Female sex hormone. Polarised light micrograph of crystals of beta-Estradiol. This is the most potent of the six naturally-occurring estrogens, the class of hormones associated with the development of female reproductive structures and secondary sex characteristics. Beta-Estradiol is secreted by the ovaries, and is the major controlling hormone in the regulation of the menstrual cycle. It is formed by enzyme-catalysed synthesis from pregnenolone. Magnification: x12 at 35mm size.