Melanosomes show pigmentation differences

Arecent study on differences in skin pigmentation between blacks and Chinese subjects has yielded new information, including that blacks have evidence of melanosomes that form nuclear caps, while Chinese do not.

MissionLead author Manoj Misra, Ph.D., and colleagues set out to study beyond what was already in the literature about ultrastructural differences in skin pigmentation. Specifically, they looked at differences in the melanosomes between the two ethnicities.

It is well documented in the literature that melanosomes vary in size, packing (or density) and number among different ethnicities, according to Dr. Misra, head of microstructure imaging, Unilever Research and Development, Edgewater, N.J.

"We processed these biopsies for electron microscopy, and employed two different methods which had not been used previously for determining the distribution of melanosomes: three-dimensional reconstruction and stereology," Dr. Misra explains.

For three-dimensional reconstruction, the researchers thinly sliced the biopsies, looking for cells within each slice.

"You align neighboring cells in different slices computationally and identify cell structures. Then, using computer-based algorithms, one reconstructs the distribution of melanosomes within a cell," he explains. "If you look in the literature, you find these melanosomes generally form a nuclear cap. These caps absorb light and prevent the damage to the DNA and the nuclei. We found that in African Americans, you see a predominant cap over the nuclei. But if you look at the lighter-skinned Chinese subjects, that is not quite the case. The cap was not seen in the biopsies."

Distribution differencesThe researchers were also interested in how, between the two ethnicities, melanosomes might be distributed differently as one moves up from the basal cells at the bottom of the epidermis (where the melanosomes are transferred to basal cells) to the suprabasal layer.

They used stereology, which would make it possible to uniquely identify a melanosome in a plane.

"If you use stereology you can ensure that you do not over count melanosomes because you have to differentiate between the ones which are uniquely present within a slice or the ones which are present in neighboring slices," Dr. Misra tells Dermatology Times.

They determined the density of melanosomes, which is the total number of melanosomes in a unit volume or a cubic micron, in the basal, suprabasal and stratum corneum skin layers.

"There was a significant difference in the density of melanosomes in the three layers," Dr. Misra says. "The CPV, as expected, was significantly higher for African Americans than for Chinese," he says. "There was a decrease in the melanosomal density as you go upward from the basal cells to the suprabasal and stratum corneum layers. That was in both ethnicities."

Skin colorThe researchers also looked into measuring the skin color of the study subjects.

Using L*, an indicator of skin lightness, the researchers found that L* values for blacks were in the 38 to 39 range and for Chinese in the 64 to 65 range.

"We found good correspondence between CPV and L* for all the biopsies," Dr. Misra says.

"This new information builds on the current foundation of knowledge that we have about people with different skin types," says Stacy Hawkins, Ph.D., authority manager for Unilever Research and Development. "The more dermatologists know about the root causes of skin pigmentation differences, the better they can guide patients."

Disclosure: Drs. Misra and Hawkins are employed by Unilever.