Figure 2.

Estradiol is a gonadal steroid; estradiol is a neurosteroid. A, All steroids are synthesized via the progressive modification and reduction of the precursor cholesterol. Estradiol is the finished product of as little as 6 or 7 enzymatic reactions, with aromatization from testosterone being the penultimate conversion. B, Estradiol is a gonadal steroid because it is synthesized primarily in the adult ovary, but is also made in the neonatal male brain from testicularly derived androgen. C, Estradiol can also be a neurosteroid, meaning it is made in the brain without dependence on precursors from the periphery. All of the necessary steroidogenic enzymes have been identified in the brain and evidence suggests estradiol can be made in some regions de novo from cholesterol, thus fitting the definition of a classic neurosteroid.