9 The Reproductive System

Reproductive anatomy and functions

The reproductive system produces gametes and sex hormones. In humans, fertilization usually occurs when a sperm joins an oocyte in a uterine tube. Reproductive organs also support early development and, in a pregnant person, gestation and birth. Anatomy and reproductive traits vary among individuals; the structures described here are common patterns, not a definition of anyone’s gender.

Testicular structures

The testes, also called testicles, are gonads located in the scrotum. Seminiferous tubules within each testis produce sperm, which mature and are stored in the epididymis before traveling through the ductus deferens. During ejaculation, sperm pass through the ejaculatory ducts and urethra. The seminal vesicles and prostate add fluids that, together with sperm, form semen. The scrotum helps keep the testes cooler than core body temperature, supporting sperm production; the testes also produce testosterone.

Ovarian structures

The ovaries are gonads that produce oocytes and hormones, including estrogen and progesterone. After ovulation, an oocyte is usually swept into a uterine tube, where fertilization may occur. The uterus is a muscular organ, and its inner lining, the endometrium, changes during the reproductive cycle and can support implantation.

The cervix is the lower, narrow part of the uterus and opens into the vagina. The vagina is a muscular canal that carries menstrual flow out of the body and can serve as a birth canal. The vulva is the collective term for external genital structures, including the labia and clitoris.

Hormonal control and gamete production

Reproductive activity is regulated by the hypothalamus and pituitary gland. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which prompts the pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Sex hormones feed back to the brain and pituitary, adjusting further hormone release.

In testes, LH stimulates testosterone production, while FSH supports sperm production. In ovaries, FSH supports follicle development, while LH helps trigger ovulation and formation of the corpus luteum.

Gamete production

Gametes are haploid, meaning each has 23 chromosomes—half the number in most human body cells. At fertilization, the chromosomes from two gametes combine, restoring the diploid number of 46 in the resulting zygote.

Spermatogenesis takes place in the seminiferous tubules and begins at puberty. Diploid spermatogonia divide and develop through meiosis into haploid spermatids; these then change shape into sperm cells, or spermatozoa. Sperm mature further in the epididymis. Each sperm has a head containing genetic material, a midpiece that supplies energy, and a tail that propels it.

Oogenesis begins before birth. Immature egg cells develop in ovarian follicles, where primary oocytes pause in meiosis. Typically, during a cycle after puberty, one oocyte resumes development and completes the first meiotic division, producing a large secondary oocyte and a small polar body. The secondary oocyte is released at ovulation and pauses again; it completes meiosis only if fertilization occurs. Unlike sperm production, oogenesis divides the cell’s contents unevenly, so most cytoplasm remains in the developing oocyte.

Ovarian and uterine cycles

The ovarian and uterine cycles are coordinated by hormones. Cycle length varies; 28 days is a commonly used example, not a rule. Day $1$ is the first day of menstrual bleeding.

1. Menstrual and follicular phases: When pregnancy has not occurred, the corpus luteum from the previous cycle breaks down, lowering progesterone and estrogen. The functional layer of the endometrium is shed as menstrual flow. Meanwhile, FSH stimulates ovarian follicles to develop. A developing follicle produces estrogen, which helps rebuild the endometrium.

2. Ovulation: Usually one follicle becomes dominant. Sustained high estrogen leads to a surge in LH, which triggers release of the secondary oocyte. Ovulation often occurs near the middle of a 28-day cycle, but its timing varies with cycle length.

3. Luteal and secretory phases: The emptied follicle becomes the corpus luteum, which produces progesterone. Progesterone makes the endometrium more suitable for implantation. If pregnancy does not occur, the corpus luteum regresses, hormone levels fall, and the next menstruation begins. If an embryo implants, hormonal signals maintain the corpus luteum early in pregnancy and help preserve the endometrium.

Fertilization and early development

After ejaculation into the vagina, sperm may travel through the cervix and uterus into a uterine tube. During this journey, sperm undergo capacitation, changes that enable them to fertilize an oocyte. If a sperm reaches the oocyte, it passes through the surrounding cells and protective outer layer. Entry of one sperm triggers changes that help prevent other sperm from entering. The genetic material from the two haploid gametes combines, forming a diploid zygote. Fertilization usually occurs in the wider, outer portion of a uterine tube.

As it moves toward the uterus, the zygote divides repeatedly without growing much in overall size. The resulting ball of cells is first called a morula and then a blastocyst. The blastocyst has an inner cell group that develops into the embryo and an outer cell layer that contributes to supporting structures, including the fetal portion of the placenta. Around the end of the first week after fertilization, the blastocyst may attach to and embed in the endometrium; this is implantation.

During the third week after fertilization, gastrulation forms three primary cell layers: ectoderm, mesoderm, and endoderm. These layers give rise to the body’s tissues and organs. The embryonic period is commonly described as weeks $3$–$8$ after fertilization, when the basic structures of major organ systems form. From the ninth week after fertilization until birth, the developing human is called a fetus; growth and organ maturation continue during this period.

The placenta supports exchange of oxygen, nutrients, and wastes between the pregnant person and fetus. The umbilical cord connects the fetus to the placenta.

Pregnancy dating

Pregnancy is often dated differently from embryonic development. Clinical gestational age is counted from the first day of the last menstrual period, usually about two weeks before fertilization. Thus, gestational age is typically about two weeks greater than age measured from fertilization.