Ovarian Cycle: Hormonal Regulation (Nursing)

00:02 So, how do we hormonally establish this ovarian cycle.

00:08 Beginning at puberty, the ovaries are going to secrete a small amount of estrogen and this is going to inhibit hypothalamic release of the gonadotropin-releasing hormone.

00:21 However, as puberty gets closer in your leptin levels, which is a hormone that is produced by our adipose or fat cells it starts to become adequate the hypothalamus becomes less estrogen sensitive and so now, gonadotropin-releasing hormone is able to be released.

00:43 Once this is released, it's going to stimulate the release of follicle-stimulating hormone and luteinizing hormone by the anterior pituitary.

00:54 These two hormones will then act on the ovaries.

00:59 These events will continue until an adult cyclic pattern is achieved and then menarche the first menstruation occurs.

01:12 So let's look at this a little closer.

01:14 The gonadotropin-releasing hormone is going to stimulate follicle stimulating hormone and luteinizing hormone secretion.

01:24 These two hormones will then stimulate the follicle to grow, mature and secrete the sex hormones estrogen and progesterone.

01:35 Follicle-stimulating hormone specifically stimulates the granulosa cells to release estrogen while luteinizing hormone prods the thecal cells to produce androgens, which the granulosa converts to estrogen.

01:55 This leads to a negative feedback loop where the gonadotropin-releasing is going to be inhibited by increasing plasma levels of estrogen which is going to exert a negative feedback on the release of follicle-stimulating hormone and luteinizing hormone.

02:13 And as well the granulosa cells begin to secrete inhibin, which further inhibits the follicle stimulating hormone.

02:24 Only the dominant follicle can withstand this dip and follicle-stimulating hormone, and the other developing follicles are going to deteriorate.

02:37 Also an ovarian cycle there is some positive feedback as well.

02:43 Positive feedback that stimulates the gonadotropin release includes estrogen levels, which continue to rise as a result of the continued release by the dominant follicle.

02:56 When the levels reach a critically high level there's going to be brief positive feedback that occurs on the brain and the pituitary.

03:05 This positive feedback leads to a luteinizing hormone surge.

03:13 The luteinizing hormone surge is now going to trigger ovulation and the formation of the corpus luteum so high estrogen levels trigger the release of stored luteinizing hormone and some follicle-stimulating hormone by the anterior pituitary at mid-cycle.

03:36 The luteinizing hormone surge is then going to trigger the primary oocyte to complete meiosis 1 to become the secondary oocyte.

03:45 The secondary oocytes enters meiosis II and continues on and stops at metaphase II.

03:55 Luteinizing hormone also stimulates other events that can lead to ovulation.

04:00 This includes an increase in the local vascular permeability, which triggers an inflammatory response that promotes the release of metalloproteinase enzymes that weaken the ovarian wall.

04:17 Also, luteinizing hormone stops the blood flow to the protruding follicle wall.

04:23 This causes the wall to become thin and bulge and eventually it becomes weak enough that it ruptures forming a hole.

04:32 The oocyte with its corona radiata surrounding it is then going to exit from the ovary and ovulation is now accomplished.

04:45 Shortly after ovulation the estrogen levels are now going to decline.

04:51 Also the luteinizing hormone is going to be transferred from the ruptured follicle into the corpus luteum.

04:59 The luteinizing hormone stimulates this corpus luteum to secrete the hormone progesterone and also some estrogen almost immediately.

05:09 The progesterone helps to maintain the stratum functionalis of the endometrium of the uterus.

05:19 The progesterone maintains a pregnancy if it is to occur.

05:26 Also in this process there is negative feedback as well.

05:31 Negative feedback is going to inhibit the luteinizing hormone and the follicle stimulating hormone release.

05:39 This happens from a rising plasma level of progesterone and estrogen.

05:45 Inhibin is also going to be released from the corpus luteum and the granulosa cells further enhancing this inhibitory effect.

05:56 Declining luteinizing hormone ends the luteal activity and inhibits follicle development.

06:05 So what happens if fertilization and pregnancy do not occur, the corpus luteum is going to degenerate when the luteinizing hormones start to fall.

06:16 This causes a sharp decrease in the estrogen and progesterone which in turn ends the blockage of follicle-stimulating hormone and luteinizing hormone secretion.

06:27 This will then start the process all over again since now the negative feedback loop has circled it's way back around.

06:37 The oocyte is actually activated about 12 months prior to when ovulation occurs, but it only takes 14 days for it to mature before ovulation.