Histogenesis of congenital and acquired melanocytic nevi: a unifying concept

Histogenesis of cutaneous malignant melanoma: The vast majority do not develop from melanocytic nevus but arise de novo as melanoma in situ

It has been considered that most cutaneous malignant melanomas arise from pre-existing melanocytic nevus. Many clinical and histopathological studies seem to support this concept. Dysplastic nevus originally proposed by Clark's group is a key entity of the intermediate lesion between benign nevus and melanoma. The latest edition of the WHO Classification of Skin Tumours (2018) has excluded Clark nevus (dysplastic nevus of mild atypia) from dysplastic nevus, the latter being now classified into the low- and high-grade by the degrees of nuclear atypia. The World Health Organization classification regards dysplastic nevus of both grades as the intermediate lesion between common acquired nevus and the radial growth-phase melanoma. An extensive genetic study recently performed by Bastian's group indicated the existence of intermediate lesions between nevus and melanoma. In spite of these findings, some investigators doubt the concept of the intermediate lesions including dysplastic nevus and insist that the majority of melanomas arise de novo as melanoma in situ, not in association with a preceding nevus. The concept of de novo genesis of melanoma may be supported by a recent meta-analysis study revealing that 71% of melanomas likely arose de novo and 29% from pre-existing nevus. In this review article, from the viewpoint of de novo genesis of melanoma, the author critically discusses the relevant findings of melanoma genesis and proposes a new framework to interpret the morphological and genetic data alternatively. Clarification of the oncogenic process of melanoma has great impact not only on clinical dermatology but also on basic oncology.

A Unifying Concept of Uveal Pigment Cell Distribution and Dissemination Based on an Animal Model: Insights into Ocular Melanogenesis

Pigmented cells are derived from neural crest cells, which migrate along the peripheral nerve sheets into their specific final region. During their migration, cells progressively acquire pigment-producing capabilities, maturation, and the shape of melanocytes. These insights, along with specific clinical characteristics of melanocytic nevi, have led to new concepts of cutaneous, periocular, and iris nevogenesis. To further elucidate the specific ocular embryogenic melanoblast distribution and dissemination - that could explain the distinct distribution of uveal melanocytic neoplasms - we investigated the ocular pigmentation of dogs affected by a specific mutation called Merle, which results in either pigment- (wild type) or non-pigment- (mutated type) producing cells. Based on our observations, we propose a unifying concept of uveal pigment cell distribution and dissemination, which postulates melanoblast migration and maturation following the trigeminal V1 branch and, later, their entrance into the eye along the ciliary nerves and their finest iris branches. Our concept provides an explanation not only for the specific distribution of ocular melanocytic lesions, including uveal and iris nevi, but also for the different locations depending on the metastatic potential of the ocular melanoma. Though speculative, the higher metastatic potential of posterior uveal melanomas compared to iris melanomas may be related to a less differentiated stage in the maturation of migrating melanocytes in the posterior segment compared to the anterior segment of the eye. However, there is a need of further studies focusing on cell differentiation markers of melanocytes at different locations in the eye.

Giant congenital melanocytic nevus

Giant congenital melanocytic nevus is usually defined as a melanocytic lesion present at birth that will reach a diameter ≥ 20 cm in adulthood. Its incidence is estimated in <1:20,000 newborns. Despite its rarity, this lesion is important because it may associate with severe complications such as malignant melanoma, affect the central nervous system (neurocutaneous melanosis), and have major psychosocial impact on the patient and his family due to its unsightly appearance. Giant congenital melanocytic nevus generally presents as a brown lesion, with flat or mammilated surface, well-demarcated borders and hypertrichosis. Congenital melanocytic nevus is primarily a clinical diagnosis. However, congenital nevi are histologically distinguished from acquired nevi mainly by their larger size, the spread of the nevus cells to the deep layers of the skin and by their more varied architecture and morphology. Although giant congenital melanocytic nevus is recognized as a risk factor for the development of melanoma, the precise magnitude of this risk is still controversial. The estimated lifetime risk of developing melanoma varies from 5 to 10%. On account of these uncertainties and the size of the lesions, the management of giant congenital melanocytic nevus needs individualization. Treatment may include surgical and non-surgical procedures, psychological intervention and/or clinical follow-up, with special attention to changes in color, texture or on the surface of the lesion. The only absolute indication for surgery in giant congenital melanocytic nevus is the development of a malignant neoplasm on the lesion.

A hypothesis on the morphologic differences between Unna and Miescher nevi on the head and neck, based on embryologic bases

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.

Rapidly lethal metastatic melanoma arising from a large congenital melanocytic naevus

A case of fatal metastatic melanoma arising from a very large congenital melanocytic naevus (VLCMN) is reported. Large congenital naevi (LCMN) are naevi >20 cm in diameter. VLCMN is used in this report to mean an extensive LCMN involving a large percentage of the body, including smaller so-called satellite naevi. A 19-year-old man with a large congenital melanocytic naevus (LCMN) presented with a new nodule on the left chest wall, which was diagnosed as a thick melanoma with synchronous axillary lymph-node metastasis. The patient developed widespread distant metastasis within weeks after surgical resection of the primary site and lymph-node basin, which was unresponsive to systemic chemotherapy and whole brain radiation therapy, and he died 7 months after diagnosis of the primary melanoma.