 Both share the presence of small lipid droplets and numerous iron-rich mitochondria, giving the brown appearance.Brown fat cells come from the middle embryo layer, mesoderm, also the source of myocytes (muscle cells), adipocytes, and chondrocytes (cartilage cells).The classic population of brown fat cells and muscle cells both seem to be derived from the same population of stem cells in the mesoderm, paraxial mesoderm.