Female Reproductive System #2

All of these statements about female gametogenesis are correct, EXCEPT:

A.  Just like in the case of the spermatogonia, the oogonia population is renewed throughout adult life.
B.  Primary oocyte is a diploid cell.
C.  Primary oocyte remains in prophase I until ovulation.
D.  Secondary oocyte comes out of metaphase II only if fertilization occurs.

Answer: A.

SHORT-AND-SWEET:

Female gametogenesis beings during the fetal life, with ovarian germ cells dividing mitotically to produce oogonia.  However, while the female fetus is still in utero, the germ cells will produce a finite number of oogonia and then stop the production (unlike spermatogonia production, which continues throughout the adult life -- answer A).  Diploid oogonia will then undergo meiosis, and arrest in prophase I as diploid primary oocyte.  Ovulation will trigger primary oocyte to continue through meiosis, but it will arrest again in metaphase II as the haploid secondary oocyte, to continue development into haploid mature ovum only if fertilization occurs.  

THE WHOLE STORY:

Before reviewing female gametogenesis (oogenesis), let's make sure we have solid understanding of the more fundamental matters.

First, let's clarify the cellular DNA content.

• With the exception of gametes, all other healthy human cells have 23 pairs of chromosomes -- each pair consisting of one Mom chromosome and one Dad chromosome, giving a grand total of 46 chromosomes per cell.  These cells are considered DIPLOID (2n).
• When the DNA replicates, each of the parent's chromosomes will consist of two identical sister chromatids.  That means that the cell which has undergone DNA replication will still have 46 different chromosomes, but each chromosome will consist of two sister chromatids (92 chromatids total per cell).  However, this cell is still considered DIPLOID (2n), because it has 46 chromosomes total.
• HAPLOID (n) cells, such as gametes, will contain only one copy of each chromosome (essentially, 23 chromatids).

Moving on to discuss M&M -- mitosis and meiosis.  These are the two types of cellular division.  Both are preceded by one round of DNA replication.

• Mitosis consists of one round of cell division following DNA replication, and it produces two daughter cells which are identical to their mother cell (all diploid, with 23 pairs of chromosomes).  Most of the cells in the human body which can replicate do so by mitosis.
• Meiosis, which occurs only in reproductive cells, consists of two sequential rounds of cell division (meiosis I and meiosis II) following DNA replication.  The final result of meiosis are four haploid gametes.  In male, these are four sperm cells, whereas female produces one ovum and three polar bodies.  During meiosis I there will be an opportunity for exchange of Mom's and Dad's genetic material, which ensures genetic diversity. 

Now that we understand the basics, let's look at what happens during oogenesis.  
Female gametogenesis begins during the fetal period.  The ovarian germ cells divide mitotically, producing multiple identical diploid oogonia.  Once they have produced several million oogonia, they stop dividing -- and ALL BEFORE BIRTH!  This is one of the key differences from the male gametogenesis, where the germ cells continue producing spermatogonia throughout adult life.  This helps us pick out answer choice A as the correct answer.  (If you were on the actual test you would go on to the next question......  How nice.  Wish every other question was this easy!)

But let's review the rest of the oogenesis.  In order to produce its progeny, oogonium cell will enter meiosis -- meiosis I, to be more specific.  However, it will not get far.  It will arrest in the first phase of meiosis I, prophase I, as the PRIMARY OOCYTE.

How "much" DNA does the primary oocyte have?  Well, we know that the DNA was replicated prior to meiosis, so the cell will have total of 46 chromosomes (92 sister chromatids).  Such cell is still considered diploid.  There is no division of genetic material during prophase I, which means that primary oocyte will also be diploid (eliminate answer choice B).

The lazy primary oocyte will remain in prophase I for years, until puberty and the first ovulation (eliminate answer choice C).  Just before ovulation the "selected" primary follicle and its primary oocyte will complete meiosis I, producing one SECONDARY OOCYTE (and one polar body to be discarded), which heads into meiosis II.  

What about secondary oocyte's DNA content?  What transpires during the rest of meiosis I?  The DNA is first exchanged between the homologous Mom and Dad chromosomes, and then during anaphase I, the homologous chromosomes (each consisting of two chromatids) are separated, yielding two haploid daughter cells, each with 23 chromosomes (either of Mom or Dad origin, each having two chromatids).

The secondary oocyte likes to chill out too, and arrests not long after its formation -- in metaphase II (good bye, answer choice D), and it stays there until the ovulated egg is fertilized by sperm.  While the sperm is trying to break in, the haploid secondary oocyte hurries to finish meiosis II, producing haploid ootid (and another polar body), which has one copy of each chromosome.

The sperm and ootid nuclei will not fuse until the second polar body is released, and the ootid matures producing a mature ovum.  After their nuclei fuse, they will produce a diploid fertilized egg, or a zygote.

Basically, you can think of oogenesis as one big arrested development (like the TV show!), with lazy primary and secondary oocytes napping whenever they can.  But how to remember when each will take a nap?  Easy -- number 1 and number 2!
Number 1:  The PRIMARY oocyte will arrest in the FIRST phase (prophase) of meiosis I.
Number 2:  SECONDARY oocyte will arrest in the SECOND phase (metaphase) of meiosis II.

Be selected or die is the other motto of oogenesis!  Only the primary oocyte which gets "selected" for ovulation in a particular month continues its development.  All of the other ones prepping for that month's ovulation die.  Similarly, the ovulated oocyte arrested in metaphase II needs to be fertilized in order to continue its development.  If there is no fertilization, it will die a sad and lonely death.

BIG PICTURE:
1.  Diploid cells have 23 pairs of chromosomes, each pair consisting of one Mom and one Dad chromosome.  Haploid cells have 23 chromosomes, each either being of Mom origin, Dad origin, or mixed Mam/Dad origin (due to recombination which occurs during meiosis).

2.  DNA replication precedes both mitosis and meiosis.  Mitosis consists of one round of cell division, which produces two daughter cells, genetically identical to their mother cell (all 2n).  Meiosis consists of two rounds of cell division, producing four haploid cells.

3.  Oogenesis is one big arrested development show!  Diploid oogonium produces diploid primary oocyte, which arrests in prophase I.  Ovulation pushes it forward, but the oocyte arrests again in metaphase II as the secondary oocyte.  Finally, if it gets fertilized it will continue its development to mature ovum.

~The MCAT POD Team~

Female Reproductive System #1

Which one of the following is associated with the shedding of endometrium during menstruation?


A. Formation of the corpus luteum
B. Rise in estrogen and progesterone
C. Rise in hCG
D. Low LH level

ANSWER: 
D. Low LH level 


SHORT-AND-SWEET:


Menstruation is usually considered to be the beginning of the menstrual cycle, but we suggest you consider it the last phase, which happens if there is no conception.  
No conception means no placenta, which subsequently means no hCG (which eliminates C).  The formation of corpus luteum is associated with the luteal phase (as the name suggests), which occurs after the ovulation and before menstruation (which eliminates A).  Estrogen and progesterone are after ovulation produced by the corpus luteum, so when the corpus luteum dies towards the end of the cycle, levels of estrogen and progesterone drop, which leads to the endometrial shedding (this eliminates B).  
We are left with answer choice D, which is the correct answer.  Remember that at the time of menstruation levels of all hormones are low.


THE WHOLE STORY:  


Female reproductive system is one of the most dreaded topics in the MCAT Biological Sciences section, which is exactly why we included it in our blog.  While others are wasting their time in needless memorization, we will help you understand the female reproductive system.  And there will be no diagrams -- we will think of it as a story.

We will start off with the big picture:  The purpose of the female reproductive system is to enable reproduction!!!  In order for this to happen, what does this system need to provide?

1. A mature egg (oocyte)

2. A way to deliver the egg to a location where sperm can fertilize it (ovulation)

3. A nice home for an embryo (in the uterus)

Think of the uterus, ovaries, and the anterior pituitary as the key players in the female reproductive system, and in particular, think of the anterior pituitary as the brains of the operation.  What do we need to memorize about them?  (We will ask you to memorize only two things in this topics, and this is one of them.) 
   -Non-gravid uterus does not produce hormones.  
   -Ovaries produce estrogen and progesterone.  
   -The anterior pituitary produces FSH (follicle-stimulating hormone) and LH (luteinizing hormone).  


Now that we know the primary purpose of the female reproductive system, the key players, and what they produce, we can start understanding the logic behind the menstrual cycle. 


Let's start by thinking about what happens after the menstruation is over.  The uterus lining is completely bare.  The ovaries, with their numerous immature follicles, are resting, taking a nice little nap.  The anterior pituitary, the brain of the operation, is resting as well.  For now.  


The brain knows that the purpose of this system is to reproduce, and BAM! -- the brain wakes up!  It starts producing FSH to wake up the ovarian follicles, stimulating them to mature.  As the ovarian follicles mature, they produce more and more estrogen.  Estrogen in turn acts on the uterus, causing the proliferation of the endometrial lining.  

Because of the endometrial proliferation and follicular maturation, which occur during this phase, this phase is called the proliferative or follicular phase.  

The product of this phase: 1. MATURE EGG. 

At the same time estrogen, which is produced by the ovaries, exerts negative feedback on the anterior pituitary, inhibiting the secretion of FSH and LH (mechanism seen in the other endocrine systems).  However, when estrogen reaches critically high level, instead of inhibiting LH production, it causes transient LH surge (positive feedback), which subsequently causes follicle rupture, or ovulation.  The LH surge tires the anterior pituitary, which now wants to rest, leading to low LH and FSH levels.

The product of this phase:  2. MATURE EGG DELIVERED TO THE FALLOPIAN TUBE, WHERE IT CAN BE FERTILIZED BY SPERM.

Now that the oocyte is delivered to the fallopian tube, waiting to be fertilized by a sperm, all eyes turn to the uterine endometrium, which now needs to make sure to provide a suitable home for a possible embryo.  How does this happen?


This happens in coordination with the ovaries.  The cells that were surrounding the oocyte in the follicle remain in the form of corpus luteum, and they continue to produce estrogen and progesterone (as they did during the proliferative (follicular) phase).  It is the progesterone (and to a lesser degree, estrogen) secreted by corpus luteum that acts on the endometrium, causing it to develop a thick juicy lining, called the secretory lining.  The secretory lining makes for a perfect embryo home.  

Because of the secretory endometrium and the formation of corpus luteum, this phase is called secretory or luteal phase.  

The product of this phase:  3. NICE HOME FOR A POSSIBLE EMBRYO.

But what happens if there is no fertilization?  
Here is that second thing to memorize: Corpus luteum has an internal "timer", and without hormonal stimulation it will die on its own after 14 days.  As it dies, estrogen and progesterone levels drop, and without these hormones the thick endometrial lining sheds, causing menstrual bleeding.


This brings us back to our initial question about what happens during menstruation.  Like we just said, the menstruation is what occurs when corpus luteum dies, which eliminates A.  As the corpus luteum dies, estrogen and progesterone will decrease, which eliminates B.  hCG (human chorionic gonadotropin) is a hormone produced by the placenta, which forms at the start of pregnancy.  There is no menstruation during pregnancy, which eliminates C. 


Answer D is the correct answer.  Like we said, during menstruation and immediately afterwards, the female reproductive system is "resting".  This includes the pituitary, which secretes minimal amounts of LH and FSH.  It is when this brain of the operation wakes up that the cycle starts anew.     


BIG PICTURE:  

The purpose of the female reproductive system is to enable reproduction!!!  In order for this to happen, the system needs to provide:

1. A mature egg (oocyte) - oocyte maturation is stimulated by FSH (follicle-stimulating hormone).

2. A way to deliver the egg to a location where sperm can fertilize it (think ovulation) - triggered by LH surge.

3. A nice home for an embryo (in the uterus) - stimulated initially by estrogen produced by the ovarian follicle, and then by progesterone produced by corpus luterum, the ovarian follicle remnant.

~The MCAT POD Team~