Sex Steroids and Bone
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These associations remained significant after adjustment for age, years since menopause, and body mass index. Thus, endogenous steroids are significant determinants of postmenopausal BMD. Endogenous estradiol may be more important for lumbar spine BMD, whereas endogenous androgens are associated mainly with femoral neck BMD. Unable to display preview. Download preview PDF. Skip to main content. Advertisement Hide. Endogenous sex steroids and bone mineral density in healthy Greek postmenopausal women. This is a preview of subscription content, log in to check access.
Prentice A. Circulating estradiol, like T, is bound to sex hormone-binding globulin SHBG, in Figure 4 and, to a lesser extent, serum albumin. Plasma SHBG is secreted from the liver; a similar, non-secretory form is present in many tissues, including reproductive tissues and the brain.
Urinary and faecal metabolites of estrogens in animals and humans have been studied for use as possible indicators of risk for hormone-dependent cancers or for infertility. There is at present no consensus about the importance of specific metabolites or metabolite ratios as prognostic factors, with the possible exception of estriol as a marker of the well-being of the feto-placental unit World Health Organization International Programme on Chemical Safety, Estrogens have been isolated from testes of stallion, bulls, boars, dogs and men.
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Estrogens may play a role in the pathogenesis of prostatic hyperplasia common in aged dogs, and estrogens receptors are present in prostatic urethra and prostatic glands of dogs. Estrogens like androgens, are transferred from testicular vein to the testicular artery. In several species, levels of estrogens in the blood of testicular artery are consistently higher than the levels in systemic blood.
The mechanisms involved in the transfer of estrogens from vein to artery in the pampiniform plexus and its physiology role are not clear. Estrogens may be playing important role in regulating the pituitary-gonadal axis.
Regulation of bone metabolism by sex steroids — Mayo Clinic
In several species, estrogens inhibit Leydig cell secretion of testosterone Pineda, as it will be mentioned. Sex steroids: Source, Target tissues and Physiological Functions. Modified from Hu et al. Progesterone is produced in the ovaries, the adrenal glands suprarenal , and, during pregnancy, in the placenta. Progesterone is also stored in adipose fat tissue.
Progesterone is synthesized by the ovarian corpus luteum, but during pregnancy the main source of P4 is the placenta as in woman,mare and ewe; in cow, the time of placenta takeover is months of pregnancy. In other species goat, sow, queen, bitch,rabbit, alpaca,camel, llama there is no placenta P4 production at all, the ovarian CL is in charge of the entire P4 for gestation. In mammals, P4, like all other steroid hormones, is synthesized from pregnenolone, which in turn is derived from cholesterol.
Androstenedione can be converted to testosterone, estrone and estradiol Figure 1 Wikipedia. Important functions of P4 are 1 inhibition of sexual behavior; 2 maintenance of pregnancy by inhibiting uterine contractions and promoting glandular development in the endometrium; and 3 promotion of alveolar development of the mammary gland.
The synergistic actions of estrogens and progestins are notable in preparing the uterus for pregnancy and the mammary gland for lactation Table 1. In at least one plant, Juglans regia , progesterone has been detected.
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In addition, progesterone-like steroids are found in Dioscorea mexicana. It contains a steroid called diosgenin that is taken from the plant and is converted into progesterone. Diosgenin and progesterone are found in other Dioscorea species as well. The switch from the principal steroid product of the maturing follicle estrogens to that of the developing and mature corpus luteum P4 is one of the amazing hallmarks of the ovary sex steroids production occurring during luteinization as described later.
Ovarian granulosa cells and testicular Leydig cells are responsive primarily to LH and synthesize androgens. Ovarian thecal cells and testicular Sertoli cells as well as Leydig cells respond to FSH with conversion of androgens into estrogens Paromatase activity. FSH also stimulates Sertoli cells to synthesize inhibin, activin, and other local bioregulatory factors Norris, Anatomically in the female hypothalamus, there are two GnRH neurons centers. The first, the surge center, consists of three nuclei called the preoptic nucleus, the anterior hypothalamic area and the suprachiasmatic nucleus.
This center releases basal levels of GnRH until it receives the appropriate positive stimulus. This stimulus is known to be a threshold level of estrogen in the absence of P4. When the estrogen concentration in the blood reaches a certain level, a large quantity of GnRH is released from the terminals of neurons, the cells bodies of which are located in the surge center. In natural condition, the preovulatory surge of GnRH occurs only once during the estrous or menstrual cycle.
The second, the tonic center, releases small episodes of GnRH in a pulsatile fashion similar to a driping faucet. This episodic release is continuous and throughout reproductive life and during the entire estrous cycle Senger, The female in various species have two important periods that mark the reproductive cycle: follicular and luteal phases.
The follicular phase begins after luteolysis that causes the decline in P4. The follicular phase is dominated by E2 produced by ovarian follicles and ends at ovulation. In women, the follicular phase is divided into menses and proliferative period 5 and 9 days respectively ; luteal phase is the secretory phase 14 days. In domestic animals, the follicular phase is divided in pro-estrus 2 days and estrus 1 day , and the luteal phase in metestrus 4 days and diestrus 14 days.
When recruited follicles develop dominance, they produce E2 and inhibin that suppresses FSH secretion from the anterior lobe of the pituitary. Thus, E2 in low concentrations causes a negative feedback suppression on the preovulatory center. That is, low estrogen reduces the level of firing GnRH neurons in the preovulatory-surge center.
Regulation of Bone Metabolism by Sex Steroids
However, when E2 levels are high estrus , as they would be during the mid-to late follicular phase figure 2 , the preovulatory center responds dramatically by releasing large quantities of GnRH. This stimulation in response to rising concentrations of E2 is referred to as positive feedback. During the middle part of the cycle, when E2 levels are low and P4 is high metestrus, diestrus , there is negative feedback on the preovulatory center, thus preventing high amplitude pulses of GnRH.
Interesting, when comparing human vs. As reviewed by Murphy, luteinization is a remarkable event involving cell proliferation, cell differentiation, and tissue remodeling that is unparalleled in the adult mammal. It comprises two major processes: a the terminated proliferation plus rapid hypertrophy and differentiation of the steroidogenic cells of follicle into the luteal cells of the CL.
Luteinization is both a qualitative and quantitative change because the mammalian CL produces up to fold greater amounts of steroid P4 than the follicle. Luteolysis results in cessation of P4 production, in structural regression to forma corpus albicans and into a follicular development and entrance into a new follicular phase. Female cyclicity and gonadal steroids. Each cycle consists of a follicular and a luteal phase. The follicular phase is dominated by the hormone E2 from ovarian follicles. E2 causes marked changes in the female tract for pregnancy.
Anestrus stands for periods of time when estrous cycles cease. Pregnancy, season of the year, lactation, forms of stress and pathology cause anestrus. Amenorrhea refers to the lack of menstrual periods and is caused by many of the same factors that cause anestrus.
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A menstrual cycle consists of the physiological events that occur between successive menstrual periods about 28 days. No endometrial sloughing menstruations occurs in animal with estrous cycles. Lutealphase is dominated by P4 from corpus luteum.
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As the main steroid produced during luteal phase is the P4 it is important to mention about the manipulation of the estrous and menstrual cycles by exogenous administration of P4. Exogenous P4 suppresses estrus and ovulation. When this exogenous P4 is removed or withdrawn, the animal will enter pro-estrus and estrus within 2 to 3 days after removal. This application is intended to increase the convenience of artificial insemination programs and to facilitate fertility in domestic husbandry animal improving pregnancy rates.
In contrast, the use of exogenous P4 in humans oral, transdermal,injectable, implants is intended to block ovulation and minimize pregnancy probability contraception Senger, Upon stimulation by LH, the Leydig cells of the testes produce androgens. Dihydrotestosterone is found in high enough concentration in peripheral tissue to be of functional importance.
Functions of T, as states before, include 1 development of secondary sex characteristics; 2 maintenance of the male duct system; 3 expression of male sexual behavior libido ; 4 function of the accessory glands; 5 function of the tunica dartos muscle in the scrotum; and 6 spermatocytogenesis. The role of T in regulating the release of hypothalamic and gonadotropic hormones is similar to that described for P4 in the female. Thus, reciprocal action of T with the hypothalamic and gonadotropic hormones is necessary for regulation of normal reproduction in the male Figure 3 Gyeongsang National University.
Luteinizing hormone acts on the Leydig cells within the testes. These cells are analogous to the cells of the theca interna of antral follicles in the ovary. They contain membrane bound receptors for LH. The production of T takes place by the same intracellular mechanism as in the female. The Leydig cells synthesize and secrete T less than 30 minutes after the onset of an LH episode Figure 3. This T secretion is short and pulsatile, lasting for a period of 20 to 60 minutes. It is believed that pulsatile discharge of LH is important for two reasons.
First, high concentration of T within the seminiferous tubule is essential for spermatogenesis Senger, Second, Leydig cells become unresponsive to sustained high levels of LH believed to be caused by reduction in the number of LH receptor. In fact, continual high concentrations of LH result in reduced secretion of T. Intratesticular levels of T are times higher than that of systemic blood.
However, testicular T is diluted over times when it reaches the peripheral blood Senger, The role of the pulsatile nature of T is not fully understood. It is believed that chronically high systemic concentrations of T suppress FSH secretion. Sertoli cells function is FSH dependent. Thus, their function is compromised when FSH is reduced. The periodic reduction in T allows the negative feedback on FSH to be removed.