00:00
In this lecture, we're going to review the normal menstrual cycle. This is essential for you to
understand so you don't have to memorize things and can figure out questions on the USMLE just
by understanding the normal menstrual cycle. First, if you're getting a little tired during your
studying that's normal. I wanted to leave you with an inspirational quote. "If sometimes you feel
yourself a little useless, offended, and depressed always remember that you were once the fastest
and most victorious sperm out of hundreds of millions." Are you ready to get started? First
let's review oogenesis. In the first trimester already oogonium are making other oogonium through
mitosis. Into the second trimester, there becomes an arrest period or arrest phase. The oocyte
is suspended in. This is the depleting phase. In the third trimester actually nothing changes, not
until puberty when we start to ovulate do we ovulate a metaphase II egg. This egg losses its
first polar body. Then, if that egg undergoes fertilization, a second polar body is lost. Only then
is the egg haploid. This haploid cell then is fused with the sperm making another haploid cell to
together deploy. Let's review the normal menstrual cycle. There are 2 phases: follicular and luteal.
01:39
In the midcycle, we typically have ovulation. Here you'll see a 28-day cycle which is commonly
used to test on the USMLE. We have a rise in FSH so that we have recruitment of follicles from
the ovary. Then, we have a midcycle rise of luteinizing hormone. This causes the oocyte, which
which has been arrested in development, to resume meiosis and complete meiosis I. Then we also
have a corresponding rise in estradiol. Once the corpus luteum is made after ovulation, progesterone
increases in the luteal phase. How long do you think it takes to make an egg? It's okay if you
don't know, that won’t be an exam question. It takes nearly 300 days. I just want you to appreciate
that there are different stages of follicles within the ovary. The immature follicles in the resting
phase that constitute our ovarian reserve are primordial follicles. The next stage are preantral
follicles followed by antral follicles. As you can see here in C, there is an antrum or a fluid-filled
cavity. As the oocyte gets larger, it becomes a mature follicle ready for ovulation. How does
this occur? Well, we get LH input to LH receptors on granulosa cells after ovulation. This causes
luteinization of the granulosa cell. This happens because cholesterol is a precursor for estradiol
and progesterone. Now let's talk about fertilization and implantation. So, if you have normal
ovulation you'll release an MII egg or metaphase II egg. That egg has already released a polar
body and is receptive to fertilization. This is what it looks like when we take them out artificially
for IVF or in vitro fertilization. In the tube, the fallopian tube that is, sperm will swim to this egg
and fertilize it. Then, after fertilization, you have 2 pronuclei: 1 from the egg and 1 from the
sperm and you can see here there are 2 polar bodies. This is a sign of fertilization. After this
time, the zygote will split into a 2-cell embryo, then it will become a 4-cell embryo and on to an
8-cell embryo. This is an example of 2 beautiful 8-cell embryos. Then it will become a morula.
04:30
Next, a blastocyst. A blastocyst is an important type of embryo. Here, we can already see an
inner cell mass. This is kind of like a little tight ball of cells that will become the fetus. The cells
that surround the blastocyst will become placenta. They are called trophectoderm. This is essentially
what fertilization looks like using electron microscopy. Again, let's review normal fertility. If
you're ovulatory you will have an LH surge followed by ovulation, followed by fertilization in the
oviduct or in the fallopian tube, then a morula will occur in the uterus. Implantation will be done
by the blastocyst and hopefully we can now review some stages of implantation. There are 3 stages
to be correct. There's apposition which is essentially the blastocyst finding the endometrium
and staying there. Then there is a process called adhesion where there are certain molecules
that cause the blastocyst to actually adhere to the endometrium. Then there's invasion. Here
are several cartoons to show you how that occurs. Thank you for listening and good luck on
your exam.