Chapter 6: Alterations to Components of the Epidermis and Dermis

An 8-year-old girl presents to her dermatologist with hypopigmented patches on the trunk. A biopsy of one of the lesions is performed. Immunohistochemical staining with Melan-A is illustrated below. Which entity on the clinician’s differential diagnosis is NOT supported by the biopsy findings?

A) Piebaldism

B) Vitiligo

C) Hypopigmented burn scar

D) Ash leaf spots of tuberous sclerosis

E) Waardenburg syndrome

The correct answer is D.

Leukoderma is a clinical term that designates disorders characterized by skin lightening. The term does not reflect the cause of hypopigmentation, which may be the result of decreased epidermal melanin content or secondary to a decreased blood supply to the skin. Hypomelanosis denotes reduced melanin pigment within the skin, whereas amelanosis refers to the total absence of melanin pigment. Depigmentation implies total loss of skin color, whereas pigmentary dilution describes a generalized partial lightening of the skin and hair.

Hypomelanosis can be classified into two histopathological categories: melanocytopenic or melanopenic. Melanocytopenic hypomelanosis is due to partial loss or total absence of epidermal and/or follicular melanocytes. The normal ratio of melanocytes to keratinocytes within the epidermis is approximately 1:10, with the exception of facial and genital skin, where it is about 1:4. This Melan-A stained biopsy exhibits a striking absence of melanocytes. Melanopenic hypomelanosis, on the other hand, is characterized by a normal number of epidermal and/or follicular melanocytes, but they either fail to synthesize normal amounts of melanin pigment or fail to transfer it to surrounding keratinocytes. All of the listed choices are examples of melanocytopenic hypomelanosis, with the exception of the ash leaf spots of tuberous sclerosis, which represent melanopenic hypomelanosis.

Piebaldism is an autosomal dominant condition characterized by congenital circumscribed patches of leukoderma and a white forelock, termed poliosis. Melanocytes are absent within involved sites. There are scattered areas of hyperpigmentation that contain a normal number of melanocytes. The condition results from mutations in the KIT proto-oncogene, which encodes a cell surface receptor tyrosine kinase important for the production of embryonic growth factors. Functional KIT receptor is required for melanocyte development both before melanoblast migration from the neural crest as well as postnatally.

Waardenburg syndrome (WS) is an autosomal dominant or less frequently autosomal ...