A Hypothesis on the Morphologic Differences Between Unna and Miescher Nevi on the Head and Neck, Based on Embryologic Bases

Abstract

Unna and Miescher nevi show very different morphologic features. The main difference is that melanocytes involve mainly the papillary dermis in Unna nevi, whereas they widely penetrate the reticular dermis in Miescher nevi. The reason for this behavior is not totally understood, but anatomical location might play a role, since because Unna nevi are mainly found on the trunk, whereas Miescher nevi are mainly found on the face. We decided to test this hypothesis in relative easy way: dermis from the frontal, temporal, maxillary, and mandibullary regions derives from the neural crest, whereas the dermis of the parietal/occipital regions originates from the paraxial mesoderm (somites and somitomeres). Therefore, we studied the morphology of 137 acquired melanocytic nevi from the head and neck and classified their locations in 7 areas: occipital, temporal, parietal, frontal, face, high neck, and low neck. From such areas, we distinguished 4 groups: area A (parietal + occipital + low neck); area B (face + temporal + frontal + high neck); area 1 (parietal and occipital); and area 2 (temporal and frontal). In region A, 97.30% of the nevi were of Unna type. In region B, 89.00% were of Miescher type. Region A had 76.60% of Unna type nevi, whereas region B had 98.89% of Miescher nevi. In area 1, 100% of the nevi were of Miescher type. In area 2, 86.67% of the nevi were of Unna type. Region 1 had 86.67% of the cranial Unna nevi, whereas region 2 had 100% of the cranial Miescher nevi. Moreover, 90.9% of the nevi from the low neck were of Unna type. In the high neck, 20% of nevi were of Unna type. Finally, 90.90% of Unna nevi were in the low neck, whereas 80% of Miescher nevi were in the high neck. We concluded that these findings supported the hypothesis that the embryologic differences of these areas of head and neck might play a role in the morphology of Unna and Miescher nevi.

The High Multiplicity of Prenatal (Congenital Type) Nevi in Adolescents and Adults

“…Variations in nevus morphology were noted, as reported by others who studied nevi having features suggesting origin in utero despite being clinically “acquired” [37–40]. In particular, it was noted if nevi were flat or nodular [39].…”

“…Unna's nevi expand the papillary dermis, have a bumpy surface, are broader than deep, and have a predilection for the neck, trunk, and limbs. An embryologic basis for the difference from Miescher's nevi has been suggested [40]. 620.…”

“…1F), Unna's nevi (Fig. 1G), or an admixture of both Miescher and Unna type features [37–40]. Occasionally, a flat nevus was nodular in the center (Fig.…”

“…We suggest that the major mechanism may be the same process of transepidermal elimination by which inert substances lacking migratory properties (eg, foreign bodies, elastosis, collagenosis, fat necrosis) work their way up from reticular dermis to epidermis [50]. A contribution from Hochsteigerung is suggested by the earlier presentation of Unna's nevi that have reached the epidermis as compound nevi (average 28 years of age); compared to those that remain intradermal (average 37 years of age) and by the documented evolution of eruptive nevi from flesh-colored intradermal nevi to pigmented compound nevi [37][38][39][40][41][42].…”

“…Some nevi show an overlap of Unna's and Miescher's type features [39]. The difference between Unna's and Miescher's nevi may have an embryologic basis [40].…”