What is so special about tests on skin samples (explants)?

A sample of skin obtained through a biopsy (removal of a fragment of body tissue in view of examining it under a microscope) is placed in a specific nutritive medium in which it is kept alive.

Such samples are for instance useful when testing a cosmetic product’s efficacy in a Science Laboratory (ex vivo testing). 

Their use is particularly relevant in that they are largely comparable to tests carried out on human subjects.