Estrogen
Basic Information 
Physiology
• Estrogens are a group of steroid hormones and function as the primary female sex hormone. The three primary naturally occurring estrogens are estradiol (most potent naturally occurring estrogen), estriol, and estrone. They derived their name from their role in the estrus cycle.
• Estrogens are produced primarily by developing follicles in the ovaries, the corpus luteum (CL), and the placenta. Synthesis of estrogens starts in theca interna cells in the ovary, by the synthesis of androstenedione from cholesterol. Androstenedione then crosses the basal membrane into the surrounding granulosa cells, where it is converted to estrone or estradiol, either immediately or through testosterone, under the action of aromatase. Being lipophilic, estrogens enter all cells freely and interact with a cytoplasmic target cell receptor. The steroid receptor complex can then enter the nucleus and regulate gene transcription.
• Estrogens have numerous roles, but in most females, the primary role is involved with the reproductive cycle. Estrogen production from the developing follicle leads to the luteinizing hormone (LH) surge that results in ovulation. During diestrus, estrogen production from the CL (along with progesterone) prepare the endometrium for the possibility of an implanted embryo. During pregnancy, the majority of the estrogens are produced by the fetoplacental unit. Subsequently estrogen concentration can be used to indirectly assess the health of the pregnancy. Estrogens also play a vital role in parturition (increasing oxytocin receptors, promoting synthesis of prostaglandins).
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