Estrogens are female sex hormones
There are two major female sex hormones: estrogen and progesterone. Both of these are steroid hormones (like testosterone). Together, these two hormones are jointly referred to as "estrogens," even though only one of them actually shares the same name.
The primary source of these hormones is the ovaries, the female gonads (we saw that androgens were primarily produces in male gonads, too). Other tissues can also make more limited amounts of estrogens, like the adrenal cortex and adipose tissue (as well as the placenta during pregnancy). These hormones are only produced if the tissue that makes them is stimulated by gonadotropins. And the gonadotropins (FSH & LH) are only released if GnRH (from the hypothalamus) is released. Before puberty, GnRH is not really released. But, as a female child reaches puberty, her GnRH levels continue to rise, causing her gonadotropins to be released, and prompting the production and release of estrogens. These estrogens cause the girl's female secondary sex characteristics to develop.
Here are some of the female secondary sex characteristics:
Did I realize that I skipped over the female primary sex characteristics? Yes! How come? Because the default sex is the female sex. The male only forms when exposed to androgens. Without androgens, as is the case for a genetically-female fetus, the female primary sex characteristics (mainly her female reproductive organs) form. Pretty neat, huh?
How are the estrogens made?
I told you above that the estrogens are mainly made in the ovaries. How does this occur? Well, in males, androgens were made pretty continuously in the testes; females do not produce their estrogens continuously-- estrogens are cyclical.
I have included a portion of Figure 22.29 here to try to show you how the estrogens are made.
On the ovulation page, the development of the follicle and release of the secondary oocyte are described. The follicular cells (those that surround the oocyte) of the follicle that develops are the cells that secrete the estrogens. Before ovulation they make more estrogen than progesterone, and after ovulation they make more progesterone than estrogen. But these are the cells that make the hormones. Therefore, the larger the developing follicle gets, the more hormone it makes; similarly, when the follicle shrivels up at the end of the cycle (into the corpus albicans), the hormone production slows way down. As you look at this figure, you can see how at the far left (when the follicle is small) and at the far right (when the follicle has shrivelled), the levels of estrogens are low. Only when the the follicle is large (as it is in the middle of the cycle) are the hormone levels high.
© 2011 STCC Foundation Press