Non-invasive, label-free visualization of melanin granules using Nanolive cell imaging

What is the function of melanin?

Melanin describes the group of natural pigments that is responsible for skin and hair pigmentation and photoprotection of the skin and eye. Deficiency in melanin can cause several disorders and diseases including vitiligo, deafness, and Parkinson’s disease, and so, there is great interest in understanding its formation, growth, and distribution.

How can you measure melanin content?

Absorption spectroscopy is currently the most widely used method for melanin quantification because it is simple and non-invasive (1). The downside of this approach is that it is destructive and so it produces limited endpoint data (2), which prevents the analysis of dynamic processes such as changes in cell morphology or confluency (3). Nanolive label-free live cell imaging offers a unique solution. Because melanin granules are electron dense, they have a higher refractive index (RI) value than other cell organelles making them easily distinguishable using Nanolive imaging.

Analyze the distribution and motility of melanin granules with Nanolive cell imaging

Here, we imaged epithelial cells every 2 secs for 9 mins. The small, bright ovoid spots in the cell’s cytosol are melanin granules. Several observations can be made in the video:

  1. Melanin granules are present throughout the cell but tend to cluster around the cell’s nucleus.
  2. The granules are highly mobile and can travel in both directions.
  3. Movements occur at variable speeds.

The last two observations agree with findings made using a live-cell fluorescent melanosomal marker (4).

What’s next?

The epithelial cells we imaged here were healthy. In the future it would be interesting to image malignant melanoma cells and compare how the number, morphology, distribution, and behavior of melanocytes changes in diseased cells. It would also be interesting to determine whether a “healthy” phenotype can be rescued by drug treatments.
Bibliography

[1] Hu DN. 2008. Methodology for evaluation of melanin content and production of pigment cells in vitro. Photochem. Photobiol. 84(3):645-9.

[2] Wakamatsu K, Ito S. 2002. Advanced chemical methods in melanin determination. Pigment Cell Res. 15(3):174-83.

[3] Chung S, Lim GJ, Lee JY. 2019. Quantitative analysis of melanin content in a three-dimensional melanoma cell culture. Sci. Rep. 28;9(1):1-9.

[4] Bruder JM, Pfeiffer ZA, Ciriello JM, Horrigan DM, Wicks NL, Flaherty B, Oancea E. 2012. Melanosomal dynamics assessed with a live-cell fluorescent melanosomal marker. PLoS One. 22;7(8):e43465.

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