Sex vs Gender
Gender and sex are often used interchangeably but are two different concepts. Sex refers to the combination of the sex chromosomes – the 23rd pair. A female receives an X chromosome from both parents, making her genotype (genetics) XX. Males receive an X from their mom and a much smaller Y chromosome from their dad, giving them the XY genotype. But a person’s genotype is just one part of determining a person’s gender.
Gender identity refers to how people view themselves about their sex. The exact cause of a person’s gender identity is fully understood, but it is most likely a combination of genetics, hormone concentrations during gestation, and other environmental factors. Most peoples’ gender corresponds with their genotype. At least one million adults identify as transgender, and 400,000 are nonbinary (male or female). Most surveys lump all people whose gender identity is different from their biological sex as “transgender,” even though this is inaccurate. In reality, some individuals identify as a third gender. For example, three genders have been recognized in India for at least 4000 years. Hijras did not identify as male or female and played an important role in Indian society until British Colonization. Many cultures have recognized multiple genders on all continents (except Antarctica) and the Pacific Islands.
Male or Female?
Sex chromosome pairing occurs during conception. If a sperm with an X chromosome fertilizes an egg (females only have X chromosomes), then the embryo’s chromosomal sex will be female (XX). If the sperm carries a Y chromosome, the embryo’s chromosomal sex will be male (XY). However, for the first 12 weeks of gestation, XX and XY appear to have the same genitalia.
What?
XX and XY fetuses younger than 12 weeks have genital swellings and general folds, a phallus, a urogenital sinus, and gonads (testes or ovaries) near the kidneys. Around the eighth week of gestation, a special sauce of hormones is released, causing the differentiation of male and female sex characteristics. By week 20, the external male and female genitalia are defined, and a gynecologist can differentiate between the two sexes.
So, sex differentiation is complete by the 20th week of pregnancy?
No, not entirely. The testes descend into the scrotum at the end of pregnancy (weeks 32-36).
Can a male be born with undescended testes?
Yes. About 30% of premature and 3% of full-term cisgender male babies are born with cryptorchidism (undescended testes). Almost all babies’ testes will descend in the first six months after birth. Simple outpatient surgery can correct cryptorchidism for the 1% of babies whose testes do not descend.
What happens if the child does not receive the surgery?
Males with cryptorchidism are more likely to be infertile and prone to hernias, testicular twisting, and testicular cancer.
Cisgender Male Anatomy
In the rest of this chapter, the term male will refer to a cisgender male.
Males contain organs that make testosterone and sperm (testes), semen (prostate and seminal vesicles), and a solution that raises the pH of the urethra (Cowper’s gland). Sperm are made, and the testes mature in a sac that sits atop the testes called the epididymis. The mature sperm travel via the vas deferens to the ejaculatory duct inside the prostate. Along the sperms’ travel, they pick up protective and nutrient-rich fluid from the prostate gland and seminal vesicles. The mixture of sperm and the prostate and seminal vesicle secretions is semen. The Cowper’s gland readies the urethra by secreting a solution that neutralizes any remaining urine left in the urethra. Semen ejects into the urethra and then out of the body via smooth muscle contractions during ejaculation.
The Primary Sex Organs
The male primary sex organs, or gonads, are the testes. The testes produce testosterone and sperm. The penis, prostate, Cowper’s gland, vestibular glands, and scrotum are accessory sex organs that we’ll discuss later in this chapter.
Interstitial cells in the testes make testosterone. Testosterone is a steroid that is a male’s primary sex hormone. Testosterone has many functions in the body, such as:
- Libido (sex drive)
- Development of accessory sex organs (penis)
- Sperm production
- Erythrocyte production
- Bone and muscle mass and strength
- Body fat distribution
- Mental health
Testosterone is governed by negative feedback.
- When plasma testosterone levels drop below homeostasis, the hypothalamus secretes gonadotropin-releasing hormone (GnRH)
- GnRH stimulates the pituitary gland, which releases luteinizing hormone (LH) – follicle-stimulating hormone (FSH) is also released, but it does not play a role in testosterone production
- LH stimulates the testes, increasing testosterone production.
- When plasma testosterone levels reach homeostasis, testosterone inhibits the secretion of GnRH.
Males have higher plasma testosterone levels than females – yes, females make testosterone. Higher plasma testosterone is why males have more red blood cells, thicker skin, more muscle mass, less fat mass, and greater bone density.
Males Have a Hormone Cycle, Too
One of the many problems of living in a society where many of the laws and social norms are defined by males is that men get to define maleness and femaleness. One of the biggest myths is that only females have a monthly hormonal cycle. Men appear to have a monthly hormonal cycle, too. A male’s hormonal cycle is sometimes more variable than a female’s, especially during puberty. The primary reason why the female cycle dominates the narrative is that only females menstruate. If males bleed from their penises every 28 days, the hormonal female stereotype would probably not exist.
Does a male show fewer emotional changes through their cycle?
No, but yes.
What?
Females “show” more emotions because of the narrative in American society. America has defined females as rotating between Jeckel and Hyde throughout their 28-day cycle – it’s even been used in milk commercials. However, male and female emotional cycles are about the same in societies that do not demonize the menstrual cycle. Plus, American males are usually taught to suppress their emotions. Therefore, culture has a greater influence on the behavioral aspect of hormonal cycles than biology.
Spermatogenesis
Meiosis is a type of cell division that makes gametes. A gamete is a sex cell. Male gametes are sperm. Meiosis only occurs in the testes, and it is called spermatogenesis. Spermatogenesis begins at puberty and continues throughout life, though the amount of sperm produced declines with age.
A sperm cell is the smallest human cell. Sperm are haploid cells, meaning they only contain half the genetic information that makes a person (female eggs have the other half). Since males are XY, 50% of the sperm contain an X chromosome, and the other half contain a Y chromosome. Therefore, males are the ones who determine a child’s biological sex.
Sperm are the only human cells with a flagellum (tail). Mitochondria in the midpiece give the flagellum the energy it needs to move through the female reproductive tract. The head of the sperm consists of the acrosome and the nucleus. The acrosome contains enzymes that dissolve the protective layers of an egg’s membrane. The nucleus contains the DNA, the only part that enters an egg during fertilization.
Hormones regulate the rate of spermatogenesis. The primary organs involved are the hypothalamus (a part of the brain), the pituitary gland, and the testes. Here are the basic steps in the feedback loop:
- The hypothalamus releases gonadotrophin-releasing hormone (GnRH)
- GnRH stimulates the pituitary gland to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
- LH stimulates testosterone secretion in the testes
- Testosterone and FSH intitate spermatogenesis
- When testosterone and sperm count reach homeostasis, testosterone and the hormone inhibin inhibits the release of GnRH
- The process repeats when blood testosterone drops below homeostasis
Accessory Sex Organs
The epididymis is a sac that sits atop each testis and is the site of sperm maturation (“sperm school”).
Mature sperm travel from the epididymis to the ejaculatory duct via the vas deferens. The vas deferens are severed and cauterized during a vasectomy, which is male sterilization.
The ejaculatory duct resides in the prostate. Sperm from the vas deferens and the seminal vesicles and prostate secretions mix here to form semen. During ejaculation, the ejaculatory duct releases semen into the urethra.
Urine and sperm travel through a male’s urethra. The urethra starts at the urinary bladder and ends at the distal end of the penis. Urine travels through the entire length of the urethra, but semen travels through the distal two-thirds. Urine is generally acidic, and sperm require a neutral environment. Before ejaculation, the Cowper’s gland (bulbourethral gland) secretes an alkaline solution that neutralizes the acid in the urethra.
The seminal vesicles and prostate gland make the viscous solution that comprises <95% of the semen’s volume. The viscous solution contains pH buffers, protective enzymes, and fructose (sugar) that nourish and protect the sperm on their journey to the egg.
The penis is homologous to the female clitoris. Its primary job is to release sperm near the cervix (the opening to the uterus). During arousal, the penis engorges with blood, becomes erect, and becomes flaccid after ejaculation. The glans penis is the distal portion of the penis and the most sensitive part. A sensitive layer of moveable skin protects the glands penis called the prepuce or foreskin. Circumcised males have had their foreskin usually removed within the first month after birth.
The scrotum is a sac that houses the testes and is homologous to females’ labia. Spermatogenesis works best at about 3 degrees Fahrenheit lower than body temperature. The scrotum expands when the testicular environment rises above 95 degrees Fahrenheit, and the scrotum contracts when the temperature dips below 95 degrees Fahrenheit.
