According to Wikipedia:
Mona Lisa has no clearly visible eyebrows or eyelashes. Some researchers claim that it was common at this time for genteel women to pluck these hairs, as they were considered unsightly. In 2007, French engineer Pascal Cotte announced that his ultra high resolution scans of the painting provide evidence that Mona Lisa was originally painted with eyelashes and with better visible eyebrows, but that these had gradually disappeared over time, perhaps as a result of overcleaning. For modern viewers the nearly-missing eyebrows add to the slightly abstract quality of the face.