Video Transcript
The diagrams show some key events that occur in the menstrual cycle. Starting with day one, put the stages in the correct order.
The menstrual cycle in females is incredibly important in preparing the body for
pregnancy. It is typically 28 days long. Let’s review the stages and the different hormones involved in order to answer this
question.
The menstrual cycle begins with menstruation on cycle days one to five. This is when the uterine lining from the previous cycle is shed. Since we’re looking for the correct order of these stages, we’ll indicate this as
number one. During menstruation, the pituitary gland begins secreting follicle-stimulating
hormone, also known as FSH, which stimulates the development of a follicle from the
ovaries. This follicle contains an egg cell.
The next stage is called the proliferative phase and occurs between cycle days six to
14. In the diagram, you can see an egg cell contained in a follicle in one of the
ovaries. During the proliferative phase, the developing follicle begins to secrete another
hormone called estrogen. Estrogen has two roles. It stimulates the cells of the uterine lining to proliferate, which is why this phase
is called the proliferative phase. And it stimulates the secretion of another hormone called luteinizing hormone, or LH,
from the pituitary gland. Since letter C corresponds to the second stage in our diagram, let’s indicate this as
number two.
The next stage is called ovulation, and this occurs around cycle day 14. Estrogen causes luteinizing hormone levels to peak around this time, which triggers
the follicle to release the mature egg cell from the follicle. This released egg can then be picked up by the fallopian tube. You can see the released egg in the fallopian tube in the diagram. Since letter D indicates the next stage, let’s mark this on the diagram.
The final stage is called the secretory phase and occurs between cycle days 14 to
28. During the secretory phase, the leftover follicle that once contained the egg becomes
a structure called the corpus luteum, which is labeled here.
The corpus luteum secretes progesterone, which is a hormone that maintains the thick
uterine lining. The actions of estrogen still work to thicken the lining, which you can see here. The egg that was released during ovulation may or may not be fertilized by a sperm
cell as it travels through the fallopian tube. If it is fertilized, then the resulting embryo implants in the thickened lining to
form a pregnancy. And the menstrual cycle pauses until after pregnancy. This is because the implanted embryo secretes another hormone that maintains the
corpus luteum to continue secreting progesterone.
If the egg is not fertilized and there is no implantation of the embryo, then the
corpus luteum degrades and progesterone levels begin to fall. Without the influence of progesterone, the thick uterine lining cannot be maintained
and it sheds during menstruation, which brings us back to cycle day one. The secretory phase is indicated as stage A in the diagram, so let’s label that.
We now have the stages of the menstrual cycle in the correct order. The order is B, C, D, A.
