What would cause a single white eyelash?

Question

I couldn't find a paper who could give me the explanation. Unfortunately, according to my search, it seems there are not many reported case, particularly for the younger population (<30). Would stress cause this?

Answer 1

1. What Causes Hair to Have Colour and How is Colour Lost?

The colour of hair is due to the presence of melanin, which may be found as eumelanin or pheomelanin. Melanin is produced by melanocytes and is transported to keratinocytes. The hair then grows out from a root at the bottom of the hair follicle. A hair without melanin is a grey hair. The exact specifics of how hairs go grey is quite complex and poorly understood.
The growth and pigmentation of hair grows are both regulated by many factors, including genes such as MC1R and DLX3. This provides some explanation why people of particular ethnicities or families can go grey at different times - their genetic makeup changes how quickly they go grey.
Another factor affecting hair pigmentation is reactive oxygen species (ROS). The generation of ROS (oxidative stress) has been shown to have a role in the greying of hair.. Moreover, ROS are generated by smoking, which could lead to premature greying.
Other factors that have been found to be associated with premature hair greying include UV light, vitamin B12, iron, copper, ferritin, calcium and vitamin D3 (source).

2. What Would Make an Individual Eyelash go Grey?

As far as I know, the regulation of eyelash growth and pigmentation is very similar to that of hair on the scalp.
Given that the regulation of hair pigmentation is controlled by genetic factors, it could be that there is a change in the function of the genes at that particular hair follicle. This could possibly involve mutations which change signaling pathways, melanin generation, etc. such that melanin is no longer transported into the hair.
There could also be an extrinsic (outside of the body) factor which has locally stopped melanin production. Perhaps at some point, there was an elevated level of ROS or UV light at that particular hair follicle. This could have introduced mutations in the genes, or could have changed how the hair was pigmented. Alternatively, there could be death of melanocytes in that specific area, which could stop the generation of melanin. There could also be a local change in the levels of the factors associated with greying (e.g. a local vitamin B12 deficiency).

3. Does Stress Cause Premature Greying?

Surprisingly, despite common knowledge telling us that hair makes us go grey, there is conflicting evidence about this. Stress has been shown to make us go bald earlier but whether it also makes us go grey is not conclusively known. But given that stress has a clear negative impact on health, it's probably best not to be too stressed regardless.