Female sex hormone. Polarised light micrograph of crystals of Oestradiol, also known as beta- Estradiol. This is the most potent of the six naturally-occurring oestrogens, the class of hormones associated with the development of female reproductive structures and secondary sex characteristics (such as breast formation). Oestradiol is secreted by the ovaries, and is the major controlling hormone in the regulation of the menstrual cycle. In men excessive production of oestradiol leads to feminization. Magnification: x20 at 6x7cm size.