Anatomy and Physiology 2e · The Endocrine System
Gonadal and Placental Hormones
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In 30 seconds
The Gonads Testes (male) and ovaries (female); reproductive organs that also secrete hormones Full entry → — the testes in males and the ovaries in females — produce gametes, but they are also endocrine glands. They secrete the sex steroid hormones: Testosterone Main androgen, made by interstitial (Leydig) cells Full entry → in males, and Estrogens Female sex steroids (mainly estradiol), made by granulosa cells Full entry → plus Progesterone Sex steroid from the corpus luteum and placenta Full entry → in females. These hormones drive the development of reproductive structures, produce the secondary sex characteristics of puberty, help maintain bone and muscle, and regulate the menstrual cycle through feedback loops.
During pregnancy, the placenta becomes a temporary endocrine organ. It secretes hormones that keep the Corpus luteum Follicle remnant that secretes progesterone and estrogen Full entry → alive, adjust the mother's metabolism for the fetus, and eventually take over estrogen and progesterone production so pregnancy can continue to term. This topic covers the hormones of the testes, ovaries, and placenta, and the feedback loops that coordinate them.
Why this matters
Gonadal hormones are the engine behind puberty, fertility, pregnancy, and much of adult health. Home pregnancy tests detect hCG Placental hormone that keeps the corpus luteum alive Full entry →, a placental hormone; fertility problems often trace to a breakdown in the hypothalamus–pituitary–gonad (HPG) feedback loop; and these hormones help maintain bone density in both sexes — which is why bone loss becomes a concern after menopause. For exams, expect questions about which cell secretes which hormone and which feedback loop operates at each point of the cycle. Tracing a hormone from source, through target, and back to its feedback effect is the core skill of this topic.
The college version
Core Concepts
The testes: testosterone and the supporting cells
Two cell populations work as a team in the testes. Interstitial cells (Leydig cells), between the seminiferous tubules, produce testosterone when stimulated by LH from the anterior pituitary. Testosterone drives development of the male reproductive tract, the pubertal changes in voice and body hair, and the anabolic increases in muscle and bone; together with FSH, it is required for spermatogenesis. Sustentacular cells (Sertoli cells) line the tubules, nurture developing sperm, respond to FSH, and secrete Inhibin Sertoli-cell hormone that inhibits FSH Full entry →, which feeds back to slow FSH release and keep sperm production steady.
The ovaries: estrogens, progesterone, and the two-cell partnership
The ovary produces its steroids through two cooperating cell types. Thecal cells, under LH, produce androgens; those androgens diffuse into granulosa cells, which convert them into estrogens (mainly estradiol) under FSH — the two-cell model. Estrogens drive development of the female reproductive tract, breast growth, the endometrial thickening of the proliferative phase, and bone maintenance. Progesterone, secreted mainly by the corpus luteum (the follicle remnant after ovulation), prepares the endometrium for implantation, quiets uterine contractions, and prepares the breasts for lactation. If no pregnancy occurs, both hormones fall and menstruation begins.
The HPG axis and feedback
The hypothalamus secretes GnRH Hypothalamic releasing hormone for FSH and LH Full entry →, which drives the anterior pituitary to release FSH and LH. In males, rising testosterone inhibits GnRH and LH, and inhibin inhibits FSH — a stable negative-feedback loop. In females the same axis runs on a monthly rhythm with one famous exception: as the dominant follicle matures, rising estrogen flips from negative to positive feedback, triggering the LH surge that causes ovulation. After ovulation, the corpus luteum's progesterone and estrogen suppress FSH and LH until it degenerates and the cycle restarts. Knowing when feedback is negative versus positive is a classic exam distinction.
The placenta: a temporary endocrine organ
The placenta secretes: hCG, which rescues the corpus luteum so it keeps making progesterone through the first trimester (and is detected by pregnancy tests); estrogens and progesterone, which the placenta produces in enough quantity by about week 12 to take over from the corpus luteum; hPL Placental hormone adjusting maternal metabolism Full entry → (human placental lactogen), which adjusts maternal metabolism to favor fetal nutrition and prepares the breasts for lactation; and Relaxin Hormone that relaxes pelvic structures Full entry →, which softens the cervix and relaxes pelvic ligaments for birth.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| FSH and LH in males | Interchangeable gonadotropins | LH stimulates testosterone from interstitial cells; FSH stimulates sustentacular cells, which support sperm and secrete inhibin |
| Testosterone source | "The testes" generally | Specifically interstitial (Leydig) cells; sustentacular cells make inhibin, not testosterone |
| Estrogen | Progesterone | Estrogen builds the endometrium and drives the LH surge; progesterone stabilizes the endometrium and maintains pregnancy |
| Positive feedback | Negative feedback | The HPG axis is normally negative; the estrogen surge → LH surge before ovulation is the classic positive-feedback exception |
| hCG | A pituitary hormone | hCG comes from the placenta/trophoblast, not the anterior pituitary — it appears only in pregnancy |
| Placenta takes over immediately | Placenta takes over ~week 12 | The corpus luteum carries progesterone through the first trimester; the placenta then takes over |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your body sends special "message" chemicals to make you grow and change. The testes and ovaries send messages that make a boy's voice deepen or a girl's body change at puberty, and they also keep bones strong. When a baby grows inside a mom, the placenta sends its own messages to keep the baby safe — and a pregnancy test works by spotting one of those messages in the mom's urine.
Worked example
After fertilization, the embryo implants and its trophoblast cells secrete hCG, which travels in the blood, is filtered into the urine, and is bound by the test's antibodies. Its purpose is functional: hCG tells the corpus luteum to keep secreting progesterone, preventing the endometrial breakdown that would end the pregnancy. If no embryo implants, hCG never appears, the corpus luteum degenerates, progesterone and estrogen fall, and menstruation begins. So the same molecule is simultaneously the pregnancy's "rescue signal" to the ovary and the diagnostic marker on the test strip. By about week 12 the placenta makes enough progesterone and estrogen on its own, the corpus luteum is no longer needed, and hCG levels fall — a commonly described pattern, though individual experiences vary.
Key takeaways
- Testes: interstitial (Leydig) cells make testosterone in response to LH; sustentacular (Sertoli) cells support sperm, respond to FSH, and secrete inhibin (inhibits FSH).
- Ovaries: thecal cells make androgens under LH; granulosa cells convert them to estrogens under FSH (two-cell model). Corpus luteum makes progesterone.
- HPG axis: hypothalamus → GnRH → FSH/LH → gonads → sex steroids, with negative feedback — except the estrogen surge → LH surge positive feedback that causes ovulation.
- Placenta: hCG maintains the corpus luteum (basis of pregnancy tests); placenta takes over estrogen/progesterone by ~week 12; hPL adjusts maternal metabolism; relaxin prepares for birth.
- Sex steroids also maintain bone density, muscle, and metabolic health in both sexes.
- Testosterone is required for spermatogenesis; estrogen contributes to growth-plate closure at the end of the adolescent spurt.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Which testicular cell type secretes testosterone, and which pituitary hormone stimulates it?
Show answer
Interstitial (Leydig) cells secrete testosterone, stimulated by LH from the anterior pituitary.
In the two-cell model of the ovary, what does each cell type contribute?
Show answer
Thecal cells produce androgens under LH; granulosa cells convert those androgens into estrogens under FSH.
What is the single exception to negative feedback in the HPG axis, and what does it trigger?
Show answer
The estrogen surge in the late follicular phase triggers an LH surge through positive feedback; the LH surge causes ovulation around mid-cycle.
What does hCG do, and why does a pregnancy test detect it in urine?
Show answer
hCG, from the trophoblast, keeps the corpus luteum secreting progesterone. It enters the mother's blood and urine within days of implantation, which is what pregnancy tests detect.
What happens to estrogen and progesterone at the end of a non-pregnant cycle?
Show answer
The corpus luteum degenerates, estrogen and progesterone fall, and the uterine lining is shed as menstruation.
Why does bone health become a concern after menopause?
Show answer
Estrogens help maintain bone density; after menopause their production drops sharply, and the accelerated bone loss that commonly follows is why bone health monitoring matters in later life.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Gonads
- Testes (male) and ovaries (female); reproductive organs that also secrete hormones
- Testosterone
- Main androgen, made by interstitial (Leydig) cells
- Estrogens
- Female sex steroids (mainly estradiol), made by granulosa cells
- Progesterone
- Sex steroid from the corpus luteum and placenta
- Inhibin
- Sertoli-cell hormone that inhibits FSH
- GnRH
- Hypothalamic releasing hormone for FSH and LH
- Corpus luteum
- Follicle remnant that secretes progesterone and estrogen
- hCG
- Placental hormone that keeps the corpus luteum alive
- hPL
- Placental hormone adjusting maternal metabolism
- Relaxin
- Hormone that relaxes pelvic structures
Sources & references
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