Melanoma depth of invasion, light micrograph. The thickness of melanoma is the single most important prognostic indicator. Clark level is a staging system that describes the depth of invasion of the dermis by melanoma. This light photomicrograph shows infiltrating clusters of tumour cells filling up the papillary dermis and approaching the interface with the reticular dermis (Clark level 3). The reticular dermis can be recognized by the horizontally oriented elastic fibres.