Sections View Full Chapter Figures Tables Videos Full Chapter Figures Tables Videos Supplementary Content +++ MELANOCYTES ++ Epidermal melanocytes are dendritic cells derived from neural crest cells Their migration and differentiation is influenced by a number of signaling molecules: steel factor (stem cell factor, c-Kit ligand), Wnt, bone morphogenic proteins (BMPs), and hepatocytes growth factor Provide melanin for 36 neighboring basal and spinous layer keratinocytes = one epidermal melanin unit Number and distribution are the same in all skin types and ethnicities Concentration and distribution/retention of melanosomes in keratinocytes cause different skin colors Types of melanin: Pheomelanin: red-yellow Eumelanin: brown-black Melanosomes Membrane-bound organelles, site of melanin synthesis and storage Found in melanocytes; they move from melanocytes to keratinocytes Types of melanosomes: Eumelanosomes: large, elliptical in shape and contain organized fibrillar glycoprotein matrix needed for eumelanin synthesis Pheomelanosomes: smaller, spherical in shape, loose fibrillar glycoprotein matrix Four stages of melanosome maturation: – Stage I melanosomes (premelanosomes) ▴ Found in the cytoplasm of melanocytes ▴ Amorphous matrix; contain unprocessed glycoprotein known as Pmel17 (gp100) – Stage II melanosomes ▴ Found in the cytoplasm of melanocytes ▴ Fully formed melanosome matrix, fibrils elongate and assemble into parallel sheets ▴ Contain tyrosinase ▴ No active melanin synthesis in eumelanosomes; melanin synthesis (not melanogenesis) in pheomelanosomes – Stage III melanosomes ▴ Found in the cytoplasm or dendrites of melanocytes ▴ Tyrosinase activity becomes positive and round or oval, electron dense, deposits of melanin found on the internal filament network ▴ Melanization begins at this stage – Stage IV melanosomes ▴ Found in the cytoplasm or dendrites of melanocytes ▴ Round or oval, electron opaque ▴ Fully melanized ▴ Possess melanin, no enzymatic activity Tyrosinase Cofactor: copper (Cu2+) Catalyzes two reactions – Hydroxylation of tyrosine to DOPA (dihydroxyphenylalanine)—this is the rate limiting step – Oxidation of DOPA to DOPAquinone +++ PIGMENTED LESIONS +++ Melasma ++ Melasma may be caused by the presence of more biologically active melanocytes in the affected skin rather than increase in melanocytes (Fig. 9-1) Predominantly affects Fitzpatrick skin types III and IV. Genetic and hormonal influences in combination with UV radiation May be precipitated by the following: oral contraceptive pills, pregnancy, thyroid dysfunction, cosmetics, and phototoxic or photoallergic medication reaction Clinical findings Brown reticulated hyperpigmented macules and patches, can be confluent or punctate Clinical subtypes Centrofacial: forehead, cheeks, upper lip, nose, and chin Malar: symmetrical involvement of the upper cheek area Mandibular: ramus of the mandible Depth may be epidermal, dermal, or mixed Diagnosis Wood's light (wavelength, 340–400 nm): identifies depth of pigment; epidermal pigment enhanced, dermal pigment is not Treatment Topical: broad spectrum sunscreen is cornerstone of therapy. Topical therapy with a triple combination agent appears to be the most clinically effective: hydroquinone, tretinoin, and topical steroid. Other commonly used, but less studied agents: azelaic acid, kojic acid, ascorbic acid, and arbutin Chemical peels: glycolic acid may be the most efficacious α-hydroxy peeling agent. Should be used in combination with daily use of depigmenting agent Device-based: carbon dioxide fractional ablative laser appears ... GET ACCESS TO THIS RESOURCE Sign In Username Error: Please enter User Name Password Error: Please enter Password Forgot Username? Forgot Password? Sign in via OpenAthens Sign in via Shibboleth Get Free Access Through Your Institution Contact your institution's library to ask if they subscribe to McGraw-Hill Medical Products. Access My Subscription GET ACCESS TO THIS RESOURCE Subscription Options Pay Per View Timed Access to all of AccessDermatologyDxRx 24 Hour $34.95 (USD) Buy Now 48 Hour $54.95 (USD) Buy Now Best Value AccessDermatologyDxRx Full Site: One-Year Individual Subscription $995 USD Buy Now View All Subscription Options