How does estradiol level rise pre-LH surge?

[seratia]

In the follicular phase, once the estradiol level reaches a certain threshold, it causes a switch from negative feedback to positive feedback on LH & FSH. That is why we have a surge near the end of the follicular phase.

My question is, what makes estradiol rise and reach that threshold to begin with, if it is under negative feedback initially. Shouldn't it not rise, because of negative feedback on LH & FSH?

 

[jim mcnamara]

See: https://embryo.asu.edu/pages/estrogen-and-menstrual-cycle-humans for a detailed explanation.