Pseudopods
Pseudopods are finger-like projections of dark pigment (brown to black) at the periphery of the lesion. They have small knobs at their tips, and are connected to either a central pigment network or central pigmented blotch. To be considered pseudopods the bulbous knobs must have a maximum diameter that is larger than the network from which it originates. Menzies and colleagues found pseudopods to be one of the most specific features of superficially spreading melanoma. On histopathology, pseudopods correspond to junctional confluent nests of melanocytes at the periphery of the lesion. Confluence of nests is also a criterion in histopathology, typical for dysplastic nevi or malignant melanoma. Depending on which area of the pseudopod is sectioned on histology, the pathologist will see this structure either as a circular cluster of cells of varying diameters or as an oval-shaped cluster of cells. Only if the pathology section passes exactly through the longitudinal axis of a pseudopod will the pathologist be able to recognize this structure as an elongated tubular cluster of cells.
Needless to say, the statistical probability of this happening is very low unless of course the pathologist uses dermoscopy at the grossing table to orient and oversee the step sectioning process. On occasion, peripheral globules and pseudopods can look similar to each other at first glance. However, with peripheral globules one will observe a small clear space separating the globule from the main tumor mass.
These peripheral globules are presumed to correspond to junctional melanocytic nests at the tips of rete ridges and they are usually associated with growing/enlarging nevi. In contrast, pseudopods emanatefrom, and are directly connected to, the main tumor mass through a stalk. Radial streaming appears as radial parallel linear extensions at the periphery of the lesion. On histopathology, radial streaming also correlates with confluent junctional nests of pigmented melanocytes.
