Physiology 12 Reproductive

GnRH

Hormone released by the hypothalamus that stimulates the anterior pituitary to release FSH and LH.

Why must GnRH be released in pulses?

Continuous GnRH causes pituitary desensitization and suppresses FSH/LH release, while pulsatile GnRH maintains normal reproductive function.

Anterior pituitary gonadotropins

FSH and LH.

FSH (male)

Stimulates Sertoli cells to support spermatogenesis and produce inhibin.

LH (male)

Stimulates Leydig cells to produce testosterone.

Sertoli cells

Support developing sperm, form the blood-testis barrier, phagocytose defective sperm, secrete inhibin, and respond to FSH.

Leydig cells

Produce testosterone in response to LH stimulation.

Blood-testis barrier

Protects developing sperm from the immune system because sperm develop after immune tolerance has formed.

Inhibin

Hormone that selectively inhibits FSH release through negative feedback.

Testosterone functions

Stimulates spermatogenesis, male reproductive organ development, secondary sex characteristics, muscle growth, libido, and bone density.

DHT (dihydrotestosterone)

More potent androgen formed from testosterone by 5α-reductase; important for prostate growth and external genitalia.

5α-reductase inhibitor

Drug that decreases DHT production and is used to treat benign prostatic hyperplasia (BPH).

Seminiferous tubules

Site of sperm production within the testes.

Why are the testes located in the scrotum?

Spermatogenesis requires a temperature approximately 1–2°C below core body temperature.

Epididymis

Site of sperm maturation and storage where sperm gain motility.

Vas deferens

Transports mature sperm from the epididymis toward the urethra.

Seminal vesicles

Produce fructose-rich fluid that provides energy for sperm and contributes most of semen volume.

Prostate gland

Produces alkaline fluid and enzymes that improve sperm survival.

Bulbourethral glands

Produce mucus that lubricates the urethra and neutralizes acidic urine.

Semen composition

Approximately 90% glandular secretions and 10% sperm.

Vasectomy

Cutting the vas deferens to prevent sperm from entering semen while maintaining normal testosterone production.

Sequence of spermatogenesis

Spermatogonia → Primary spermatocyte → Secondary spermatocyte → Spermatid → Spermatozoa.

Cryptorchidism

Failure of one or both testes to descend into the scrotum, causing infertility due to elevated temperature.

Hypogonadism

Reduced testosterone production leading to infertility, low libido, decreased muscle mass, and delayed puberty.

Benign prostatic hyperplasia (BPH)

Enlargement of the prostate that compresses the urethra and causes urinary symptoms.

Nitric oxide (NO) role in erection

Relaxes smooth muscle, increases blood flow into erectile tissue, and compresses veins to maintain erection.

How do PDE5 inhibitors (Viagra) work?

Prevent cGMP breakdown to prolong nitric oxide-mediated vasodilation.

Oogenesis sequence

Oogonia → Primary oocyte → Secondary oocyte → Ovum (only after fertilization).

When are females born with their eggs?

All primary oocytes are formed before birth and no new eggs are produced afterward.

Granulosa cells

Respond to FSH, convert testosterone into estrogen using aromatase, and produce inhibin.

Theca cells

Respond to LH and produce testosterone that granulosa cells convert into estrogen.

Follicle maturation sequence

Primordial → Primary → Secondary → Antral → Mature (Graafian) → Corpus luteum.

Corpus luteum

Temporary endocrine structure that secretes progesterone and estrogen after ovulation.

Follicular phase

Days 1–14 of the ovarian cycle when follicles mature and estrogen levels rise.

Ovulation

Release of the secondary oocyte triggered by the LH surge.

Luteal phase

Days 15–28 when the corpus luteum secretes progesterone and estrogen.

Dominant hormone during follicular phase

Estrogen.

Dominant hormone during luteal phase

Progesterone.

Why does estrogen switch from negative to positive feedback?

Sustained high estrogen levels stimulate the hypothalamus and pituitary, producing the LH surge.

LH surge function

Triggers ovulation and formation of the corpus luteum.

Menstrual phase

Shedding of the functional endometrium due to falling progesterone.

Proliferative phase

Estrogen stimulates regeneration and thickening of the endometrium.

Secretory phase

Progesterone stimulates endometrial glands to prepare for implantation.

Hormonal birth control mechanism

Provides progesterone (± estrogen) to maintain negative feedback and prevent the LH surge and ovulation.

Fertility drugs

Increase GnRH, FSH, or LH activity or reduce estrogen negative feedback to stimulate ovulation.

Sperm lifespan

Approximately 4–6 days in the female reproductive tract.

Egg lifespan

Approximately 1–2 days after ovulation.

Capacitation

Process in the female reproductive tract that enables sperm to fertilize the egg.

Acrosome reaction

Release of digestive enzymes that allow sperm to penetrate the zona pellucida.

Zona pellucida

Glycoprotein layer surrounding the egg that sperm bind to before fertilization.

Cortical reaction

Blocks additional sperm from entering the egg to prevent polyspermy.

Polyspermy

Fertilization by multiple sperm, resulting in a nonviable embryo.

Sequence after fertilization

Zygote → Cleavage → Morula → Blastocyst → Implantation → Embryo → Fetus.

Morula

Solid ball of totipotent cells formed about 3 days after fertilization.

Blastocyst

Fluid-filled stage composed of the inner cell mass and trophoblast.

Inner cell mass

Develops into the embryo.

Trophoblast

Develops into the placenta and secretes hCG.

Implantation

Embedding of the blastocyst into the endometrium.

Placenta functions

Transfers oxygen and nutrients, removes fetal waste, produces hormones, and separates maternal and fetal blood.

Do maternal and fetal blood mix?

No; substances diffuse across the placental barrier without direct blood mixing.

hCG (human chorionic gonadotropin)

Hormone produced by the trophoblast that maintains the corpus luteum during early pregnancy.

Why is hCG important?

It maintains progesterone production until the placenta can produce sufficient hormones.

Progesterone during pregnancy

Maintains the endometrium and suppresses uterine contractions.

Estrogen during pregnancy

Supports uterine growth and breast development.

Prolactin

Stimulates milk synthesis after childbirth.

Oxytocin

Stimulates uterine contractions during labor and milk ejection during breastfeeding.

Colostrum

First milk produced after delivery containing antibodies and immune cells.

Endometriosis

Growth of endometrial tissue outside the uterus causing pain, inflammation, and infertility.

Ectopic pregnancy

Implantation outside the uterus, most commonly in the fallopian tube.

Pre-eclampsia

Pregnancy complication characterized by hypertension and fluid retention that may progress to seizures.

Menopause

End of ovarian follicle function resulting in decreased estrogen and increased FSH/LH.

Perimenopause

Transition period before menopause characterized by irregular menstrual cycles.

SRY gene

Gene on the Y chromosome that initiates testis development.

AMH (Anti-Müllerian hormone)

Causes regression of the Müllerian ducts during male development.

Wolffian ducts

Develop into the male reproductive tract under testosterone stimulation.

Müllerian ducts

Develop into the female reproductive tract unless inhibited by AMH.

Androgen insensitivity syndrome

XY individual with nonfunctional androgen receptors resulting in female external genitalia despite testes and AMH production.

Congenital adrenal hyperplasia

Excess androgen production in XX individuals causing masculinization and ambiguous genitalia.

Stages of childbirth

Cervical dilation → Delivery of the baby → Delivery of the placenta.

Main hormones involved in labor

Oxytocin and prostaglandins stimulate uterine contractions.

High-yield comparison: Spermatogenesis vs. Oogenesis

Spermatogenesis is continuous after puberty and produces four functional sperm; oogenesis begins before birth, pauses until puberty, and produces one functional ovum.

High-yield comparison: FSH vs. LH

FSH primarily stimulates gamete-supporting cells (Sertoli and granulosa), while LH stimulates hormone-producing cells (Leydig and theca).

High-yield comparison: Estrogen vs. Progesterone

Estrogen promotes follicle growth and endometrial proliferation, while progesterone maintains the endometrium and supports pregnancy.

High-yield concept: What happens if LH decreases?

Testosterone production falls, causing decreased spermatogenesis and infertility.

High-yield concept: What happens if FSH decreases?

Sertoli cell function decreases, reducing sperm production.

High-yield concept: What happens if progesterone falls?

The endometrium is shed, resulting in menstruation.

High-yield concept: What happens if hCG is absent in early pregnancy?

The corpus luteum degenerates, progesterone falls, and miscarriage may occur.

High-yield concept: What happens if the cortical reaction fails?

Multiple sperm fertilize the egg (polyspermy), producing a nonviable embryo.

High-yield concept: Why is aromatase important?

It converts testosterone into estrogen, which is essential for female reproduction and male bone health.