As it turns out, the most commonly used substrate for primary muscle cell growth is a low percentage (0.1 – 0.2%) of porcine gelatin, derived from pigs.  Alternatively, Matrigel, which is derived from mouse sarcoma cells, has a similar Young’s Modulus (measure of stiffness) as the human satellite cell niche (0.45 kPa versus 0.5 kPa, which I include in my manuscript's discussion section here [23]) and can also be used as a differentiation substrate.  So we are again left wondering if in vitro meat is really devoid of animal products.  Luckily, efforts have already been made in this area as well through the use of synthetic hydrogels to mimic desired stiffness and elasticity features of certain cell types such as muscle [41].  Alternatively, recombinant proteins such as laminin-521 have fared well in promotion of skeletal muscle growth [42]. Again I speculate on the use of hydrogels as a cell substrate here, but mainly want to make the point that this area can be free of animal products as well. Additionally, it's quite possible that in order to properly scale production, substrate-free bioreactors will be the primary choice for growing in vitro meat.