As the progesterone level rises, it circulates around the body in the blood. When a high concentration of progesterone reaches the hypothalamus in the brain, the effect is to stop the hypothalamus from producing GnRH. This type of control system, where the rise in one body chemical in this case, progesterone stops the production of another body chemical in this case, GnRH , is called a negative feedback mechanism. But the corpus luteum has a limited lifespan, and after it stops producing progesterone, the negative feedback on the hypothalamus stops, and this allows it to begin producing GnRH again.
So the ovarian cycle begins all over again. Next, we turn our attention to the events occurring in the uterus during the same typically day period as the events just described in the ovaries. The uterine cycle indicates the cyclical changes that occur in the uterus in response to the female sex hormones, progesterone and oestrogen. If fertilisation does not occur after ovulation, the corpus luteum will degenerate and production of progesterone will decrease, so the stimulus for maintaining the thick endometrium will disappear.
The reduction in progesterone causes the shedding of the thick endometrial lining. The muscular wall of the uterus the myometrium contracts to help cut off the blood supply to the endometrium, causing it to break away from the uterus. The endometrium was richly supplied with blood vessels to nourish the fetus if a pregnancy occurred, so when it disintegrates and passes down the vagina, some blood is mixed in with it. The mixture of tissue and blood passes out through the vagina as the menstrual fluid or menstrual flow , usually for a period of about three to five days.
Menstruation usually occurs at monthly intervals throughout the reproductive years, except during pregnancy when it is completely suppressed and the woman cannot get pregnant again until after the baby is born. Breastfeeding a baby also suppresses menstruation, but there is a risk that ovulation and pregnancy may still occur.
The concentration of oestrogen in the blood is rising during this period, following the end of menstruation, as the ovaries prepare for the next ovulation at around day During this phase, the blood concentration of progesterone increases, which causes even more blood vessels to grow into the endometrium. This makes the endometrium receptive to the fertilised ovum. If the ovum is fertilised and the embryo implants in the endometrium and a placenta develops, it produces a hormone called human chorionic gonadotropin HCG throughout pregnancy.
HCG signals the corpus luteum to continue to supply progesterone to maintain the thick, nourishing endometrium throughout the pregnancy. Continuous levels of progesterone act as a negative feedback mechanism on the hypothalamus and pituitary gland, preventing the release of FSH and LH, and hence further ovulation ceases. The corpus luteum degenerates and the level of progesterone declines; the endometrium disintegrates and the woman menstruates — a signal that she did not become pregnant during that menstrual cycle.
You probably know that menarche the first menstruation starts on average between the ages of 12 to15 years in Ethiopia. But in some cases it can start as late as 17 to 20 years, or as early as 8 to 9 years. Some of the factors that affect the age of menarche are biological, and some are cultural. Menarche begins when the hypothalamus in the brain is sensitised to begin producing GnRH at around the age of 12 to15 years.
But evidence suggests that GnRH may begin at an earlier age in girls who are well nourished and exposed to sexual motivating factors, such as watching sexual films and talking about sex.
In malnourished girls, who have little exposure to sexual motivating factors, menarche may be delayed until the age of 17 to 20 years. Disease conditions that affect the hypothalamus and pituitary gland, or the ovaries and uterus, can also affect the age of first menarche. Around the age of the menarche, the female sex hormones, oestrogen and progesterone, are responsible for the development of secondary sexual characteristics in the female. These include:. Together with the menarche, the appearance of the secondary sexual characteristics marks the period known as puberty — the period of life typically between the ages of 10 to15 years during which the reproductive organs grow to adult size and become functional.
You already learned about them in Study Session 3 — they are the anatomical structures of the external female genitalia e. Menstruation continues every month, except during pregnancy, until the woman reaches the menopause at around the age of 48 to 50 years, when menstruation ceases. By the time she reaches the menopause, her ability to bring ova to maturity has come to an end.
In Study Session 5, we describe what happens when an ovum is fertilised and it implants in the uterus and develops into a fetus.
Now that you have completed this study session, you can assess how well you have achieved its Learning Outcomes by answering these questions. You can check your answers with the Notes on the Self-Assessment Questions at the end of this Module.
The left-hand column of Table 4. Complete the right-hand column by entering the correct period in days measured from the last normal menstrual period LNMP. A woman may feel cramping pains in her abdomen when she is menstruating because the muscular walls of the uterus the myometrium contract to help cut off the blood supply to the endometrium, causing it to break away from the uterus. Can you suggest how the relatively high concentration of artificially produced oestrogen and progesterone in contraceptive pills prevents pregnancy in women who take the pills regularly as prescribed?
The relatively high concentration of artificially produced oestrogen and progesterone in contraceptive pills prevents pregnancy by activating a negative feedback mechanism on the hypothalamus in the brain. High levels of oestrogen and progesterone mimic the situation that occurs in pregnancy.
This in turn stops the ovaries from maturing any more ova, so ovulation does not occur, and the woman cannot get pregnant as long as she takes the contraceptive pills regularly as prescribed. In a pregnant woman, the placenta continuously produces the hormone HCG, which causes the corpus luteum in the ovary to produce progesterone throughout the pregnancy.
Progesterone maintains the endometrium as a thick fatty layer, so menstruation ceases during pregnancy because the endometrium remains attached to the uterus, where it assists in nourishing and protecting the growing fetus. In short this allows you to use the content throughout the world without payment for non-commercial purposes in accordance with the Creative Commons non commercial sharealike licence.
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Anatomy and Physiology of Animals/Reproductive System
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Hormonal Regulation of the Female Reproductive System. Antenatal Care Module: 4. Learning Outcomes for Study Session 4 When you have studied this session, you should be able to: 4. SAQ 4. In a recent in vivo study Genaro et al. This increment did not take place in group C, where the testis had been pre-incubated with hydrocortisone.
In the latter case, pre-incubation with cortisol suppressed the stimulating action of hCG. The acute stress was effective in altering the gland control, leading to increased testosterone release from the testis in vitro to the incubation medium. Adrenal activity exerts suppressing effects on the testicular function. Paradoxically, however, it can also have a stimulating effect over the reproductive function Brown et al.
Stimulation of the reproductive axis by adrenal hormones under acute stress suggests that the response may depend on the duration and intensity of the stimulus Armario et al. On the other hand, Fenske performed in vitrostudies showing that the mechanism of testicular function inhibition by cortisol does not depend on plasma LH concentrations since cortisol would be capable of acting directly on Leydig cells.
Therefore, such effect of hydrocortisone is not able to prevent Leydig cell sensitization by acute stress, which reinforces the idea that this sensitization is mediated by a factor other than hydrocortisone. Studies involving these hormonal axes are still mandatory to increase our knowledge on the reproductive function of the Felidae family, which should enable one to use this information for a better handling of reproduction and captivity, especially in the case of the endangered species.
The domestic cat can thus be employed as an important model in these studies. The results showed that fragment of testis from cats under stress secreted higher levels of testosterone in vitro than those from animals without stress. The three groups presented variations in testosterone concentrations in response to hCG; however, secretion by group S was significantly higher than those by groups A and C.
With respect to group S, it is noteworthy that the suppressing action of hydrocortisone led to a reduction in the testosterone in pure TCM when cortisol was included in the medium, despite the presence hCG. Thanks are also given to Ms. Zanon for excellent assistance during the radioimmunoassay.
Armario A. Differences in Prl and LH to acute stress between peripuberal and adult male rats. Baldwin C. Adrenocortical function in the domestic cat during treatment with levonorgestrel. Bambino T. Direct inhibitory effect of gluco-corticoids upon testicular luteinizing hormone receptor and steroido-genesis in vivo and in vitro.
Barber P. Disorders of the parathyroid glands. Feline Med. Bridges P. Elevated temperature heat stress in vitro reduces androstenedione and estradiol and increases progesterone secretion by follicular cells from bovine dominant follicles. Domestic Anim. Brown J.
Evaluation of the pituitary-gonadal response to GnRH, and adrenal status, in the leopard Panthera pardus japonensis and tiger Panthera tigris. Faulborn K. Effects of an intravenous injection of tetracosactid on plasma corticosteroid and testosterone levels in unstressed male rabbits. Acta Endocrinol.
Animal Reproductive Structures and Functions | Biology
Fenske M. Role of cortisol in the ACTH-induced suppression of testicular steroidogenesis in guinea pigs. Ferasin L. Iatrogenic hyperadrenocorticism in a cat following a short therapeutic course of methylprednisolone acetate.