Aplasia cutis congenita with dermal melanocytosis.

Aplasia cutis congenita is a rare disorder in which a localised area of skin is absent at birth. It most commonly affects the scalp. Two types of aplasia cutis congenita are distinguished: nonmembranous and membranous. A 10-year-old girl with no medical history presented with an area of congenital alopecia. On physical examination, a greyish-bluish to brown-coloured alopecic atrophic patch, measuring 40 mm 9 20 mm of diameter, was observed on the vertex. There was no hair collar sign (Fig. 1a). Her parents described the appearance of the lesion at birth, as a roundish erosion covered by a thin transparent membrane that gradually increased in size and turned brown. Trichoscopy showed an alopecic plaque, of a grey-blue colour, brownish at the periphery and white in the centre with some telangiectatic vessels. An absence of follicular openings and a lack of skin appendages were noticed. In the margin, hair bulbs with dark pigmented proximal ends were visible through the translucent epidermis (Fig. 1b). Because of the uneven and irregular pigmentation, a diagnostic biopsy was performed to rule out an unusual melanoma. The patient and the parent consent were obtained. Histopathological examination revealed flattening of the epidermis with loss of the rete ridges and a lack of appendages. Elongated dendritic cells containing melanin granules were scattered in the dermis (Fig. 2a,b). These dermal cells were positive for HMB-45 on immunohistochemical staining, consistent with the presence of melanocytes in the dermis (Fig. 2c). All these findings led to the diagnosis of membranous scalp aplasia cutis congenita associated with dermal melanocytosis. Because of parental pressure for a therapeutic solution, the cicatricial plaque was completely removed. Membranous scalp aplasia cutis congenita presents as a small ovoid defect covered by a thin translucent membrane. The distribution of lesions along the fusion lines and the frequent association with hair collar sign suggest that such lesions may represent a forme fruste of neural tube defect. In our case, it was associated with dermal melanocytosis. This association is very rare. During normal embryonic development, melanoblasts originate from embryonic neural crest cells. After the closure of the neural tube, they migrate from the neural crest and differentiate into melanocytes. Their migration, proliferation and differentiation into melanocytes are dependent on mediators produced by cells of the ectoderm, dorsal neural tube and keratinocytes such as the family of glycoproteins Wnt. Therefore, a preceding ectodermal fusion defect may affect the migration and proliferation of melanoblasts by the activation of Wnt signalling pathway due to the lack of antagonist signalling molecules produced by neighbouring cells. Thus, dermal melanocytes fail to reach the epidermis during migration and persist in the dermis. In addition, the presence of glial cells in the dermis, suggests that the glial tissue containing melanocytes remained in the upper dermis as an embryonic remnant. Aplasia cutis congenita is characterised trichoscopically by an absence of follicular openings, a lack of skin appendages and a visible vascular network. Rakowska described radially arranged hair bulbs at the hair-bearing margin visible through a translucent epidermis. This aspect was (a)