Adolescence begins with the onset of puberty, a developmental period in which hormonal changes cause rapid physical alterations in the body, culminating in sexual maturity. Although the timing varies across cultures, the average age range for reaching puberty is between 9–14 years for females and between 10–17 years for males (Marshall & Tanner, 1986). For both males and females, these changes include a growth spurt in height, growth of pubic and underarm hair, and skin changes (e.g., pimples). Hormones drive these pubescent changes, particularly the increase in testosterone for males and estrogen for females. This chapter will focus on the physical and body chemistry changes associated with puberty. Chapter 14 will expand on these topics as part of more in-depth information on sexual development and topics such as gender identity and sexual orientation.

Sex differentiation, also referred to as sexual differentiation, is the process in which genitals and reproductive organs develop within the womb. This occurs as a result of complex hormonal processes that alter neutral tissues to develop along female or male lines, or some combination of the two. Secondary sex characteristics begin to develop further as part of the sex differentiation process during puberty. Therefore, prenatal development, as well as changes that further occur during puberty, create a cascade of events that results in physical changes to the body. Chromosomes, genes, gonads, hormones, and hormone receptor sites play key roles within the endocrine system that influence sex development. Sex is both a genetic and environmental experience in which epigenetic factors can alter the way that genes function, resulting in changes to hormone production and hormone receptor sites at different points across the lifespan.

Puberty is the stage of development at which individuals become sexually mature. Though the outcomes of puberty for males and females are very different, the hormonal control of the process is very similar. In addition, though the timing of these events varies between individuals, the sequence of changes that occur is predictable for male and female adolescents. A concerted release of hormones from the hypothalamus (GnRH), the anterior pituitary (LH and FSH), and the gonads (either testosterone or estrogen) is responsible for the maturation of the reproductive systems and the development of secondary sex characteristics, which are physical changes that serve auxiliary roles in reproduction.

The first changes begin around the age of eight or nine when the production of LH becomes detectable. The release of LH occurs primarily at night during sleep and precedes the physical changes of puberty by several years. In prepubertal children, the sensitivity of the negative feedback system in the hypothalamus and pituitary is very high. This means that very low concentrations of androgens or estrogens will negatively feedback onto the hypothalamus and pituitary, keeping the production of GnRH, LH, and FSH low.

As an individual approaches puberty, two changes in sensitivity occur. The first is a decrease of sensitivity in the hypothalamus and pituitary to negative feedback, meaning that it takes increasingly larger concentrations of sex steroid hormones to stop the production of LH and FSH. The second change in sensitivity is an increase in sensitivity of the gonads to the FSH and LH signals, meaning the gonads of adults are more responsive to gonadotropins than are the gonads of children. As a result of these two changes, the levels of LH and FSH slowly increase and lead to the enlargement and maturation of the gonads, which in turn leads to secretion of higher levels of sex hormones and the initiation of spermatogenesis and folliculogenesis.

In addition to age, multiple factors can affect the age of onset of puberty, including genetics, environment, and psychological stress. One of the more important influences may be nutrition. Historical data demonstrate the effect of better and more consistent nutrition on the age of menarche (menstruation) in females in the United States, which decreased from an average age of approximately 17 years in 1860 to the current age of approximately 12.75 years in 1960, as it remains today (Martinez, 2020). Some studies indicate a link between puberty onset and the amount of stored fat in an individual. This effect is more pronounced in females, but has been documented in males as well. Body fat, corresponding with secretion of the hormone leptin by adipose cells, appears to have a strong role in determining menarche. This may reflect to some extent the high metabolic costs of gestation and lactation. In females who are lean and highly active, such as gymnasts, there is often a delay in the onset of puberty.

Signs of Puberty

Different sex steroid hormone concentrations between the sexes in general and within each individual uniquely also contribute to the development and function of secondary sexual characteristics. Examples of secondary sexual characteristics are listed in Figure 12.1. Each individual’s hormone concentrations will depend upon genetics, diet, stress, and more. Thus, secondary sex characteristics will progress along a continuum of possibilities based on these nature multiplied by nurture combinations.

Something to keep in mind as well is that people can experience varying degrees of muscular, breast, and hair development depending on their genetic and environmental makeup. For instance, some males at the beginning of puberty may see breast tissue growth as well as other body fat increases, and some females may have facial and body hair growth.

One factor in this complex process is how unused testosterone is converted to estrogen, making it not about how much testosterone is present but rather how that testosterone is being used or unused. When testosterone is either excessively or insufficiently converted to estrogen, it can have significant impacts on both boys and girls. In boys, an over conversion of testosterone to estrogen can lead to conditions such as gynecomastia, where breast tissue develops, causing physical and psychological discomfort. Excess estrogen may also interfere with muscle development and result in premature closure of growth plates, potentially leading to a shorter stature. On the other hand, inadequate conversion of testosterone to estrogen can impair bone health, increasing the risk of osteoporosis and fractures. It can also cause delayed or abnormal sexual development and contribute to emotional and behavioral issues due to hormonal imbalances. In girls, excessive conversion of testosterone to estrogen can result in early puberty, causing the early development of secondary sexual characteristics and potentially disrupting growth patterns. Elevated estrogen levels may also lead to menstrual irregularities and increase the risk of estrogen-related health conditions, such as certain cancers. Conversely, insufficient conversion of testosterone to estrogen can delay puberty, affecting the timely development of secondary sexual characteristics and menstruation. Additionally, low estrogen levels can compromise bone density, raising the risk of osteoporosis, and impact emotional stability and cognitive function, potentially leading to mood disorders and difficulties with memory and concentration. Maintaining a proper balance in the conversion of testosterone to estrogen is therefore crucial for ensuring healthy development and overall well-being in both sexes.

It is incorrect to view testosterone as a strictly “male” hormone and estrogen as a strictly “female” hormone. Intersex individuals, females, and males all have estrogen and testosterone impacting their changing bodies during puberty. To understand the person-level variations in hormones and their impact on development, individuals will need to analyze their specific hormone levels, the functionality of genes involved in sex differentiation, and environmental stressors and nutrition that may have epigenetic effects on their genes.

As a female reaches puberty, typically the first change that is visible is the development of the breast tissue. This is followed by the growth of axillary and pubic hair. A growth spurt normally starts at approximately age 9–11 and may last 2 years or more. During this time, a female’s height can increase 3 in. (7.6 cm) a year. The next step in puberty is menarche, the start of menstruation.

In males, the growth of the testes is typically the first physical sign of the beginning of puberty, which is followed by growth and pigmentation of the scrotum and growth of the penis. The next step is the growth of hair, including armpit, pubic, chest, and facial hair. Testosterone stimulates the growth of the larynx and thickening and lengthening of the vocal folds, which causes the voice to drop in pitch. The first fertile ejaculations typically appear at approximately 15 years of age, but this age can vary widely across individual males. Unlike the early growth spurt observed in females, the male growth spurt occurs toward the end of puberty, at approximately age 11–13, and a male’s height can increase as much as 4 in. (10.2 cm) a year. In some males, pubertal development can continue through the early 20s.

Male Anatomy

Males have both internal and external genitalia that are responsible for procreation and sexual intercourse. Males produce their sperm on a cycle. However, unlike the female’s ovulation cycle, the male sperm production cycle is constantly producing millions of sperm daily. The male sex organs are the penis and the testicles, the latter of which produce semen and sperm. The semen and sperm, as a result of sexual intercourse, can fertilize an ovum in the female’s body; the fertilized ovum (zygote) develops into a fetus, which is later born as a child.

Male Reproductive Anatomy

Male reproductive anatomy involves the organs and glands that produce sperm, create semen to transport sperm, and conduct this liquid semen out of the body. Semen production involves the work of accessory glands, each responsible for the production of one or more key ingredients of semen.

Sperm production:

• Testis: Males typically have two testes (also called testicles), which, in humans, descend from the abdomen during fetal development and are enclosed outside the abdomen in the scrotum. Each testis houses many tube-like structures (the seminiferous tubules) in which sperm are made. Specialized cells (the Leydig cells) in the testes produce testosterone.
• Scrotum: This structure is a pouch of skin that holds the testes that contracts or expands to adjust the distance the testes are from the body to regulate their temperature.

• Seminiferous tubules: These structures within the testes are the actual sites of sperm production.
• Epididymis: This rubbery device sits astride the testis. Sperm mature here and are stored prior to ejaculation (when sperm-bearing semen leaves the body, typically during orgasm).

Semen production:

• Seminal vesicles: These two glands produce an alkaline (basic) fluid that can neutralize the acidity of the vagina. This fluid contains fructose and other nutrients to provide energy for the sperm.

• Bulbourethral (or Cowper’s) glands: These two glands provide a mucus-rich alkaline fluid that lubricates the inside of the urethra to allow for easier passage of sperm and neutralizes the urethra (urine residue is acidic). Some of this fluid exits the penis prior to ejaculation (this pre-ejaculate fluid can also contain sperm). The remainder of the fluid combines with the semen ejaculate.
• Prostate gland: This organ wraps around the urethra and provides muscular contractions that help propel semen during ejaculation and block urine flow from the bladder during ejaculation. It also provides fluid in the ejaculate that contains enzymes and zinc that aid in sperm motility.

Sperm/semen transport:

• Ductus (or vas) deferens: This pair of muscle-lined tubes carry sperm from the epididymis of each testis into the abdominal cavity where they loop over the bladder and join with the ducts from the seminal vesicles to form the ejaculatory ducts. The muscles that line the ductus deferens contract to propel semen during ejaculation.
• Ejaculatory ducts: These ducts are formed by the joining of the vas deferens with the duct from the seminal vesicle. Each ejaculatory duct empties into the urethra.
• Penis: This is the organ that encircles the urethra as the urethra exits the abdomen. This organ changes from flaccid (soft and limp) to erect (rigid and standing away from the body) during sexual arousal or spontaneously. In uncircumcised males, the penis has a fold of skin called a foreskin that covers the head of the penis during the flaccid state and retracts behind the glans (or head) of the penis during the erect state.

• Urethra: This tube runs from the bladder through the penis and enables urine and semen to exit the body.

Female Anatomy

Female external genitalia is collectively known as the vulva, which includes the mons veneris, labia majora, labia minora, clitoris, vaginal opening, and urethral opening. Female internal reproductive organs consist of the vagina, uterus, fallopian tubes, and ovaries. The uterus hosts the developing fetus, produces vaginal and uterine secretions, and passes the male’s sperm through to the fallopian tubes, while the ovaries release the eggs. A female is born with all her eggs already produced. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the fallopian tubes. Females have a monthly reproductive cycle. At certain intervals, the ovaries release an egg, which passes through the fallopian tube into the uterus. If the egg meets with sperm during its passage, the sperm might penetrate and merge with the egg, fertilizing it. If not fertilized, the egg and the tissue that was lining the uterus are flushed out of the system through menstruation (around every 28 days).

The female reproductive anatomy includes external structures (the clitoris and vulva), structures involved in supporting the health and maturation of eggs and in fetal development (ovaries, corpus luteum, and uterus), and structures involved in the transport of sperm, eggs, and babies (vagina, cervix, and oviduct).

Female anatomical structures can be broken down into the following:

Egg production and fetal development:

• Ovary: Females have two ovaries that are the site of egg production prior to birth. When an egg is selected for maturation, the ovaries are also the site of the corpus luteum. The ovary produces the hormones estrogen, progesterone, and testosterone.
• Corpus luteum: This temporary structure is the site of egg maturation within the ovary. After ovulation (release of the egg) the corpus luteum produces progesterone to maintain a possible pregnancy.
• Uterus: This muscle-lined, triangular organ is where a fertilized egg implants and develops. The uterus develops a thick blood lining and sheds this lining on a monthly cycle.

Transport of eggs, sperm, and embryos:

• Vagina: This highly expandable pouch structure serves as the opening of the female reproductive tract to outside the body. The vagina is the point of sperm entry and the point of exit for unfertilized eggs, menstrual discharge, and babies through vaginal delivery during a pregnancy.
• Cervix: The cervix is the opening between the vagina and the uterus. The size of this opening varies from tightly closed, to open for the passage of sperm, to open wide enough for a baby to pass through.
• Oviducts (sometimes called fallopian tubes): These ducts transport mature eggs from the ovary toward the uterus. If a sperm and egg are in the oviduct at the same time, the egg can be fertilized by a sperm.

Exterior structures:

• Vulva: This is a general term for the exterior parts surrounding the vagina, including the labia majora and labia minora, which are the folds of skin on either side of the clitoris, urethra, and vagina. Often this term is overlooked, with folks referring to the vulva as “vagina,” which is the internal structure. It’s OK and more accurate to say vulva when referring to the external structure. Importantly, the structure and appearance of the vulva may vary widely. There’s no “one size fits all” when it comes to the vulva, and diversity in appearance needs to be celebrated.
• Clitoris: This sensitive, nerve-rich organ is analogous to the head of the penis. The part of the clitoris that is visible outside the body is dorsal to (closer to the belly) the urethra and the vagina. The interior part of the clitoris extends internally along either side of the vagina.

Unfortunately, the structure of the clitoris is not well known by many people, including individuals with a clitoris. Historically, and even in some contemporary settings, this feature of anatomy has been muted or correlated with antiquated notions of female sexuality, such as hysteria or neurosis.

Female Reproductive Physiology

Female reproductive anatomy and physiology has many similarities to that of the male. As described earlier, females also use LH and FSH secretion from the pituitary, which is triggered by GRH from the hypothalamus, to stimulate hormone production by the gonads. Negative feedback also plays a role in regulating hormone production. However, in females, the interplay among the hormonal signals is more complicated than in males. While male hormonal feedback and signaling provide a relatively steady level of the sex hormone testosterone, females experience a monthly cycle during which the circulating hormone levels rise and fall as changes occur in the ovaries and the uterus. This surging of hormones, along with the changes in the ovaries and uterus, require the more complicated physiological controls described next.

Menstruation

The banner in figure 12.2 was carried in a 2014 march in Uganda as part of the celebration of Menstrual Hygiene Day. Menstrual Hygiene Day is held on May 28 each year and aims to raise awareness worldwide about menstruation and menstrual hygiene. Maintaining good menstrual hygiene is difficult in developing countries like Uganda due to taboos against discussing menstruation and lack of widely available menstrual hygiene products. Poor menstrual hygiene, in turn, can lead to embarrassment, degradation, and reproductive health problems in females. May 28 was chosen as Menstrual Hygiene Day because of its symbolism. May is the fifth month of the year, and most women average five days of menstrual bleeding each month. The 28th day was chosen because the menstrual cycle averages about 28 days.

The monthly female reproductive cycle can be divided into three phases: the follicular phase, ovulation, and the luteal phase. For each of these phases, there are concurrent changes happening in the uterus and in the ovaries.

The name “follicular phase” is in reference to the egg-containing follicle in the ovary that matures during this phase. This phase begins on day 1 of a female’s reproductive cycle. Day 1 is defined as the first day of menstruation (the first day of a period). Menstruation occurs for about the first five days of the follicular phase. During these days, if a female is not pregnant, low circulating levels of the hormone progesterone trigger the breakdown of the endometrium (the lining of the uterus). This blood-rich tissue exits the uterus through the cervix and then leaves the body through the vagina. During menstruation, low circulating levels of estrogen and progesterone stimulate GnRH production (from the hypothalamus in the brain), which leads to LH and FSH secretion by the pituitary gland. FSH signals the maturation of several follicles within the ovaries. These follicle cells produce a steadily increasing amount of estrogen.

Ovulation refers to the rupture of a mature follicle within the ovary. This ruptured follicle releases an oocyte (an unfertilized egg) into the abdominal space. Because this rupture is an actual breakage, some females will feel a twinge or slight pain during ovulation. Ovulation generally happens around day 14 of the reproductive cycle in one ovary.

During the luteal phase, the now empty follicle within the ovary collapses. This collapsed mass of cells is called a corpus luteum. The corpus luteum produces progesterone that enters the blood circulation. Progesterone signals the hypothalamus to signal the pituitary to reduce FSH and LH production, which prevents other follicles from maturing. If the oocyte in the oviduct is not fertilized, the corpus luteum degrades, causing a drop in progesterone, which triggers the beginning of menstruation and the return to the follicular phase of the reproductive cycle (back to day 1 after about a 28-day cycle). If the oocyte is fertilized, then that begins the cellular process of fetal development (pregnancy).

Intersex Anatomy

About 1 in 1,000–1,500 people will be born noticeably intersex, such as having partial elements of both a penis and vulva. However, other intersex conditions may not show up until later, such as during puberty or when trying to conceive children, making the number of intersex individuals higher in actuality. Some estimates show that some intersex conditions can be as high as one in 66. Taken together, some researchers, such as Anne Fausto-Sterling (2000), argue that the number of intersex people is actually closer to about one in 100 (Intersex Society of North America, 2008b). Here is a video of four individuals sharing some of their experiences (figure 12.3).

https://www.youtube.com/watch?v=cAUDKEI4QKI

The term intersex is an umbrella term that encompasses many different variations in sex. Some intersex individuals have variations of their sex chromosomes. For example, the SRY gene on an individual’s Y chromosome may be missing or may have been translocated onto an X chromosome. Thus, in some instances, a male can have XX chromosomes, and a female can have XY chromosomes. Other variations are possible as well, including X0, XXY, XYY, and XXX. According to Dr. Charmian Quigley with the Intersex Society of North America (2008a):

The last time I counted, there were at least 30 genes that have been found to have important roles in the development of sex in either humans or mice. Of these 30 or so genes 3 are located on the X chromosome, 1 on the Y chromosome and the rest are on other chromosomes, called autosomes (on chromosomes 1, 2, 3, 4, 7, 8, 9, 10, 11, 12, 17, 19). In light of this, sex should be considered not a product of our chromosomes, but rather, a product of our total genetic makeup, and of the functions of these genes during development. (para. 11–12)

Therefore, we need to be careful when we oversimplify this process and relate sex to XX or XY chromosomes only.

Transgender Anatomy

Further neurobiological research has been conducted to explain why many transgender people experience gender dysphoria, which is an intense feeling that the sex they were assigned at birth based on genital presentation does not match the way they feel about themselves. The contrast between the genital-based presentation of sex and the brain’s sex can be explained from a neurobiology perspective by taking a closer look at sex differentiation when we are still in the womb. During prenatal development, sex differentiation of the genitals takes place during the first two months of pregnancy, while sex differentiation of the brain occurs around four months of pregnancy (Swaab & Garcia-Falgueras, 2009). This discrepancy between the development of the genitals and the development of the brain has led biological researchers to believe this may be the reason why some individuals are transgender or experience gender dysphoria. Something to keep in mind: Humans actually show a lot less sex dimorphism than many other species, which means that there are actually a lot of similarities between males and females, making this type of research rather difficult as differences are often very subtle if they are even present at all.

Hormone replacement therapy and hormone blockers have been used to promote desirable changes in the physical bodies of some transgender individuals. These treatments have had positive results, adding merit to the perspective that there may be biological reasons and solutions for people experiencing gender dysphoria from a biopsycho perspective. In transgender men, testosterone therapy will result in enlargement of the clitoris, leading it to resembling a small penis; increased facial hair and body hair similar to that of other males in the individual’s family; redistribution of body fat and increased muscle mass; and deepening of the voice due to thickening of the vocal cords. In addition, the breast tissue will begin to atrophy, and the individual will experience more oily skin. In transgender women, hormone blockers and estrogen hormone therapy will cause breast tissue growth and redistribution of body fat and reduced muscle mass. Furthermore, the individual’s body hair will thin and grow more slowly, their penile tissue (if not having regular sex or masturbating) will begin to atrophy, and their skin will become drier. Electrolysis for hair removal may be desired, and voice lessons can help train the individual to speak in a register more socially expected for a woman.

Hormone blockers prior to puberty are especially beneficial in preventing these long-lasting impacts and aid the transition process for transgender individuals. However, many parents feel conflicted about this and may not consent on their children’s behalf. Surgeries may also be used by some transgender individuals. Within transgender communities, these procedures are commonly referred to as “top surgery,” which is surgery to either remove or enhance breast tissue, and “bottom surgery,” which alters the genitals. Creating a vagina can be done utilizing the existing tissue. The procedure has positive results for many individuals and allows them to experience sensation, but this will depend on scar tissue. Creating a penis requires the addition of testicular implants and a penile pump for erection purposes.

Every transgender person will have to decide what steps they want to take to help them feel as comfortable as possible in their bodies. If someone does not use hormones or have surgeries, it does not make them any less transgender than someone who does. Also, keep in mind that access to health care is a privilege and not a right in our country, which can be a barrier for many individuals in receiving the care they need. Family, culture, and society also act as barriers in this process. It is never appropriate to question a transgender person about their genitals, surgeries, or hormones; doing so would be an example of a microaggression. This is othering and dehumanizing and reduces transgender individuals to their genitals and body rather than seeing them as a whole person with feelings and the right to privacy. Create a space where someone feels comfortable to share their experiences with you on their own rather than questioning them. Here are some resources:

• National Center for Transgender Equity [Website].
• Trans Student Educational Research [Website].
• Supporting the Transgender People in Your Life: A Guide to Being a Good Ally [Website].

If you or someone you know is interested in learning more about how hormone replacement therapy impacts the body, check out the links provided by the University of California San Francisco Transgender Care [Website]. UCSF Transgender Care also has more information on the types of surgeries [Website] that are available for transgender individuals.

What about when nonbinary individuals, such as those who identify as agender, bigender, or genderqueer, experience gender dysphoria? Or, what about when transgender individuals don’t experience gender dysphoria? What might be some biological explanations? We need more research to answer these questions. However, this raises another question to think about further: if gender is a social construct, then does biology even really matter anyway, and are researchers trying too hard to find biological answers? This goes back to the nature versus nurture debate once again. We will discuss gender as a social and cultural phenomenon in chapter 14, to explore this question further from other perspectives.

Licenses and Attributions for Puberty

Introduction to Human Sexuality by Ericka Goerling, PhD and Emerson Wolfe, MS is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J. E., Womble, M., & DeSaix, P. (2013). Anatomy and physiology. OpenStax. Licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

Cotner, S. & Wassenberg, D. (2018). The evolution and biology of sex. PressBooks. https://open.lib.umn.edu/evolutionbiology/ (for sections on Female Anatomy and Male Anatomy). Licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

Miller, C. (2016). Human biology. PressBooks. https://humanbiology.pressbooks.tru.ca/. Licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

OpenStax College. (2013). Anatomy & physiology. OpenStax. http://cnx.org/content/col11496/latest/. Licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Adaptations: Reformatted. Modified content for language, application to subject and cohesion.

Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J. E., Womble, M., & DeSaix, P. (2013). Anatomy and physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/27-3-development-of-the-male-and-female-reproductive-systems#fig-ch28_03_01. (Adapted for the section on Puberty.)

Cotner, S. & Wassenberg, D. (2018). The evolution and biology of sex. PressBooks. https://open.lib.umn.edu/evolutionbiology/

Figure 12.2. Image by Satirtha—The Helping Hand: a nonprofit organization (link not available) on Wikimedia Commons is licensed CC BY-SA 4.0.