Reproductive physiology

Question 1

What is the difference between biological sex, sexual reproduction and sexual intercourse?

Answer 1

• Biological sex = identifies gender, result of chromosomes, production of gametes
• Sexual reproduction = produces offspring that differ genetically form both parents
• Sexual intercourse = required for sexual reproduction, sexual activity, sexual pleasure, human bonding

Question 2

What brain pathways are associated with pleasure? How do they contribute to securing procreation? What role do they play in human bonding + parental behavious?

Answer 2

(1) Mesolimbic dopaminergic system (Pleasure and reward pathway)

• Pleasure, reward, fertility, reproduction and parenting pathways are all linked
• Therefore pleasure pathway has a role in human bonding and parental behaviours
• Activation of the pleasure pathway encourages intercourse → survival of the human race

(2) Nigostriatal tract = movement control
(3) Neuroendocrine p/ws regulating fertility + parenting)

Question 3

How does the brain control penile erection?

Answer 3

• Efferent: increased parasympathetic activity to smooth muscle of pudental artery to cause erection
• Afferent: via pudental nerve, e.g. by tactile stimulus
  
  • Increased nitric oxide synthase (NOS) activity → increased NO → increased cGMP production → dilation of arterial smooth muscle → increased blood flow also compresses venous outflow of blood → leads to increase in size of penis
  • In female, clitoris increases in size due to same mechanism

NOTE:

• Cyclic GMP is normally de-activated by a phosphodiesterase enzyme, and this will reverse the changes leading to penile erection.
• Viagra inhibits the phosphodiesterase, thus potentiating the effects of cyclic GMP.

Question 4

Contrast between the male and female reproductive functions.

Answer 4

Question 5

Label the diagram.

Answer 5

Question 6

Give the functions of the following structures:-

• Seminiferous tubules
• Leydig (interstitial) cells
• Sertoli cells
• Epididymis

Answer 6

Found in the testis

• Seminiferous tubules = Produce sperm
• Leydig (interstitial) cells = just outside tubules, LH responsive to secrete testosterone, , Important for sexual differentiation and spermatogenesis
• Sertoli cells = large cells in seminferous tubules, FSH responsive and make inhibin (for -ve fb on HPG axis), Support spermatogenesis, nutritional and hormonal support for spermatogenesis

Outside testis

• Epididymis (convoluted duct behind the testis, along which sperm passes to the vas deferens) = Sperm are released from the testis and stored here prior to ejaculation
  
  • At ejaculation, sperm are released into the contractile Vas Deferens then pass through urethra.
  • During ejaculation sperm are mixed with fluid from the seminal vesicles and the prostate gland.

Question 7

What happens to the testosterone produced by Leydig cells?

Answer 7

• Testosterone released into testes + vascular system
• Testosterone is transported in the seminiferous tubules bound to androgen-binding protein
• Testosterone is transported in the blood mainly bound to sex hormone binding globulin (SHBG), some to albumin and a small amount is free and bioactive

Question 8

Describe the negative feedback from the testes to hypothalamo-pituitary axis.

Answer 8

• Sertoli Cells (stimulated by FSH) - produce INHIBIN - negative feedback effect on hypothalamus and pituitary
• Leydig Cells (stimulated by LH) - produce TESTOSTERONE - negative feedback effect on hypothalamus and pituitary
• These Decrease the AMPLITUDE of the GnRH pulses.

Question 9

Define gametogenesis in males

Answer 9

1. Primordial germ cells (44X/Y) 2. Spermatogonium (44X/Y) Mitosis 3. Primary spermatocyte (44X/Y) Meiosis 1 4. Secondary spermatocyte (22X/22Y) Meiosis 2 5. Spermatids (22X/22Y) 6. Spermatozoa (22X/22Y) Into lumen –> epididymis –> vas deferns –> urethra

Question 10

Label the diagram.

Answer 10

Question 11

Give the functions of the following structures:-

• Ovary
• Fallopian tube
• Uterus

Answer 11

• Ovary = Produce mature oocytes monthly, Produces steroids needed for female reproductive function (progesterone and oestrogens)
• Fallopian tube = where released oocyte is fertilised, pathway through which fertilised oocyte (conceptus) reaches uterus
• Uterus – where conceptus normally implants and is supported throughout pregnancy

Question 12

What are the functions of the granulosa and thecal cells?

Answer 12

• Within the follicle, the oocyte is surrounded by granulosa cells. As follicle develops, oocyte grows, granulosa cells proliferate
• Granulosa cells:
  
  • Follicular phase = FSH stimulates the granulosa cells to produce aromatase and convert androstendione (produced by theca cells) into oestradiol
  • After ovulation = produces progesterone
• Theca cells:
  
  • Follcular phase = LH stimulates theca cells to produce androstenedione (for granulosa cells) AND progesterone

Question 13

Define gametogenesis in females

Answer 13

OOGENESIS

1. Begin with oogonia from division of germ cells. Oogonia divide mitotically to produce primary oocytes (diploid).
2. Primary oocytes enter the first meiotic division straight away but are halted in prophase of the first meiotic division.
3. The oocytes form a layer of cells around them, forming primordial follicles. These cells remain dormant for 12-50 years. More cells die of atresia.
4. Puberty - release of FSH rescues a group of the dormant cells and they continue development.
5. They complete the first meiotic division around the time of ovulation.
6. If fertilisation takes place, they will complete the second meiotic division to form an ovum.

Question 14

Which axis controls the menstrual cycle?

Answer 14

• Hypothalamic-pituitary-ovarian (HPO) axis
• LHRH released from the hypothalamus same as GnRH, and stimulates LH + FSH
• LH and FSH stimulate theca and granulosa cells in ovaries (not the uterus)

Ovaries produce progesterone and oestrogen (steroids) that influence the uterus

• Theca and granulosa cells secrete steroid hormones oestrogen and progesterone which act on the uterus (endometrium)
• Progesterone and oestrogen also have negative feedback on both the hypothalamus and the pituitar

Question 15

Describe the ovarian cycle.

Answer 15

Follicular

• There is a slightly elevated level of FSH at the start of the cycle.
• This rescues some of the follicles from dormancy so they continue development.
• The follicles start producing 17 beta oestradiol which inhibits gonadotrophins (FSH and LH)

Ovulatory

• The Graffian follicle is no longer FSH-dependent and can survive with it’s own local production of oestrogen.
• The other cells are FSH-dependent and so undergo atresia when the FSH is removed.
• Eventually oestrogen levels remain high enough for long enough that they switch to a positive feedback effect and stimulate a surge in LH and FSH.
• Surge in LH stimulates ovulation

Luteal

• Empty follicle becomes a corpus luteum and produces 17 beta oestradiol and progesterone, which has a negative feedback effect on gonadotrophins
• If fertilisation doesn’t take place, oestrogen and progesterone levels fall and negative feedback on FSH and LH decreases so levels rise again.

Question 16

What conditions must be in place for the rising oestrogen concentration switch from negative feedback to positive feedback on gonadotrophins?

Answer 16

• Absence of progesterone
• Oestrogen concentration must be high enough for a minimum of 36 hours

Question 17

Describe the endometrial cycle.

Answer 17

Menstrual

• Day 1-5 of cycle
• Shedding of blood and endometrial lining of uterus
• Remaining basal endometrium very thin

Proliferative and repair - ESTROGEN

• Day 5-14 of cycle
• Stimulation of endometrial cell proliferation, increase in thickness, increase in number and length of glands, increased in length of arteries

Secretory – PROGESTERONE

• Day 15-28 of cycle
• Production of nutrients and other factors
• Epithelial glands widen, endometrium thickens, increased coiling of spiral arteries
• Endometrial lining maintained by progesterone
  
  • Importance of PROGESTERONE
  • If there is no pregnancy, progesterone falls and endometrium sheds →
  • If there is a pregnancy, corpus luteum will continue to maintain progesterone levels → no menses

Question 18

How do hormonal levels fluctuate over the menstrual cycle?

Answer 18

1. FSH: steady in follicular, drop + peaks just before ovulation then steadily decreases during luteal, starts to increase back to normal at end of luteal
2. LH: As with FSH but starts below FSH, peak is larger at ovulation then drops to below FSH
3. 17-B oestradiol: gradually increases during follicular, peaks few days before ovulation, then dips and then increases + dips again in luteal phase
4. Progesterone: starts off right below oestrogen, stays steady until little peak at ovulation, then dips and increases, overtakes oestrogen near end of luteal phase

Question 19

How do hormones feedback to the HPG axis?

Answer 19

Follicular: oestradiol -ve fb to anterior pituitary + hypothalamus Midcycle: oestradiol +ve fb to anterior pituitary + hypothalamus Luteal: progesterone -ve fb to anterior pituitary + hypothalamus

Question 20

How does fertilisation occur?

Answer 20

1. Sperm enters female tract near CERVIX following intercourse
2. Cervical mucus is normally hostile to sperm changes mid cycle to allow sperm entry into uterus
3. Passage of sperm through uterus → fallopian tube
4. Survival of the fittest → fertilization window is a few days
5. CAPACITATION: mediated by progesterone release from oocyte → increased motility of sperm and chemotaxis
6. Fusion of egg and sperm within ~24hrs post ovulation
7. ACROSOME REACTION
   
   • Digests zona pellucida to allow sperm entry
   • Hardening of zona pellucida → prevents polyspermy
8. CALCIUM INFLUX
   
   • Resumption of meiosis
   • Release of second polar body
9. Initiation of embryo mitotic cleavage following fusion of maternal and paternal pro nuclei

Question 21

Summarise the preimplantation development and events immediately after implantation of the foetus.

Answer 21

Preimplantation development

• Normally occurs within the Fallopian tube
• Over a period of 6 days
• Series of cleavage divisions, which sequentially double the number of cells in the conceptus (2, 4, 8, 16 cells) to produce a ball of undifferentiated cells (the Morula)
• The Morula differentiates - inner and outer cell layer
• This then develops into the Blastocyst which has (1) outer layer of trophectoderm (2) an inner cell mass, and (3) a fluid-filled cavity.
• The Blastocyst then hatches from the Zona Pellucida (within which it has developed up to this time, about day 6 after fertilisation)

Implantation

• Blastocysts begins to implant in the uterine lining>complete 10 days post-fertilisation.
• By this time the inner cell mass, which was a group of undifferentiated cells has become a bilayer disk, composed of hypoblast and epiblast cells.

Gastrulation

• Bilayer > trilaminar embryo
• 3 layers of Germ Cells (Ectoderm, Mesoderm and Endoderm), occurring during days 14-18 postfertilisation