Again, this is currently speculative, unless you know something that researchers in the field don't (or have yet to publish):
Garza, L. A, Liu, Y., Yang, Z., Alagesan, B., Lawson, J. A, Norberg, S. M., … Cotsarelis, G. (2012). Prostaglandin D2 inhibits hair growth and is elevated in bald scalp...