Melanosome Distribution in Keratinocytes in Different Skin Types: Melanosome Clusters Are Not Degradative Organelles

Melanin within keratinocytes is devoid of melanosome markers.

“melanin within keratinocytes was shown by us and the group of Graça Raposo to be devoid of melanosome markers [ 18 , 19 ].”

Melanin-containing organelles display a similar size in light and dark skins.

“even though melanin-containing organelles display a similar size in light and dark skins [ 18 , 88 , 89 , 90 , 91 ]”

Melanin accumulates in moderately acidic compartments with low degradative capacity, as shown by low Lysotracker and DAMP accumulation and low DQ-BSA fluorescence, despite the abundance of these organelles within keratinocytes.

“Moreover, we and Graça Raposo’s group showed that melanin accumulates in moderately acidic compartments (presenting low levels of Lysotracker and DAMP [3-(2,4-dinitroanilino)-3′-amino-N-methyldipropylamine] accumulation) with low degradative capacity, as shown by the low levels of dye quenched-bovine serum albumin (DQ-BSA) fluorescence, despite the abundance of these types of organelles within ker”

MKSs are likely unique melanin-storage compartments presenting low acidity and degradative capacity but maintaining characteristics of LEs/lysosomes, including membrane markers LAMP1/2 and CD63.

“MKSs are likely unique melanin-storage compartments presenting low acidity and degradative capacity but maintaining characteristics of LEs/lysosomes, including the membrane markers LAMP1/2 and CD63 [ 18 , 27 ]”

Melanocores are observed in the interstitial space between melanocytes and keratinocytes.

“More recently, we and others provided further evidence supporting this model, in which melanocores were observed in the interstitial space between melanocytes and keratinocytes [ 18 , 19 ].”

In light skins, melanin is present in clusters.

“In light skins, melanin is present in clusters, whereas dark skins predominantly contain single melanin granules that are usually larger”

Melanin is not found in double membrane organelles (autophagosomes) within keratinocytes.

“Notwithstanding these observations, melanin is not found in double membrane organelles, i.e., autophagosomes, within keratinocytes”

Melanin organization within keratinocytes differs between phototypes.

“Importantly, it is well established that melanin organization within keratinocytes differs between phototypes [ 18 , 88 , 89 , 90 , 91 ].”

Melanin-containing compartments lack the classical autophagosomal protein LC3.

“and melanin-containing compartments lack the classical autophagosomal protein LC3 [ 18 , 104 ].”

In dark skins, melanin predominantly consists of single melanin granules that are usually larger.

“In light skins, melanin is present in clusters, whereas dark skins predominantly contain single melanin granules that are usually larger”

Melanin is found in single membrane compartments inside keratinocytes.

“Indeed, melanin is found in single membrane compartments inside keratinocytes, supporting exo-/phagocytosis or the fusion of melanocyte and keratinocyte membrane transfer models [ 17 , 18 , 19 ]”

LAMP1 and CD63 are present in the membrane of single and clustered melanocores, independently of skin phototype.

“Graça Raposo’s group showed that, independently of the skin phototype, the lysosomal-associated membrane protein 1 (LAMP1) and the tetraspanin cell surface antigen 63 (CD63)—markers of late endosomes (LEs)/lysosomes—are present in the membrane of single and clustered melanocores [ 18 ]”