Did you know that over 40% of American adults do not know that the sex of a fetus is determined by the sex chromosome of the sperm, and not the egg? It is a somewhat shocking statistic, but awareness about reprodudctive biology is low even in the developed world. Before we dive into what intersex, or DSD is, we first need to know about general reproductive anatomy and human sex (medically speaking).
You may have learned about it in sex ed, during high school biology class, or just have a general idea from life experiences. Most of us have heard at some point that “a male body has a penis” and “a female body has a vagina”. We may have also generally observed that children can’t be differentiated sex wise by their bodies (boys and girls look and sound generally the same before puberty), but we know generally who is an adult man or a woman regardless of clothing most of the time. We know that boys grow taller and broader and their voices deepen during puberty, and we know that girls grow breasts, hips and start menstruating, from our sex ed classes. We understand that testicles make testosterone for male bodies, and ovaries make estrogen for female bodies. But this is only parts of what “sex” is, and likely the only parts we were exposed to because of how it affects our lives. We are rarely taught about the medical definitions of sex, unless we major in biology or go to medical school.
Biological or anatomical sex, as we know it, is comprised of the five sex characteristic categories. Sex chromosomes, gonads, internal reproductive structures, genitalia, and sex hormone profile. Below is the breakdown of each characteristic in the typical male versus typical female body.
Typical Male
- XY
- Testicles
- Prostate, seminal vesicles
- Penis, scrotum (male genitalia)
- Testosterone dominant
Typical Female
- XX
- Ovaries
- Uterus, cervix, Fallopian tubes
- Vulva: clitoris, labia majora/minora, vagina
- Estrogen and progesterone dominant
Sex is first expressed in genitalia at birth, and then a second time in the changes the body undergoes during pubertal development (the process through which someone goes from child to adult, and gains the development and capacity to reproduce).
In a typical male body, pubertal development is testosterone dominant and progresses through 5 roughly estimated stages in development (though this varies from person to person). These are called the Tanner Stages, and they are listed below as a general way of understanding what is expected during male puberty.
Tanner 1: No changes
Tanner 2: Pubic hair begins to develop, height growth begins to speed up, and the facial structure appears to change. The scrotum reddens and the testicles enlarge.
Tanner 3: Pubic hair continues to grow and become darker, the growth spurt begins. The genitalia enlarge further, and the teen may begin to have wet dreams (night time ejaculation). The voice begins to crack and deepen. Pubertal gynecomastia occurs in 50% of boys at this time.
Tanner 4: Pubic hair fills out and becomes wiry. The peak of the growth spurt begins. Shoulders and rib cage begins to broad. The voice has permanently deepened and no longer cracks. The genitalia continues to enlarge.
Tanner 5: Pubic hair reaches its adult stage and takes on a triangular shape in the groin. Height growth begins to slow and will stop in a few years. The genitalia has reached full adult size. Sperm production has begun to take place by this stage. Facial hair will begin to come in, and so will body hair on the torso. At this point, pubertal gynecomastia recedes.
In a typical female body, pubertal development is governed by relative absence of testosterone and the slow increase of estrogen and progesterone. It also progresses roughly through 5 stages, and they are listed below. Of course, not everyone experiences the stages in the exact order, or in the same timeline (stalling can be common), and that is largely dependent on genetics and environmental factors.
Tanner 1: No changes
Tanner 2: Pubic hair begins to develop, height growth speeds up, and small breast buds begin to form under the nipples
Tanner 3: Pubic hair continues to grow and becomes thicker, and the peak of the growth spurt begins. Some genital changes begin to occur. The breasts enlarge beyond breast buds, and now are visible protrusions from the chest.
Tanner 4: Pubic hair darkens and thickens and now covers the pubic bone. Growth begins to slow down, but the hips begin to widen. The areola begins to change and will become bigger, elevating to a small mound on top of the breasts. Near the end of this stage, menstruation begins.
Tanner 5: Pubic hair reaches adult stage and reaches the medial thigh. Growth slows and then stops, and the menstrual cycle stabilizes after one to two years. The areola on the breasts mature and lose the temporary elevation, and the breasts reach adult size and shape.
So how is sex differentiated? What happens in the womb that causes one to be male or female? Sex differentiation starts before the egg is fertilized — typically, the sex of the fetus is determined by the sperm. The egg and the sperm each carry half of a full set of chromosomes (23 each). The egg will always express an X chromosome, and the sperm expresses either an X or a Y. After the egg is fertilized, a Y sperm will complete an XY sex karyotype, while an X sperm will complete an XX sex karyotype.
Onwards, the sex determinant gene SRY on the XY chromosomes will activate to differentiate testes from the primitive gonads, and the absence of SRY on the XX chromosomes will differentiate the primitive gonads into ovaries. From here on out, the testes will start producing testosterone which will differentiate seminal vesicles and the prostate from the viscera. They will also release a hormone called anti mullerian hormone, which will prevent the development of a uterus. Finally, conversion of testosterone into dihydrotestosterone will cause the growth of the penis and the fusion of labioscrotal (the primitive genital form) folds into the scrotum. In the fetus with ovaries, the lack of anti mullerian hormone will cause mullerian structures (uterus, fallopian tubes, cervix) to form, and the lack of testosterone will cause genitalia to form into the labia, vagina and clitoris.