Re: why is melanin only accumulated in one spot to form a freckle?

Freckles are defined as localized hyperpigmentation and they are due to 
increased pigment production locally. This  is distinct from a local 
increase in the number of pigment producing cells, the melanocytes. Thus, 
the melanin doesn't go to one spot, but rather that some of the melanocytes 
which are distributed throughout the skin make more pigment than others. I 
don't believe that it is fully understood why this difference in behavior 
occurs.