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External Genital Development in Males and Females

Chapter 19 /  Lesson 8   Transcript
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  • 0:07 Males and Females All…
  • 1:19 DHT
  • 5:07 Sexual Differentiation
  • 7:51 Lesson Summary
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Lesson Transcript
Instructor: Heather Adewale

Heather has taught reproductive biology and has researched neuro, repro and endocrinology. She has a PhD in Zoology/Biology.

The developed genitals of a male look much different than the developed genitals of a female. However, they both grow from identical sets of tissues. Learn more about external genital development and find out how and when the tissue changes.

Males and Females All Start the Same

While many fetal developmental processes are similar between males and females, there is one particular process that is quite different. Yes, as you may have guessed, I'm talking about the development of the male and female external genitalia!

Now, even though you probably guessed that's what I was referring to, did you also know that both males' and females' genitals actually start out the same? You see, just like the internal gonads, the testes and ovaries, developed from the same fetal structure, the external reproductive organs also develop from the same set of tissues. Up until about 2-2.5 months after conception (or fertilization), males and females look the same, even on the outside!

Early male and female genital tissues start out the same.
Early Genital Tissues

But obviously, even though they may start out looking the same, they end up looking quite different. So, how does this happen? What is it that makes these initially identical tissues develop into gender-specific structures? Well, it all comes down to one little word - hormones, specifically the hormones produced by the gonads.

DHT

You see, once the internal gonads have developed, the male's testes start producing a hormone called testosterone. This happens between weeks 7-9 of gestation and is the beginning of the steps leading to the differences in male and female development. This process is known as sexual differentiation and can officially be defined as the process of developing sex- or gender-specific tissues, organs, and functions, and while this can refer to many different developmental processes, in this lesson, we are focusing on the sexual differentiation of the external genitals.

So, let's start at the beginning! Okay, not all the way at the beginning - that would be when fertilization occurs. Let's skip ahead to about the end of month two/beginning of month three. You may remember that the entire process of fetal development takes about nine months, or 40 weeks. By the end of month two, there are already differences between males and females. However, these differences are only on the inside. By now, the internal gonads have developed into either testes or ovaries. However, the external structures still look pretty much the same.

Both males and females start out in what is called a state of undifferentiation, where their tissues look exactly the same and even have the same potential to develop into either male or female parts! They both kinda look like the picture below - neither male nor female.

undifferentiated male and female

But, once the male's testes start producing testosterone, things begin to change! Testosterone is released from the testes into the bloodstream. Testosterone is a male steroid hormone that is part of a class of hormones called androgens. Another hormone in this class is dihydrotestosterone, or DHT for short. While testosterone is important for many aspects of male development, about 5% of testosterone is converted into DHT. And, it is DHT that's the main hormone required for normal external development of the male genitals.

Now, in order for DHT to be able to carry out its effects on the external tissues, it has to be able to bind to them. It does that by binding to receptors for androgens known as androgen receptors. You can think of it kind of like a lock and key method. The receptor is like the lock, and, like any lock, it can only be opened by certain keys. And, the keys for our androgen receptor locks are the androgen hormones, like testosterone and DHT. Each of these has a unique structure, allowing it to fit into and activate the receptor.

DHT, or the lack thereof, effects how the genitals develop.
Sexual Differentiation

Once the DHT has bound to its receptors, it begins to carry out its job. You may remember from other lessons that hormones act like messengers, and the message DHT is carrying are instructions for the genital tissues of the male. These instructions direct the development, or differentiation, of the undifferentiated tissues. Without testosterone, DHT, or androgen receptors, the undifferentiated tissues would end up following more of a female path of differentiation.

So, what are these changes I keep talking about? Well, first let's divide our tissues into three different sections: the genital tubercle at the top, the labioscrotal swellings on the side, and the urethral folds in the middle.

Sexual Differentiation

Both sexes develop urogenital and anal membranes early in sexual differentiation.
Urogenital Anal Membranes

Under the influence of androgens, mostly DHT, the male tissues will enlarge and fuse to create the penis and the scrotum. In comparison, the female tissues will develop into the female genitals in the absence of androgens.

1. First, in both sexes, the urethral folds elongate and fuse. This splits the cloacal membrane, located in between the urethral folds, into the urogenital membrane up at the top and the anal membrane below.

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Ch 19. Early Development to Childbirth
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