The menstrual cycle is a regular cycle in the female reproductive system that can be divided between the ovarian cycle and the uterine cycle. The menstrual cycle will cease after a certain point in females' lives known as menopause. Now the ovarian cycle is going to involve cyclic changes in the follicles of ovaries. Whereas, the uterine cycle is going to be the cyclic growing and shedding of the endometrial lining. Now, the ovarian cycle begins with the follicular phase. This is the part where the follicle matures and gets ready to release the ovum. FSH is going to stimulate follicle maturation and the follicle will secrete estradiol, an important estrogen. Ovulation is the release of the ovum from the follicle and it will travel into the fallopian tube. This occurs in the middle of the ovarian cycle and it's triggered when maximal estradiol concentrations cause an LH spike. And this LH spike causes the follicle to rupture and release the ovum, aka ovulation. Now the luteal phase is the final part of the ovarian cycle. And this is where the follicle transforms into the corpus luteum, that temporary endocrine structure we mentioned. The corpus luteum is going to secrete progesterone, and this is going to maintain the uterine lining, and it's going to provide negative feedback to FSH and LH. Now the corpus luteum will degrade if there is not pregnancy, if fertilization and implantation don't occur. And we'll talk about what happens if they do occur a little later.
Now the uterine cycle, as we said, is going to involve the endometrial lining. The first part of the uterine cycle is actually menstruation. This is where the endometrium is shed. So that's occurring during the follicular phase. Now, the proliferation phase is the second part of the uterine cycle. This is where the endometrium actually grows. And you can see all of this happening in this nice chart right here. Now the final part of the uterine cycle is the secretory phase, where the uterus prepares for implantation. Basically, it gets ready for a fertilized egg to make its way there.
Now, it's worth noting that some organisms actually have what's known as an estrous cycle. And during an estrous cycle, the uterus actually reabsorbs the endometrium in the absence of pregnancy, rather than, secreting or shedding the endometrial lining like we see in humans. And here you can see the various phases, up top we have our, phases of the ovarian cycle, you can see the follicular phase, the luteal phase, and right here in the middle that's ovulation. And notice how on this graph showing hormone levels, we have estradiol reaching its peak rather, right there. That triggers that LH spike that we see and that is what causes ovulation, where the ovum is released from the follicle. And here you can see an example of the shedding and the building up of the endometrial lining. Right, with that let's go ahead and flip the page.