Just like a woman’s ovaries go through the monthly cycle of recruiting eggs, her uterus also goes through a monthly cycle as it prepares for the possible implantation of a fertilized egg. This lesson will introduce you to the phases and hormones of the female’s uterine cycle.
I know you’ve all heard of PMS, right? You know – premenstrual syndrome.
I know you guys may think it’s just those weird monthly mood swings that females go through where they’re laughing one minute and crying the next. And girls, I know what you’re thinking, ‘What mood swings?’, right? Or maybe you’re remembering those cramps and achy muscles you have to deal with for a few days every month. But did you know about the physiological reason for it? Have you ever thought about what the words actually mean?Well, ‘pre’ obviously means ‘before’ and ‘menstrual’ means the menstrual cycle, but what about the word ‘syndrome’? Well, ‘syndrome’ is used to classify PMS as a condition.
However, despite what you may think, not every woman experiences symptoms of PMS, and for those that do, their symptoms vary widely. So, what exactly is this menstrual cycle that PMS precedes?
The Uterine Cycle
The uterine cycle (menstrual cycle) is the monthly series of changes that the female’s uterus, or uterine tissue, undergoes in preparation for the implantation of a fertilized egg.
And, like all other cycles in biology, there’re different phases – three in this case.Each phase involves changes in the uterine wall, which is made up of three different layers. There’s the outer perimetrium, the middle and muscular myometrium (that’s the thickest layer), and the inner and thinner endometrium.
But only one of these layers undergoes the changes during the uterine cycle, and that’s the endometrium. The endometrial tissue itself is also made up of two layers: there’s the basilar zone, which attaches the endometrium to the underlying myometrium, and the functional zone, which is the part that changes throughout the uterine cycle.
The first phase of the cycle is menses. Menses is the beginning of the uterine cycle, when the unneeded uterine tissue sheds. This is going to be followed by the proliferative phase, which is a period of tissue regeneration, and then, last but not least, the secretory phase, where the arteries and glands supplying the tissue regenerate.
All together, the uterine cycle takes an average of 28 days, but this can vary from 21-35 days based on the individual female.
Each month, the uterus prepares for the possibility of fertilization. It does so by preparing or creating a new layer of endometrial tissue. Remember how we said the uterus was made up of three layers? Well, you can think of the layers this way: the endometrial layer is the layer that’s changed every month – kind of like the sheets on a bed. And the myometrial layer – that’s like the comforter on your bed, and that doesn’t get changed as often, and, in the case of the uterus, the myometrium doesn’t get changed at all.
Each month, just like you would put new sheets on a bed, the endometrium prepares new layers of tissue.If fertilization occurs, the fertilized oocyte, or egg, travels to the uterus, where it implants itself into the uterine wall – kind of like it’s nestling itself under the sheets of a bed. From there, it continues its development for the next nine months of pregnancy.If fertilization does not occur, the uterus has to start all over again! It does this first by shedding all of its tissue in the functional zone – that’s the menses phase – and then rebuilding it during the proliferative phase. That takes about two weeks. Once rebuilt, the secretory phase helps finish up the final preparations, just in case the next oocyte is fertilized.
Uterine tissue isn’t the only thing that changes during the cycle.
The female’s steroid hormone levels also change – most importantly, the hormones estrogen and progesterone.
Both of these hormones are produced by the female’s ovaries during different parts of her ovarian cycle. You see, at the same time the uterus is progressing through its cycle, the ovaries are progressing through a cycle of their own, called the ovarian cycle. This cycle is also monthly, and it involves the stages of oocyte growth and development, preparing the oocyte for ovulation and fertilization.
Details on this can be found in other lessons, but for now, all you need to know is that estrogen is produced by the ovaries during the follicular phase of the ovarian cycle and progesterone is produced by the corpus luteum during the luteal phase of the ovarian cycle. Now, each of these hormones plays a role in the changes that the uterine tissue goes through.Estrogen levels begin to increase during the proliferative phase of the uterine cycle. Now, as you may have noticed, estrogen decreases drastically after ovulation, but don’t let that fool you. It’s still there – it’s just at lower levels. The medium to high levels of estrogen are needed to prepare the uterine tissue for implantation, but the lower levels are needed to maintain it.Okay, so how does estrogen prepare the uterus? Well, it does so by building up the tissue in the uterine wall:
- It helps to thicken the uterine wall by increasing the number and the size of cells in the functional zone of the endometrium.
- It increases blood supply to the uterus.
After estrogen has finished preparing the uterine wall, progesterone continues this process by putting on the finishing touches. This happens during the secretory phase. That’s why you see a rise in progesterone after ovulation.
- Progesterone increases vascularization of the functional zone by providing increased blood flow.
- It also inhibits uterine muscle contractions that might prevent an egg from implanting.
- It helps modify the ‘tone,’ or the consistency, of the tissue by working with estrogen to enlarge the uterine glands and increase their secretions. This helps make the lining more suitable for implantation.
Okay, so now let’s go back through a quick review of today’s lesson. We talked about the uterine cycle phases and hormones. The uterine cycle has three phases:
- Menses is the beginning of the cycle, and it starts with the shedding of the functional zone of the endometrium.
This is followed by the rebuilding of the functional zone during the next two phases.
- The proliferative phase uses estrogen to help rebuild the functional zone, increasing its thickness.
- The secretory phase uses progesterone to help prepare the uterine wall for implantation of the fertilized egg.
Now remember, this whole cycle takes about 28 days, and it’s occurring at the same time as the female’s ovarian cycle. Why is this important? Well, because the ovaries and the uterus work together in order to get the body ready for reproduction.
The ovaries prepare the oocyte for fertilization, and the uterus prepares itself to protect and nourish the fertilized oocyte.So, like many other systems in your body, they have to work together to get the job done!
This lesson will help you to:
- Define premenstrual syndrome
- Describe the uterine cycle and list the three phases
- Understand that the ovaries and uterus must work together for reproduction and what happens if the egg is unfertilized