Why is the growth rate of PromoCell's HDMEC significantly reduced when I remove hydrocortisone from the Endothelial Cell Growth Medium MV Kit (C-22120)?

Epidermal Growth Factor (EGF) and hydrocortisone have been reported to have a synergistic effect on the growth of microvascular endothelial cells. Accordingly if you remove the hydrocortisone the proliferation of HDMEC is clearly affected.