Müllerian Ducts and Sex Differentiation

Something that has always stuck with me since Intro to Biology years ago was something my professor said: females are the prototypical human sex. In other words, we all start out as females in a sense. It is not until around the 8 week benchmark in fetal development that a fetus starts to develop distinct sex characteristics. Generally speaking, from the moment of fertilization the embryo has an innate genetic sex (XX in a female; XY in a male). However, if you were to see a fetus before the 8 week mark, you would see that the genitalia is undifferentiated between XX and XY. That means that a female fetus and a male fetus look the same between the legs, so to speak.

So, why is that?  Why do female and male genitalia not differentiate from the moment of fertilization?  And what causes the differentiation when it happens?  And what did that professor mean by saying that females are the prototypical humans?  Well, it all has to do with the Müllerian ducts and hormonal reactions during pregnancy.

The Müllerian ducts are a set canals in the urogenital region of an embryo (that is, where the urinary and genital structures develop).  Depending on which way sex differentiation goes, the Müllerian ducts develop into the Fallopian tubes, uterus, and upper part of the vagina or they will begin to disappear leaving only small vestigial remains.  (The male reproductive organs develop out of the adjacent Wolffian duct.  These ducts begin to disappear during sex differentiation in females.)  Here is a diagram:

For reference, here is a list of homologous human reproductive structures (for instance, before sex differentiation, the scrotum and the labia majora are one and the same).

It is around this 8 week period that hormones are released from within a male fetus from the testes (from cells called Sertoli cells).  This is called the anti-Müllerian hormone.  The chromosomes of a male fetus receive this hormone and react by impeding the development of the Müllerian ducts.  In a female fetus, the chromosomes do not exist so the Müllerian ducts continue to develop.  From time to time, the necessary chromosome to inhibit the development of the Müllerian ducts in the male are missing.  (Remember that human chromosomes are incredibly complex, so when DNA is being “written” sometimes it can make a “typo.”)  When this happens, the fetus continues to develop the Müllerian ducts.  So, the genetically male fetus begins to grow a uterus and sometimes other female reproductive structures.  Usually, the testicles do not descend but a penis will still be present because it does not develop from the Müllerian ducts.  This is one of the many complications of determining sex at birth.  It may not be immediately clear what the child’s sex is.  This is called Persistent Müllerian duct syndrome (PMDS) and can also result as a failure of the testes to ever secrete the hormone.

So, going back to the words of my professor, all humans begin as embryos with the same “feminine” appearing genitalia.  This is why he says, with glee, that females are the prototypical humans.  Keep in mind all that the ancient Greek philosophers argued (and Freud, for that matter) that men were the essential human form and that women are defective versions of males.  Turns out, that all men start out as women in a manner.  There is a lot more to be said on this topic, but I will stop here for now.  Use the information you have learned here to impress your friends and put any obnoxious men in their place.  Please feel free to leave comments and questions.

Endometriosis

In the past two weeks, I cannot tell you how many times I have heard someone mention in conversation “endometriosis.” I have several friends and family members who have endometriosis, so it is not an altogether unfamiliar subject to me. However, I have never given much thought to endometriosis.   So, in writing this blog entry I hope to educate both myself and my readers a bit.

To start with the root, the word endometriosis comes from the Greek words meaning “inside” and “womb.”  Remember that the endometrium is the layer lining the uterus.  Here is a diagram:

The endometrium is where the embryo implants after it has been fertilized, so it is an integral part of reproducing.  Without a healthy endometrium, an embryo is not likely to implant successfully.

Interestingly, too many endometrial cells are a bad thing, and this is what endometriosis is:  the proliferation (spread) of endometrial cells outside of the uterus, especially common on the ovaries.  These cells respond as the endometrium does to hormonal changes over the course of a menstrual cycle.  So, imagine that you have endometrial cells throughout your pelvis and not just in your uterus.  Menstrual cramps would be amplified–the pain difficult to bear.  So, if you have particularly unmanageable menstrual pain during your period, please visit your doctor.  You may have a disorder such as endometriosis.  Only investigation by a medical professional can determine the source of the pain.

It is estimated that between 5-10 percent of women are affected by endometriosis.  Its effects generally do not manifest until menarche (first menstrual cycle) and usually diminish following menopause, though not always.  A large number of women who are infertile are infertile because of endometriosis (about 20-50%).  The proportion of women with chronic pelvic pain who suffer from endometriosis is much higher (about 80%).  There seems to be an increase in incidence of endometriosis in families that affected by it.  Women with an immediate relative with endometriosis has a higher risk of having endometriosis.  There is some association between endometriosis and certain types of cancer (especially ovarian), so it is important to know if you have endometriosis or if you have another malady with similar symptoms.

Endometriosis usually appears in patches within the pelvis that are often visible to the human eye during surgery because they can appear as darkened bluish-black spots.  Here is a diagram example of endometriosis:

Not all endometriosis appears as darkened spots, so it is necessary sometime to perform biopsies to be certain.  Endometriosis causes an inflammatory response that often causes scar tissue.  This scar tissue is frequently problematic in a variety of ways, including being the cause of infertility.  The cause of endometriosis is, so far, unknown but it is believed that there is more than one possible cause.  Hopefully, further research will uncover causes that can help in management and treatment of the disease.  Symptoms, too, can vary widely across endometriosis sufferers.  Abdominal pain being the most common symptom, others include:

  • nausea, vomiting, fainting, dizzy spells
  • frequent or constant menstrual flow
  • chronic fatigue
  • heavy or long uncontrollable menstrual periods with small or large blood clots
  • mood swings
  • pain in legs and thighs
  • back pain
  • mild to extreme pain during intercourse
  • mild to severe fever
  • headaches
  • depression
  • Again, it is important to remember that other disorders can have similar symptoms to endometriosis.  It is crucial to consult a doctor if you are experiencing problems in order to accurately determine what you have.  Never rely on an “Internet diagnosis.”  This article is just scratching the surface of this topic (maybe I will write “Endometriosis:  Part 2”), so please feel free to leave comments and questions!

    Vulva

    I live!  I apologize greatly for such a prolonged absence, but it was important for me to focus these months on school.  It has paid off because as of today I hold a Master of Arts degree in Bioethics and Medical Humanities.  Hurrah!  Now, I hope to devote the proper amount of time to this blog.  I began writing an article about the vulva before my hiatus and decided to finish it for my return.

    So, what is the vulva? What’s the difference between the vulva and the vagina? Isn’t it all just one, connected thing down there? Why are there different names if it’s just a vagina, after all?  What does the vulva do?  All good questions. All questions I hope to answer.

    The word vulva comes from Latin and was used to mean “womb” or, more generally, “female genitals.”  In modern usage, the word vulva refers specifically to the the external genitals of the female.  The word vagina is often used as a catchall term to refer to both the internal and external reproductive organs of a female; however, to be completely correct, the vagina is an internal structure only and the vulva is the external structure only.  These external components that make up what we call the vulva include the clitoris, the labia majora and minora (outer and inner lips), the pubic mound (mons pubis), the vestibule of the vagina (the area inside the labia minora that includes the openings for the urethra and vagina), and the vaginal orifice (the actual opening of the vagina).

    Here is a diagram:

    Its development occurs during phases, particularly the fetal and pubertal stages.  As the entrance to the reproductive tract, it protects its opening by a “double door”:  the labia majora (large lips) and the labia minora (small lips).  The vagina is a self-cleaning organ with an environment that promotes healthy microorganisms that balance each other out and guard against invading, unhealthy elements.  Cleaning your vulva is important to gynecological health.  Simply use warm water and mild soap on a daily basis.  (Remember, this is for external use!)  It is unwise to use heavily perfumed soaps as they can irritate your vulva.  It is also unnecessary to douche unless a doctor specifically recommends it.  Douches can cause irritation and flush out those healthy microorganisms allowing for infection to set in.  The vulva is more vulnerable to infections than the external genitalia of males.  So, take good care of it.

    The vulva is key to sexual functioning.  The external structures of female genitalia are very full of nerve endings allowing for pleasure when properly stimulated.  When aroused, the vulva undergoes several physical changes it making it one of the external signs that a woman is aroused.  First, moisture from the vagina reaches the vaginal orifice, moistening the vulva.  The labia majora become enlarged and spread apart somewhat and can change color somewhat (darkened from increased blood flow).  The labia minora and the clitoris also increase in size.  During orgasm, the various muscles contract, though most of these contracting muscles are not located in the vulva.  Following orgasm, stimulation of the vulva may be uncomfortable or even painful.  The increased blood flow slowly dissipates until the vulva returns to normal.

    The vulva performs different functions than the vagina, thus it is important to know the difference between the vulva and the vagina.  Especially if you are talking to a medical professional, be sure to clarify whether you mean the internal structures (vagina) or the external structures (vulva).  Have anything to say about vulvas?  Have you say and leave a comment!