What decides which microRNA needs to be activated?

[fredreload]

So after watching this video, I'm not quite sure how specific microRNA is produced for the cell type. It's like skin cell only produce skin cell regulatory microRNA and muscle cell only produce muscle cell regulatory microRNA, but what causes the RNA Polymerase 2 to select and produce the microRNA needed for the cell type?

 

 

P.S. We're getting real close guys

Edited September 17, 2016 by fredreload