Skin melanocytes: biology and development.

In the human skin, melanocytes are present in the epidermis and hair follicles. The basic features of these cells are the ability to melanin production and the origin from neural crest cells. This last element is important because there are other cells able to produce melanin but of different embryonic origin (pigmented epithelium of retina, some neurons, adipocytes). The life cycle of melanocyte consists of several steps including differentiation of melanocyte lineage/s from neural crest, migration and proliferation of melanoblasts, differentiation of melanoblasts into melanocytes, proliferation and maturation of melanocytes at the target places (activity of melanogenic enzymes, melanosome formation and transport to keratinocytes) and eventual cell death (hair melanocytes). Melanocytes of the epidermis and hair are cells sharing some common features but in general they form biologically different populations living in unique niches of the skin.

Fas and FasL expression on cells of two transplantable melanoma lines according to their different biological properties.

Fas and FasL interaction induces apoptotic cell death. In immunocompetent cells it plays a crucial role in the effector functions of the cells and in the regulation of host immune response. In tumours (e.g. melanoma), FasL expression possibly counteracts the Fas-positive effector T cells that infiltrate into tumours, and consequently the Fas/FasL interaction can contribute to the escape of tumour cells from the systemic immune response. In this study we examined differences in Fas and FasL expression on cells from the hamster melanotic melanoma line (Ma) and a more aggressive amelanotic melanoma line (Ab). We also tried to find out whether the Fas/FasL expression induces an ability to undergo spontaneous apoptosis in these two transplantable melanoma lines. Our previous studies have shown that cells of the Ma line have a higher ability to undergo spontaneous apoptosis than cells of the Ab line. Isolated transplantable melanoma cells were incubated for 4 and 24 hours and after that time the expression of Fas and FasL was estimated by flow cytometry. The results show that there was no Fas expression, although FasL was detected on both melanoma cell lines. Therefore the data reported by other authors indicate that a lack or a low level of Fas expression and an ectopic expression of FasL on melanoma cells can be an escape mechanism of the tumour, to avoid host immune responses. The content of FasL-positive melanotic melanoma cells was higher than in amelanotic melanoma cells and increased with the prolongation of the incubation time. FasL expression on amelanotic melanoma cells was detected after 24 hours at a level similar to that on melanotic melanoma cells after 4 hours incubation time. FasL expression on melanoma cells can induce apoptosis in cytotoxic T lymphocytes and NK cells which are responsible for tumour cells elimination. The results obtained suggest that the Fas/FasL system does not play any significant role in spontaneous apoptosis of two melanoma cell lines. But these results may indicate the presence of immune privilege of tumour cells with FasL expression.

Spontaneous apoptosis of melanotic and amelanotic melanoma cells in different phases of cell cycle: relation to tumor growth.

Since the spontaneous alteration of native melanotic (Ma) into amelanotic (Ab) transplantable melanoma line it has been observed that this alteration is accompanied by the acceleration of growth of Ab line. The aim of the present study was to check and estimate spontaneous apoptosis of cells from cell cycle phases. Cytometric cell cycle analysis was performed by staining cells with propidium iodide (PI). Apoptosis estimated by the TUNEL method, alterations in the plasma membrane structure (annexin V staining), changes in the mitochondrial transmembrane potential--delta psi m (JC-1 staining) showed that amelanotic melanoma cells have decreased ability to undergo spontaneous apoptosis. The obtained results showing that in the native melanotic line about 30% of cells are in S+G2/M phases and that 33% of these cells undergo apoptosis could lead to the conclusion that the slower growth of this melanoma line is the result of lower proliferation activity and higher rate of apoptosis of these tumor cells. The number of cells in S+G2/M phases in amelanotic melanoma line increases up to 40% and only 7% of them undergo apoptosis. This observation seems to suggest that the expansive growth of this melanoma line depends mainly on the decreased ability to undergo spontaneous apoptosis, especially in case of cells from S+G2/M phases. Moreover, the obtained results indicate that alteration of melanotic line into amelanotic one, accompanied by differences in many biological features also concerns basic cell processes such as cell cycle and cell death.