Estrogens cause the growth of the uterus, fallopian tubes, and vagina. Stimulation of proliferation of the vaginal epithelium is checked by the cyclical exposure to progesterone during the luteal phase in the mature female. Estrogens also are responsible for the expression of female secondary sex characteristics during puberty. These include breast enlargement, the distribution of body hair, body contours as determined by subcutaneous fat deposition, and skin texture. During development, estrogens stimulate proliferation of the ductal epithelial cells in breast tissue. Progesterone mediates lobuloalveolar development at the ends of these mammary ducts. In women, cyclical changes in the breast cell proliferation occur during the menstrual cycle, with the highest levels of proliferation occurring during the luteal phase, when circulating levels of both estrogen and progesterone are high. This has led to the idea that progesterone, as well as estrogens, exerts mi-totic effects in adult human breast tissue. The effects of estrogens and progesterone on breast development are most noticeable during puberty and pregnancy.
Estrogens can stimulate the release of growth hormone and exert a positive effect on nitrogen balance. These effects contribute to the growth spurt during puberty. Closure of the bone epiphyses signaling the end of long bone growth is also estrogen mediated.
Bone remodeling occurs throughout adult life. Osteoblasts are the bone cells that are responsible for increasing bone mass. Bone loss occurs through the activity of other bone cells, called osteoclasts. Normal bone remodeling takes place when there is a balance between osteoblast and osteoclast activities. Estrogens maintain bone mass by inhibiting bone resorption by the osteoclasts. Estrogens inhibit the production of cy-tokines by peripheral blood cells and the osteoblasts that stimulate osteoclast activity. In postmenopausal women, declining estrogen levels give rise to a net increase in osteoclast activity and loss of bone mass resulting in the serious condition osteoporosis. Also, progestins antagonize loss of bone.
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