The ovaries and the placenta (the temporary organ that serves to nourish and remove the fetus’s wastes) are the primary sources of estrogens, with the adrenal glands and the male testes also contributing small amounts. Estrogens are thought to be produced in the ovaries, specifically in the egg follicle (the saclike structure that holds the immature egg) and in the interstitial cells (certain cells in the framework of connective tissue). Blood estrogen levels appear to be highest during the ovulation process and immediately following menstruation, when the corpus luteum grows to fill the vacated egg follicle.
Ovarian steroid hormones are derived from cholesterol, the parent molecule. The hormone progesterone is produced from the steroid hormones pregnenolone and cholesterol. The androgens (male sex hormones) dehydroepiandrosterone, androstenedione, and testosterone are formed as intermediates in the conversion of progesterone to the main estrogensestradiol and estrone. Androgens are precursors to estrogens, and aromatase is the enzyme responsible for this conversion.
While adipose tissue is a significant source of estrogen in postmenopausal women, the ovaries are the primary source of aromatase. One of the most powerful estrogens, estradiol, can be created from testosterone. Even though estradiol can be converted into estrone, androstenedione is the primary precursor. Both estrone and estradiol combine to create estriol, the weakest of the estrogens.
Complexes of estrogen and its receptor are transported into the nucleus, where they regulate the transcription of specific genes to affect protein synthesis. Gene transcription is the process by which genetic instructions stored in DNA (deoxyribonucleic acid) are converted into messenger RNA (ribonucleic acid) molecules. Two types of estrogen cytoplasmic receptors, estrogen receptor-alpha and estrogen receptor-beta, exist; they are found in different tissues but can both stimulate DNA synthesis.