When does cell differentiation occur

It is to do with the genes, but stem cells can differentiate into any kind of cell; the type they become depends on environmental influences. These come from neighbouring cells, which are producing chemicals that influence the stem cell differentiation. Signals are detected through receptor molecules on the cell surface, and the activated genes direct the formation of the kinds of proteins the cell needs to specialise.

From a high concentration to a low concentration.

Are you asking about a street map usage  if so here is your answer  a street map for a very limited area could simply be a bit map or a .jpg image.  but a more general map would probably store the streets as vectors.  but you shouldn't assume that there is one way to do it