Expert: Dana Krempels, Ph.D. - 4/9/2008

Question
QUESTION: I have two questions.  

1)   There is an “acid wash” as I have heard it referred to that happens to the brains of mostly males prior to birth. Can you tell me when this occurs and what the chemical is that is involved in the process? Where would I find the documentation to back up this event?
2)   A long time ago I heard of a discovery that male in the womb has an open vagina until a certain week and then the vagina sealed up and the genitals then grew outward instead of inward. Can you tell me which week this happens in and where I can find documentation?  

Thank you for you assistance

Jim


ANSWER: Dear Jim,

1.  I have never heard of this "acid wash" in male fetuses prior to birth.  If you can find a more biologically precise term for this, I might be able to help you.

2.  The "default" pathway for all mammals during development is to become female.  In humans, the SRY gene (which triggers the male-determining cascade) turns on at about 8 weeks of gestation.

All human embryos start with two duct systems, the Wolffian ducts (which will become the male "plumbing") and the Mullerian ducts (which will become the female "plumbing").  When the male-determining cascade initiates, the Mullerian duct system (which would include what would become the vagina) degenerates and is lost.

In a female embryo, the Od (Ovary-determining) genes turn on at about week 13, and at that point, the Wolffian ducts degenerate and are lost.   

However, there are still structures in both male and female embryos that are homologous--derived from the same tissues.  The female clitoris and male penis are derived from the same embryonic source; the vagina is not at all homologous to the penis, so it can't really be said that the same tissue grows "outwards" in the male and "inwards" in a female.  

You can find an excellent overview of both the genetic and phenotypic (i.e., physical) aspects of sex determination in humans here:

http://student.bmj.com/issues/04/11/education/400.php

Hope that will help clear things up.  

Good luck!

Dana




---------- FOLLOW-UP ----------

QUESTION: Dana,

Thank you for your reply. I am looking to find out a more tyical name for the "acid wash" which takes place.

in your answer you stated that

"the Mullerian duct system (which would include what would become the vagina) degenerates and is lost."

If it totally degenerates how is it that every male has a perfectly straight scare on his scrotum here it healed closed.  This straight line that a person is born with that is clearly a spot where two sides have healed together.   This is the only “scare” that any human is born with.

From what I remember it is around the 19 week that this process takes place, it that in line with your knowledge?

Thank you again for your assistance and I  hope that you have a wonderful day.

Sincerely

Jim

Answer
Dear Jim,

The scrotal scar is due to the fusion in the male of the tissues that would have become the labia majora in the female.  They are not part of the Mullerian duct system.  A male embryo does not have an open vagina during development for this reason, but he *does* have an open urethra.  As the penis and scrotum develop normally, fusing tissues that remain open in the male, then the urethra is enveloped in those tissues.

In the female, the urethra and vaginal openings are separate because the tissues that comprise the scrotum in the male do not join and fuse over them.

I hope that helps clear things up.

Dana