Lecture 14: The Reproductive System II

What is mitosis? What is meiosis?

Both are ways for cells to divide, but:

• Mitosis = For growth, repair, and regular body cell renewal

• Meiosis = For making gametes (sperm & egg cells only) for reproduction

Outline how mitosis works. Starts with one cell (46) ends with 2 daughter cells (46)

1. Parent cell: Starts with 2n = 46 chromosomes (diploid)

2. DNA replicates: So now there’s double the info

• The number of chromosomes stays the same (46)

• BUT each chromosome now has two sister chromatids (basically a "copied" arm)

• So we went from 46 single chromosomes → 46 duplicated chromosomes

3. Cell splits into 2 identical daughter cells - They're exactly the same as the parent cell and each other

4. Each one still has 2n = 46 chromosomes

Outline how meiosis works. Starts with one cell (46) ends with 4 cells (23)

1. Parent cell: Starts as 2n = 46 chromosomes (diploid)

2. DNA replicates

• The number of chromosomes stays the same (46)

• BUT each chromosome now has two sister chromatids (basically a "copied" arm)

• So we went from 46 single chromosomes → 46 duplicated chromosomes

3. 🔄 Meiosis I: First division

• Makes 2 cells with different combinations of DNA

• Because during Meiosis I, chromosomes cross over and swap genes

4. 🔁 Meiosis II: Second division

• Each of those 2 cells splits again → total of 4 cells

5. Each cell is n = 23 chromosomes (haploid) - And each one is genetically unique

definition of n, 1n, and 2n?

what is spermatogenesis?

Spermatogenesis is the process of making sperm cells in the testes.

Where does spermatogenesis start?

The Testes

• Inside each testicle are seminiferous tubules (tiny, coiled tubes).

what is the difference between the outer vs inner (closer to lumen) portions of the seminiferours tubules

• The cells on the outside of the tube are baby/immature sperm cells.

• The inside/lumen is where fully-formed sperm go once they're made.

What cells helps the sperm to develop?

Sertoli Cells

• Nurse Cells

• Located in the seminiferous tubules.

• They help sperm develop, give nutrients, and protect them.

• Think of them like babysitters or personal trainers for sperm

how does sperm go from Primordial germ to to spermatozoa using meiosis in the testes

How cells become sperm:

• Through MITOSIS + MEIOSIS

• Primordial Germ Cell to Spermatozoa

RECALL: Pathway of sperm

**prostatatic fluids give the ability to swim

What is semen?

Semen = sperm + secretions from glands

• Cloudy white fluid consisting of sperm + secretions from accessory glands.

• Provides nutrients, acts as a transport medium to protect sperm.

• 2-5mL per ejaculation containing 100s of millions of sperm!

What are the 3 important acessory glands that contribute to sperm/semen

1. SEMINAL VESICLE (aka the mother)

What it Adds

• Alkaline fluid + sugars

Why it’s Important

• Feeds sperm

• Makes vaginal environment less acidic

• Thins cervical mucus

• Triggers reverse peristalsis (pulls sperm up female tract!)

2. PROSTATE GLAND (aka the father)

What it Adds

• Citric acid + enzymes (via ejaculatory ducts + prostatic ducts)

Why it’s Important

• Keeps semen liquid

• Protects DNA

• Turns on sperm’s swimming ability

3. BULBOURETHRAL GLAND

What it Adds

• Pre-ejaculatory fluid

Why it’s Important

• Cleans out urine (acidic!) from urethra before sperm passes through

Where does Hormone control begin in Males? What does it release

Step 1: Start at the Brain – Hypothalamus

• The hypothalamus is like the boss that sends out the first signal:

• GnRH = Gonadotropin-Releasing Hormone

• GnRH tells the anterior pituitary to get to work.

After GnRH is released, what is released as a result?

Step 2: Anterior Pituitary Releases Two Hormones

• FSH = Follicle Stimulating Hormone

• LH = Luteinizing Hormone

• Both go to the testes, but they do different jobs.

What does LH do in the testes

**Step 3: What LH Does

• LH goes to Leydig cells in the testes (outside the seminiferous tubules).

• Leydig cells release testosterone 

• Testosterone does a lot:

  • Starts, maintains, and drives spermatogenesis (making sperm)

  • Grows facial/body hair, deepens voice, builds muscle

  • Keeps up libido (sex drive) and accessory glands like the prostate

• Too much testosterone?

• Negative Feedback to shut down GnRH and LH → less testosterone made

Think: GnRH ➡ LH ➡ Leydig (Testes) ➡ Testosterone

What doe FSH do in the testes?

**Step 4: What FSH Does

• FSH goes to Sertoli cells in the testes (inside seminiferous tubules — the "nurse cells")

• Sertoli cells release ABP = Androgen Binding Protein

• ABP holds on to testosterone (released by the Leydig cell) and keeps it in place, so it can help sperm grow!

• It binds to testosterone and helps keep testosterone levels high inside the seminiferous tubules

• Because sperm cannot develop properly without high levels of testosterone in the tubules.

• Too much sperm?

• Sertoli cells release inhibin → shuts down FSH → less ABP → slows down sperm production

Think: GnRH ➡ FSH ➡ Sertoli (Testes) ➡ ABP

ABP ➕ Testosterone ➡ Spermatogenesis

What is 1 common disorder of the prostate? symptoms?

Benign Prostatic Hypertrophy (BPH)

• Common with age — affects 70% of men over 70

• Even more likely if it runs in your family

• The prostate gets bigger and squishes the urethra — the pee tube inside it

• Symptoms:

  • Weak pee stream

  • Gotta pee urgently or often

  • Trouble fully emptying bladder

  • Sometimes pain or leaking

• This is not cancer, just annoying and affects quality of life.

What is another disorder of the prostate

1. Prostate Cancer

• 1 in 8 men will be diagnosed — also pretty common.

• Symptoms:

  • Frequent urination

  • Blood in urine (bad sign — go see a doc!)

  • Pain when peeing

  • can’t empty bladder fully

• Treatment:

  • Radiation or chemo

  • Surgical removal of prostate

• ****But here’s the catch:

  • If the prostate is removed, the man can become infertile because…

  • The prostate's secretions are what:Help sperm swim AND Protect sperm DNA on the journey

  • No prostate = sperm can’t swim = sperm can’t reach the egg ❌

What is oogenesis?

Oogenesis = the process of making a mature egg (ovum) in people born female.

When does oogenesis start

IN UTERO: Before You Were Born

What proccesses occur in oogenesis IN UTERO: Before You Were Born

IN UTERO: Before You Were Born

• You started with oogonium (a baby egg cell, 2n = 46 chromosomes).

• It made primary oocytes by mitosis.

• These primary oocytes begin meiosis I... but paused halfway and got frozen in it!

• You were born with all the eggs (primary oocytes) you'll ever have — ~500,000, each paused in mitosis before meiosis I.

• So from birth to puberty, all your eggs are just chilling, frozen in time.

What proccesses occur in oogenesis during PUBERTY: The Eggs Wake Up

PUBERTY: The Eggs Wake Up

• When you hit puberty, your hormones (like FSH and LH) wake up some of those frozen primary oocytes.

• Each month, a few unfreeze and complete meiosis I.

• This gives:

• A secondary oocyte (what gets ovulated)

• And a first polar body (a tiny useless cell that gets tossed away)

• The secondary oocyte now paused in meiosis I before it starts meiosis II

  

What proccesses occur in oogenesis during OVULATION

OVULATION

• The secondary oocyte is released from the ovary into the fallopian tube.

• This is called ovulation.

• Important: It is still NOT a mature ovum! It only finishes meiosis II if it's fertilized by sperm.

What proccesses occur in oogenesis during OVULATION

FERTILIZATION

• If a sperm cell enters the secondary oocyte:

• Meiosis II finishes!

• The oocyte becomes a mature ovum

• A second polar body is tossed out

• The ovum (1n = 23) and sperm (1n = 23) combine to make a zygote (2n = 46)

• This is the moment new life begins.

What are all the polar bodies??

• You don’t want to split your cell contents equally.

• You keep all the nutrients, organelles, cytoplasm in one big cell (the egg)

• The leftover DNA gets shoved into tiny polar bodies that die.

• This ensures the ovum is strong and ready to grow a baby if fertilized.

What is a follicle?

Follicles are “nests” that support the developing egg.

What does each follicle contain?

Each follicle in the ovary contains only ONE egg (primary oocyte - that has STOPPED after mitosis0

What are the 7 stages of follicular development

Follicle Stages:

1. Primordial follicle

➤ You’re born with ALL the eggs you’ll ever have, but they’re immature.

➤ These immature eggs are inside primordial follicles, which contain primary oocytes.

➤ They stay frozen before meiosis I and don’t finish developing… yet.

**PUBERTY OCCURS

2. Primary follicle

➤ Hormones wake up some of those sleeping primordial follicles.

➤ They now turn into primary follicles (still holding the same egg — still a primary oocyte).

➤ A few of these start developing each month, but only ONE becomes dominant most of the time.

3. Secondary follicle

➤ That dominant follicle becomes a mature follicle (also called a Graafian follicle) and contains a secondary oocyte — the egg is now halfway matured!

➤ MEIOSIS 1 complete

4. Mature (Graafian) follicle

➤ Ready to ovulate — releases secondary oocyte

5. Ovulation

➤ Follicle bursts, egg is released

➤ It releases the secondary oocyte — this is ovulation.

➤ The egg is now in the fallopian tube, waiting for sperm.

➤ Meiosis II is still not finished yet — it pauses again!

6. Corpus luteum

➤ The empty follicle (left behind after egg pops out) becomes a Corpus luteum - A hormone-secreting structure (a temporary gland)

➤ It secretes:

• Estrogen

• Progesterone

• Inhibin

➤To prepare the uterus for pregnancy

7. Corpus albicans

➤ If no pregnancy happens, luteum breaks down into scar tissue

➤ The egg dies

➤ The corpus luteum shrinks and becomes a corpus albicans (basically scar tissue)

➤ Hormone levels drop

➤ You get your period (uterine lining sheds)

what would happen if there WAS fertilization

**What If There Is Fertilization?

➤ The sperm meets the secondary oocyte in the fallopian tubes.

➤ That triggers MEIOSIS II to finish

➤ The egg becomes a true ovum, and the nuclei from sperm + egg fuse.

➤ You now have a zygote (fertilized egg) — 46 chromosomes (23 from each parent).

Where does Hormone control begin in females? What does it release

Step 1: Start at the Brain – Hypothalamus

• The hypothalamus is like the boss that sends out the first signal:

• GnRH = Gonadotropin-Releasing Hormone

• GnRH tells the anterior pituitary to get to work.

After GnRH is released, what is released as a result?

Step 2: Anterior Pituitary Releases Two Hormones

• FSH = Follicle Stimulating Hormone

• LH = Luteinizing Hormone

• Both go to the OVARIES, but they do different jobs.

What does FSH do in the ovaries

**Step 3: What FSH Does

• FSH goes to the ovaries, specifically to growing follicles (the tiny egg sacs).

• FSH stimulates the growth of follicles

• Those growing follicles release estrogen and a little inhibin.

• Too many follicles growing?

• Inhibin shuts down FSH production (negative feedback).

GnRH ➡ FSH ➡ Ovarian Follicles ➡ Estrogen & Inhibin

what does LH do in the ovaries

**Step 4: What LH Does

• LH goes to the ovaries and it does two major things

1. When estrogen levels are high (positive feedback), LH surges → triggers ovulation (the egg pops out of the mature follicle)

2. After ovulation, LH helps that leftover follicle become the corpus luteum

GnRH ➡ LH ➡ Ovulation ➡ Corpus Luteum

Specialized feature of the ovaries**: What does the corpus luteum do in the ovaries?

**Step 5: What the Corpus Luteum Does

• After ovulation, the corpus luteum (the leftover egg sac) becomes a little hormone factory. It releases:

• Progesterone: Protects the uterine lining (endometrium) so it's ready for a baby

• Estrogen: Still helping thicken the lining

• Inhibin: Stops more FSH from being made—no new follicles, please!

• Relaxin: Keeps uterus chill (no contractions yet); softens joints/cervix later if pregnant

• Too much estrogen + progesterone?

• Negative feedback shuts down GnRH, LH, FSH

what are the 3 stages of the menstrual cycle?

THE MENSTRUAL CYCLE (28 Days-ish)

Split into 3 major stages:

1. FOLLICULAR PHASE (Days 1–13)

2. OVULATION (Days 14)

3. LUTEAL PHASE (Days 15 - 28)

what happens during the FOLLICULAR PHASE

• FSH rises → stimulates follicles to grow

• One follicle becomes dominant (contains secondary oocyte)

• Follicle releases estrogen

• Estrogen thickens the endometrial lining

• At first: Low estrogen levels = blocks LH, allows FSH to rise

• By the end: High estrogen levels = LH surge! (**inhibin from follicles shuts down FSH production)

what happens during the OVULATION phase

• LH surge (due to high estrogen levels) causes follicle to rupture

• Egg is released!

• Egg travels to the fallopian tube

what happens during the LUTEAL PHASE

• Follicle becomes the corpus luteum (with help of the LH)

• Corpus luteum makes: Progesterone, Estrogen, Inhibin, Relaxin

• Progesterone holds the uterine lining in place

• If no fertilization:

  • Corpus luteum shrinks

  • Hormones drop

  • Uterine lining sheds → period starts

OVERALL: what is the roles of Progesterone, Estrogen, FSH, LH

OVERALL: Recap the hormone levels during a full cycle

Clinical Correlate: What is endometriosis?

• Endometrial-like tissue grows outside the uterus

• What Happens:

  • Tissue still responds to cycle hormones

  • Bleeds during period → but nowhere to exit

• Causes scar tissue, pain, and infertility

• Symptoms:

1. Painful periods

2. Pain during sex, pooping, or peeing

3. Infertility

• Treatment:

• Birth control pills (reduce estrogen)

• Surgery (remove tissue)

What are 4 forms of contraception?

1. Intrauterine Device (IUD)

• A small T-shaped device placed inside the uterus.

• Comes in 2 main types: hormonal (secretes progestin) or copper (non-hormonal).

• How it works:

  • Thickens cervical mucus, making it hard for sperm to swim through the cervix.

  • Alters the environment in the uterus/fallopian tubes, making it hard for sperm or a fertilized egg to survive.

  • Progestin (a synthetic form of progesterone) creates a negative feedback loop — it tells the brain to stop releasing GnRH, which means less FSH and LH, so no ovulation happens.

2. Oral Contraceptive (Birth Control Pills)

• Contains estrogen and progesterone, or just progesterone.

• Mimics pregnancy hormone levels → prevents ovulation.

• High hormone levels tell the brain: "We're already pregnant, no need to ovulate!"

• Red pills in the pack are placebos (no hormones) → causes a drop in hormone levels → triggers a "fake period".

• How it prevents pregnancy:

  • ❌ No ovulation

  • ❌ Thicker mucus

  • ❌ Unfriendly environment for sperm

3. Vasectomy

• Procedure for individuals assigned male at birth.

• Vas deferens is cut and tied → no sperm can get into semen.

• Reversible sometimes, but not guaranteed.

4. Tubal Ligation

• Procedure for individuals assigned female at birth.Fallopian tubes are blocked, cut, or tied so the egg can't meet sperm.

• Permanent (irreversible).

• So even if ovulation happens, fertilization cannot.

What is menopause?

• Natural end of menstruation and fertility in people assigned female at birth.

what happens during menopause?

What happens:

1. Ovaries shrink (atrophy) and stop responding to FSH and LH.

2. Estrogen and progesterone levels drop.

• No more ovulation

• No more periods

3. FSH and LH rise in the blood (because ovaries are ignoring them now).

Descent of the Gonads - 👦 Assigned Male at Birth

• Testes start inside the abdomen during early development.

• A structure called the gubernaculum pulls the testes down through the body wall into the scrotum by 9 months of gestation.

• If testes don’t descend → infertility (they get too hot inside the abdomen for sperm production).

• Spermatic cord (bundle of “wires” - vas deferens, blood vessels, lymph vessels etc.) forms from this descent.

Descent of the Gonads - 👧 Assigned Female at Birth

👧 Assigned Female at Birth

• Ovaries also descend but only into the pelvis.

• The ovaries stay inside the body (they don’t exit like testes).

• The gubernaculum becomes:

• The ligament of the ovary (connects ovary to uterus)

• The round ligament of the uterus (anchors uterus forward)