[Lotus]

Apart from the genetic portion, I thought it was mostly progesterone & maybe prolactin, with the latter having less influence but does play a part. Of course nothing happens without E as well, but for areola & nips you need the other two just as much.

Of course Lotus will correct me...

Lol spoked,

No corrections today. (just my two cents, lol)

Prolactin, serotonin, oxytocin, blood flow/temperature change, excitation (nipple stimulation) can stimaluate areola expansion. Developing sweat glands/milk ducts could benefit areola expansion. Other options exist imo,.......(creative).

I wouldn't be at all surprised that areolas have their own independent receptors.