Estrogen and progesterone actions in endometrium and breast tissue across reproductive stages

Actions Estrogen and Progesterone in Endometrium and Breast: Puberty, Pregnancy and Puerperium Including Molecular Actions and Functionally Important Tissue Level Effects

Estrogen and progesterone are essential to cyclical changes that occur during puberty, pregnancy and peurperium. Complex molecular activity influences tissue changes in multiple reproductive organs including the uterus, endometrium, ovaries, breasts and more (Kimbrell & McDonnell, 2003).During a woman's reproductive cycle the hypothalamus secretes gonadotropin-releasing hormone or luteinizing releasing hormone which stimulates the release of luteneizing hormone and follicle stimulating hormone (Merck, 2005). This in turns promotes maturation of female ovum and stimulates the body's release of the reproductive hormones estrogen and progesterone during a woman's cycle. Estrogen and progesterone contain carbon atoms and other polycyclic compounds that are created from cholesterol and circulate within the bloodstream via plasma proteins (Merck, 2005).

Unbound estrogen and progesterone compounds stimulate the reproductive system including the endometrium, uterus and breasts to mature and reproduce (Merck, 2005). These hormones work synergistically to promote "positive and negative feedback effects on the CNS hypothalamic-pituitary unit" which in turn both stimulates and inhibits gonadotropin secretion (Merck, 2005).

Estrogen and progesterone generally act as a steroid on the human body. Estrogen is a lipophilic molecule that "passively diffuses across cell membranes and binds to estrogen receptors" (Kimbrell & McDonnell, 2003: 1671). Substances can bind to estrogen receptor molecules resulting in "conformational changes" influencing tissue activity including breast growth during cyclical periods (Kimbrell & McDonnell, 2003: 1671).

Progesterone receptors exist in two forms derived from "alternative start sites on the same gene" (Kimbrell & McDonnell, 2003: 1671) but performing different functions. Tissues targeted by the hormone progesterone invariably include the vagina, ovaries, uterus, mammary glands and hypothalamus (Kimbrell & McDonnell, 2003). Progesterone often establishes and maintains a pregnancy (Kimbrell & McDonnell, 2003).

Estrogen and Progesterone During Puberty

LH and FSH which stimulate estrogen and progesterone generally remain high during birth but fall to low levels throughout the prepubertal years, only to rise during puberty again (Merck, 2005). During puberty increases in DHEA (adrogens in the body) stimulate puberty. At this time scientists are not entirely certain what mechanisms stimulate puberty to occur fully, but know multiple factors may influence the process including intermittent releases of GnRH throughout childhood (Merck, 2005). During early puberty LH and FSH release stimulate production of estrogen which encourages the development of sex characteristics including breast tissue growth (Merck, 2005). This occurs primarily during sleep initially but then throughout the day later in puberty (Merck, 2005).

Complex hormonal and endocrinal communication must occur for puberty and pregnancy to occur in the female body. Human breast tissue being development during fetal development at roughly the sixth week of human life, but doesn't grow or develop until puberty (Rosenthal & Arsenault, 2005). It is during puberty that the pituitary glands begins the development cycle by releasing follicle stimulating hormone or FSH which signals the gonads or ovaries to produce estrogen at the beginning of the reproductive cycle and progesterone toward ovulation triggering puberty, and resulting in tissue growth (Rosenthal & Arsenault, 2005).

Under the influence of progesterone and estrogen the breasts undergo five stages of development including the prepubertal stage, the breast bud state where tissue begins to growth, breast elevation, formation of the areolar mound and adult contour when the breasts are capable of producing milk (Rosenthal & Arsenautl, 2005).

Endometrial changes also occur under the influence of increasing estrogen and progesterone, including the introduction of menarche which typically follows breast tissue growth by roughly 2 years (Merck, 2005). Other changes that result from hormonal fluctuations include body fat increases. Thereafter a cyclical process of vaginal "discharging of sloughed endometrium" occurs every month stimulating menstrual flow (Merck, 2005).

Endometrial changes occur as estrogen and progesterone secretion begins in the early half of the cycle, circulating slowly and remaining relatively constant until roughly a week prior to the LH surge where ovarian secretion of the hormone estrogen stimulates the follicle to release an egg (Merck, 2005). Thereafter progesterone surges encouraging maturation of the egg if fertilization occurs.

Estrogen and Progesterone During Pregnancy and Peurperium

The endometrium undergoes multiple changes during menstruation under the influence of estrogen and progesterone. Estrogen is more of an influence during to the endometrium and breast tissue while progesterone is equally important to support a pregnancy and influence proper tissue growth (Kimbrell & McDonnell, 2003).

The endometrium is generally thin through the follicular phase of development with a dense stroma until estradiol levels increase in the latter follicular phase involving mitoses of the endometrium; this in turn results in mucosal thickening and tubular glands lengthening (Merck, 2005). At this point the endometrium adopts a "trilaminar pattern" until after the late luteal phase when stroma "become edematous" resulting in bleeding (Merck, 2005: 1). If however pregnancy occurs the lining of the endometrium remain thick. Increasing levels of estrogen and progesterone also result in tissue changes and density increase in the breasts, much like that which is seen during puberty (Merck, 2005).

Estrogens and progesterone are considered hydrophobic molecules, hence must be transported through the blood via serum globulin (RCN, 2005). They can bind to cells including target cells and receptor proteins in the human body (RCN, 2005). During pregnancy the placenta surrounding the baby is the primary source of progesterone in the body and is essential to sustain the pregnancy (RCN, 2005). During the latter half of pregnancy however estrogen secretion via the placenta increases triggered by the fetus; in this case the placenta releases a chemical called CRH which signals the pituitary gland in the fetus to secrete ACTH, which releases the precursor to estrogen known as DHEA-S (RCN, 2005).