NPB101: Reproductive System

Furlow

What is a viviparous sexual reproductive strategy, and what are its advantages and challenges?

internal fertilization and live birth (develop in oviduct/uterus; mostly in mammals, some fishes, scorpions)

• Advantages: protection of the developing embryo inside the body.

• Challenges: higher energy investment by the parent, fewer offspring.

Oviparous Sexual Reproduction

• Egg-laying (invertebrates and some vertebrates)

• Fertilization internal or external

Describe the developmental pathway of male reproductive anatomy.

• XY sex chromosomes → testes develop from common gonad precursor

• Fetal testosterone secretion → differentiation of internal ducts (epididymis, vas deferens), accessory glands, external genitalia (penis, scrotum)

• Antimüllerian hormone causes loss of female internal structures

What hormone causes loss of female internal structures in male development?

Antimullerian hormone

Describe the developmental pathway of female reproductive anatomy.

• XX sex chromosomes → ovaries develop from gonad precursor

• No early hormone secretion → differentiation of oviducts, uterus, cervix, external genitalia (vagina, labia, clitoris)

• Male structures lost due to absence of testosterone

What causes a loss of male structures in female development?

Absence of testosterone

What is the basic anatomy of the testes?

• Seminiferous tubules → epididymis → vas deferens → urethra

• Sertoli cells: support spermatogenesis

• Leydig cells: secrete testosterone

Outline Spermatogenesis and the Role of Sertoli Cells

• One diploid spermatogonium → four haploid spermatozoa

• Sertoli cells support maturation

  1. form a tight barrier btwn developing sperm and blood

  2. provide nutrients to developing sperm

  3. phagocytose the material leftover from remodeling of spermatids

  4. secrete fluid that washes sperm into the epididymis

  5. secrete androgen binding protein to keep some local testosterone

List male reproductive tract glands

• Testes

• Epididymis

• Vas deferens

• Seminal vesicles

• Prostate gland

• Bulbourethral glands

• Urethra

  Function: contribute fluids, nutrients, and enzymes to semen

Describe the hormonal regulation of male reproduction.

• GnRH (hypothalamus) → stimulates LH & FSH (anterior pituitary)

• LH → Leydig cells → testosterone

• FSH → Sertoli cells → support spermatogenesis

• Sertoli cells → inhibin (inhibits FSH)

• Testosterone → local effects (converted to estradiol), systemic effects, feedback inhibition

How does testosterone feedback work in the male brain?

• Testosterone inhibits kisspeptin neurons in the hypothalamic "pulse center"

• High androgens in females → disrupt pulse center → LH secretion stops → loss of cycles & masculinization

What do male brains lack that female brains have?

Functional surge center

Describe the stages of oogenesis.

• Oogonia → primary oocyte (at birth) → secondary oocyte (puberty) → ovum (after fertilization)

• One primary oocyte → one ovum + 3 polar bodies

• Meiosis starts before birth, arrested in meiosis I

• Resumes and arrests in meiosis II until fertilization

Outline the ovarian cycle.

• Follicular phase (pre-ovulation) — days 1-14

• Ovulation — day 14

• Luteal phase (post-ovulation) — days 15-28

Describe early to mid-follicular hormonal regulation.

• LH stimulates thecal cells → produce androgens

• FSH stimulates granulosa cells → convert androgens to estradiol (via aromatase)

• Granulosa cells also release inhibin, which helps suppress FSH

• Estradiol + FSH together help the follicle grow and form the antrum

• Low estrogen levels at this stage give weak negative feedback to the hypothalamus (through kisspeptin neurons in the pulse center)

Late follicular phase leading into ovulation

1. High estrogen causes positive feedback via kisspeptin “surge center” in the hypothalamus

2. Surge of GnRH → LH surge and smaller FSH surge from anterior pituitary → ovulation

What does LH do in late follicular phase to ovulation hormonal changes?

• Promotes follicle rupture

• Ovum Release (ovulation)

• Corpus luteum (CL) formation

What does corpus luteum (CL) do in late follicular phase to ovulation hormonal changes?

Secrete progesterone and estradiol

What occurs during the Luteal Phase?

• Progesterone → inhibits GnRH, LH, FSH, and kisspeptin surge center to prevent another ovulation

• Prevents second LH surge (prevents second ovulation)

• CL depends on LH, dies if LH drops if no fertilization

• Menstruation begins once progesterone and estradiol levels drop

Describe the phases of the uterine cycle

• Proliferative phase

• Secretory phase

• Menstrual phase

Uterine Cycle: Proliferative Phase

Follows follicular → endometrial growth

Uterine Cycle: Secretory Phase

Luteal → nutrient/gland development

Uterine Cycle: Menstrual Phase

Hormone drop → shedding

What are systemic effects of estrogen (estradiol)?

1. Endometrium growth

2. Mammary gland growth

3. Growth plate fusion and bone density maintenance

4. Permissive for actions of progesterone (estrogen stimulates transcription receptor gene to make more progesterone)

What are systemic effects of high levels of progesterone?

1. Uterus: Inhibits estrogen induced growth

• Induces endometrial environment secretion to implantation (glycogen)

• Inhibits uterine contractions

• Forms cervical mucus plug to oppose sperm entry

• Maintains endometrium for pregnancy

• endometrial lining collapses and is lost

• Ovulation inhibited due to negative feedback of GnRH and LH secretion

What happens if fertilization and implantation occur?

• Embryo secretes hCG → maintains corpus luteum (CL)

• corpus luteum (CL) continues progesterone and estradiol production

• Placenta later takes over hormone production (independent of LH/hCG)

• LH (luteinizing hormone) remains low during pregnancy

When is the peak function of CL post ovulation?

6-7 days

When is the presence of healthy enough CL post-ovulation?

10-12 days

What happens to CL in the absence of fertilization?

accelerated decline after 10 days, lost by 13-14 days.

What secretes testosterone in males, and what hormone stimulates its secretion?

• Leydig cells in the testes

• Stimulated by luteinizing hormone (LH)

What does testosterone, secreted by Leydig cells in males, inhibit through negative feedback?

• GnRH from the hypothalamus/pulse center

• LH from the anterior pituitary
  → This reduces further testosterone production

What secretes inhibin in males, and what stimulates it?

• Secreted by Sertoli cells in the testes

• Stimulated by FSH

What does inhibin, secreted by Sertoli cells in males, inhibit?

Follicle stimulating hormone (FSH) from the anterior pituitary
→ Selectively reduces FSH without affecting LH or GnRH

What secretes estradiol in females, and what stimulates it?

• Secreted by granulosa cells in ovarian follicles

• Stimulated by FSH and conversion of androgens (from thecal cells via aromatase)

What do low levels of estradiol (estrogen) in females inhibit through negative feedback?

• GnRH pulses (via kisspeptin neurons in pulse center)

• FSH, especially in combination with inhibin
  → This helps limit recruitment of additional follicles

What do high levels of estradiol in females trigger, and how is this feedback different?

Triggers positive feedback on:

• GnRH pulse frequency

• LH secretion → leads to LH surge
  → This induces ovulation

What secretes progesterone and under what conditions?

• Secreted by the corpus luteum in the ovary

• After ovulation, in response to LH

What does progesterone secreted by the corpus luteum inhibit?

• GnRH, LH, and FSH secretion

• Kisspeptin neurons in both pulse and surge centers
  → Prevents another LH surge and ovulation during the luteal phase.

What does inhibin, secreted by granulosa cells in females, inhibit?

FSH from the anterior pituitary
→ Helps limit recruitment of new follicles during the late follicular phase.

What do high levels of androgens in females inhibit?

• GnRH and LH secretion

• Via inhibition of kisspeptin neurons in the hypothalamic pulse center
  → Can stop cycling and promote male secondary sex characteristics.

What’s a common feature Leydig cells of the testes and thecal cells of the ovaries?

They synthesize testosterone under the control of LH

Where does fertilization of the embryo occur?

Ampulla region of the oviduct/Fallopian tube

After fertilization, where does implantation of the embryo occur?

Endometrium of the uterus

How long does it usually take for an embryo to travel and implant in the endometrium after fertilization?

6-7 days

Effects of Testosterone before birth

• Differentiation of the reproductive tract and external genitalia

• Promotes descent of the testes into the scrotum

Effects of Testosterone on Sex-Specific Tissues after Birth

• Promotes growth and maturation of the reproductive system at puberty

• Is essential for spermatogenesis (conversion to estradiol)

• Maintains the reproductive tract throughout adulthood

Other Reproduction-Related Effects of Testosterone

• Develops the sex drive at puberty

• controls gonadotropin hormone secretion

Effects of Testosterone on Secondary Sexual Characteristics

• Induces the male pattern of hair growth

• Deepens voice

• Promotes muscle growth responsible for the male body configuration

Non-reproduction actions of testosterone

• Exerts a protein anabolic effect

• promotes bone growth at puberty

• closes epiphyseal plates

• induces aggressive behavior

Hormone Secretion Patterns during Pregnancy

• Increased prolactin secretion at end of pregnancy

  • caused by decrease in hypothalamic dopamine and estrogen stimulation of prolactin synthesis

• Increased oxytocin receptors in uterus and mammary glands

  • caused by positive feedback with estrogen