The body has hundreds of different kinds of receptors, each one designed for a particular kind of chemical signal. Some receive messages from the thyroid gland, which may cue cells to consume more oxygen and generate more heat. Others are tuned to the adrenal glands, which send messages that regulate blood pressure and the body's response to stress. The hypothalamus in the brain has all kinds of receptors to monitor hormone levels in the blood so the brain can signal the hormone-producing glands when adjustments are needed. McLachlan demonstrates by moving the plastic model through the air toward the receptor, showing how the estradiol docks in the pocket of the receptor like a Star Trek vehicle returning to the much larger mother ship.
Hormone molecules are tiny compared to the sprawling receptors.
This union of hormone and receptor targets genes that trigger the production of particular proteins. In the case of estrogen, these proteins accelerate cell division. So when estrogen joins with receptors in the uterus, it will cause the lining of that organ to thicken. Estrogen produces such a response in the first half of the menstrual cycle to prepare the uterus in the event an egg is fertilized when ovulation occurs at midcycle. In endocrinology textbooks, one still finds flat assertions that receptors are highly discriminating about chemical structure and will bind only to their intended hormone or a very closely related compound.