RNA content can vary widely between tissues, cell-types, physiological state, etc.
If you are accustomed to working with tissues where RNA is plentiful, such as liver, you may have unrealistically high expectations of RNA yields from tissues with lower RNA contents (e.g. skin, muscle, bone). Tables 1 and 2 provide general guidelines for estimating RNA yields from a variety of cells and tissues. Remember that RNA yields from analogous tissues in different organisms can also vary.
Table 1. Total RNA Yields from 1 mg of Various Tissues.
General Guideline for the Amount of Total and Poly(A) RNA to Expect from Tissues and Cells