Melanocytes in epidermis

Epidermal have round to oval, dark stained nuclei, smaller than those of basal keratinocytes. A clear halo of surrounding cytoplasm.

Melanocytes in hair

Melanocytes are diffentiated into

Ear

Melanins are found in Striae vascularis of Inner ear.

Eye

Melanocytes are also present in the Iris stroma in the front of Iris, Iris pigment epithelium on the back of Iris, Retinal pigment epithelium.

Adrenal glands

They are also seen in Medula and Zona reticularis.

Others

Melanin is also found in heart, liver, muscles and intestine.

Phaeomelonosomes

Eumelanosomes

Neuromelanin

1. Fontano-masson method

Melanin granules reduce ammoniacal silver nitrate to metallic silver [6–14].

2. Schmorl’s method

Melanin has ability to reduce Ferricyanide to Ferrocyanide which in presence of Ferrous ion forms Prussian blue [6–14].

3. Dopa-oxidase method [6–14]

This is the Most specific test, based on action of Dopa-oxidase upon Dopa.

4. Bleaching technique

Using strong oxidising agents like KMnO4 or Hydrogen peroxide [6–14].

5. Formaldehyde-induced fluorescence

Formalin fixation imparts a strong yellow fluorescence to unstained tissues with biogenic amines [6–14].

6. PAS (Periodic acid schiff)

Pseudomelanin (melanosis coli) is PAS +ve while true melanin is PAS –ve [6–14].

Melanogenesis

TYR Gene (located in the long arm of chromosome 11 i.e chr 11 q 14 – q 21) is involved in melanogenesis [13-17].

Genes involved in development of melanoblast

The genes/transcription factors involved in melanogenesis are Pax3, Sox10, Mitf, Edn3(endothelin 3), Ednrb (endothelin receptor B), Kit (c-Kit tyrosine kinase receptor), Kitl (Kit ligand/asSCF/steel factor) and Snai2 (also called as Slug) [13–17].

Sites of melanogenesis

Melanosomes are synthesised in [13–18]:

Steps of melanin synthesis

Stage 1

Melanosomes are spherical endosomal vacuoles lacking tyrosinase (TYR) activity (the main enzyme involved in melanogenesis) and have no internal structural components. No melanin is present yet. They contain the melanosomal protein Pmel17, which is sorted into intraluminal vesicles (ILVs) within the organelle. A partial clathrin coat is seen which is involved in sorting proteins into ILVs of vacuolar endosomes. The presence of Pmel17 gives rise to the structurally important intraluminal fibrils that characterise stage II melanosomes [13–20].

Stage 2

Melanosomes are ellipsoidal, around 0.5 micro-mm diameter. At this point, the presence and correct processing of Pmel17, an important melanosomal structural protein, determine the transformation of stage I melanosomes to elongated, fibrillar organelles known as stage II melanosomes. They contain MELANOSOMAL enzymes tyrosinase TYRP1. They exhibit minimal deposition of melanin. Melanin is deposited within cross-linked longitudinal filaments. Enzymes activity is localised in surrounding membrane. Tyrosinase and TYRP1 are traficking to melanosomes from early endosomes. They are present in tubular endosomal domain. Tyrosinase- and TYRP1-positive endosomal membranes have buds that are coated with the adaptor proteins AP1 or AP3, which play roles in sorting tyrosinase and TYRP1 to melanosomes [13–20].

BLOC1 and BLOC2 (protein complexes) plays important role in the regulation of endosome-to-melanosome transport.

Stage 3

Melanosomes are ellipsoidal. Melanin deposition increases by enzymatic activity and non-enzymatic polymerization. The pigment is uniformly deposited on the internal fibrils [13–20].

Stage 4

Melanosomes are ellipsoidal. Melanosome is fully developed and is filled with electron-opaque organelles. Melanin production is through polymerization.

Key protein involved in melanosome assembly is NCKX5, encoded by the gene SLC4A5 [13–20].

1. Genetics and endogenous factors

Migration, proliferation, differentiation and survival of melanocytes is influenced by tyrosine kinase receptor KIT and its ligand stem cell factors [7–9,11–16].

Microphthalmia transcription factor (MITF) is a nuclear protein involved in:

The two Rab27A effectors are Slac2-a (also called melanophilin) and Slp2-a; they are abundantly expressed on melanosomes and sequentially regulate melanosome transport in melanocytes. TGF-beta suppress melanogenesis by inhibiting activation of Pax3. P 53 promotes melanogenesis by suppressing TGF- beta, upregulation of alpha-MSH and KITL. E-cadherin (mediating cell-to-cell interaction) also regulates melanocytes. Dermal fibroblast is also involed in regulation of melanocytes. Notch signalling in melanocytes, an essential cell-to-cell interaction mechanism, regulates processes such as cell proliferation, cell fate decision, differentiation or stem cell maintenance [7–9,11–16].

2. Environmental or exogenous factors

3. Endocrine factors

The endocrinal products and substances responsible for melanogenesis are CRH (Corticotropin-releasing hormones), UROCORTINS (CRH-like neuro-peptide), POMC (Pro-opio-melanocortin), ACTH, alpha-MSH And beta-ENDORPHIN [7–9,11–16].

Mechanism of action of melanocytes

Melanocyte acts on Melanocyte-receptors thereby activating signaling pathways [7,9,11–16,19–21].

Role of melanocytes

Melanocytes or melanin [4,7,8,10,12–16,21,22].

Deleterious effects

In vitro studies, melanin reacts with DNA and is known to act as photosensitiser. In contrast to eumelanin, pheomelanin is prone to photodegradation and contribute to damaging effects of UVR because it can generate Hydrogen peroxide and superoxide anions and may cause mutations in melanocytes. Pheomelanin is associated with higher rates of apoptosis after UVR. Pheomelanin increases release of Histamine, thereby contributing to sun-induced erythema and oedema. Pheomelanin is UVA sensitiser that cause cell death [4,7,8,10,12–16,21,22].