within the same anatomical region. Their coexistence raises the possibility of a shared developmental disturbance, although it does not, in itself, establish that their constituent cells share a common origin or genetic alteration. In this article, we propose the concept of an ectomesenchymal developmental field disorder to describe spatially organized anomalies involving two or more tissue components whose association may plausibly be related to neural crest-derived progenitors, ectomesenchymal derivatives, or established embryonic interactions between these cell populations and neighboring tissues. Three mechanisms are considered, which may operate independently or overlap: disruption of a common progenitor, early somatic mosaicism, and reciprocal signaling between genetically distinct cell populations. We review representative models associated with GNAQ/GNA11 and PIK3CA, neurovascular interactions observed in certain plexiform neurofibromas, and interactions between the neural crest and placodes. We also propose a hierarchy of evidence, ranging from mere anatomical coexistence to molecular and functional demonstration of the underlying mechanisms. This framework complements the concepts of malformation, hamartoma, mosaicism, and syndrome, while emphasizing that neither ectomesenchymal origin nor a causal relationship can be established solely on the basis of morphology or the detection of a genetic variant in non-microdissected tissue.