Coitus and Conception

Question 1

Define sexual reproduction.

Answer 1

Formation of a unique individual from the gametes (spermatozoon and oocyte) of two other individuals

Question 2

How long does it take spermatogonia to mature into spermatozoa?

Answer 2

Up to 74 days

50 in seminiferous, 12-26 in epididymis

Question 3

What are the rates of sperm production?

Answer 3

1000 spermatozoa per second
20 yo - 6.5m/g/day
over 50 yo - 3.8m/g/day

Question 4

What are the phases of coitus?

Answer 4

Excitement phase
Plateau phase
Orgasmic phase
Resolution phase (+/- refractory period)

Question 5

What is the female sexual response?

Answer 5

Blood engorgement and erection:
- Clitoris, vaginal mucosa, breast and nipples
Glandular activity
Sexual excitement (as in male)
\+/- orgasm (not needed for reproduction)
No physiological refractory period

Question 6

What are the mechanisms of penile erection? ( The start)

Answer 6

Stimulants

• Psychogenic
• Tactile (sensory afferents of penis and perineum)

Which leads to (a spinal reflex for tactile)

Efferents

• Somatic and autonomic efferents
  
  • Pelvic nerve (PNS)
  • Pudendal nerve (somatic)

And this leads to haemodynamic changes

Question 7

What are the haemodynamic changes that occur as part of a penile erection?

Answer 7

The arteries of the corpora cavernosa needs to vasodilate
Intracellular calcium in the smooth muscle of these arteries must reduce

Inhibition of sympathetic arterial vasoconstrictor nerves

Activation of PNS

Activation of non-adrenergic, non-cholinergic, autonomic nerves to arteries, releasing NO (Nitric Oxide)

These haemodynamic changes cause tumescence

Question 8

How does nitric oxide work in relation to haemodynamic changes in penile tissue to cause tumescence?

Answer 8

Post-ganglionic PNS release ACh
ACh bonds to M3 receptor on endothelial cells
A rise in [Ca2+]intracellular, activation of NOS and formation of NO
NO diffuses into vascular smooth muscle and causes relaxation (vasodilation)

NO also directly released from nerves

Question 9

What are the main categories of causes of erectile dysfunction?

Answer 9

Psychological (descending inhibition of spinal reflexes)
Tears in fibrous tissue of corpora cavernosa
Vascular (arterial and venous)
Drugs

Factors blocking NO:
- Alcohol, ant-hypertensives, diabetes

Question 10

Describe emission in reference to male sexual response.

Answer 10

Movement of ejaculate into prostatic urethra (leakage)
Vas deferens peristalsis
Accessory gland secretion; e.g., bulbourethral

Question 11

Describe the mechanism of ejaculation.

Answer 11

Spinal (& cerebral) reflex

Sympathetic nervous system control (L1, L2)

1. Contraction of glands and ducts (smooth muscle)
2. Bladder internal sphincter contracts
3. Rhythmic striatal muscle contraction (pelvic floor, Ischiocavernosus, Bulbospongiosus, hip and anal muscles)

Question 12

Describe the normal contents of ejaculate.

Answer 12

2-4ml
20-200 x 10/\6 sperm per ml
Total sperm/ejaculate >40 x 10/\6
>60% sperm swimming forward vigorously
,30% abnormal morphology

Question 13

What is classed as low sperm count, and what is it called?

Answer 13

Abnormal oligozoospermia

Question 14

What are the glandular components of semen?

Answer 14

Seminal vesicles:
- 60% of volume
- Alkaline fluid (neutralizes the acid: male urethra, female
reproductive)
- Fructose, prostaglandins, clotting factors

Prostate
- 25% volume
- Milky, slightly acidic fluid
- Proteolytic enzymes (break down clotting proteins, re-
liquefying semen in 10-20 minutes)
- Citric acid, acid phosphatase

Bulbourethral glands (Cowper's gland)
- Very small volume
- Alkaline fluid
- A mucous that lubricates the end of the penis and 
  urethral lining

Question 15

Where is semen deposited?

Answer 15

Just before the cervix

Question 16

How does the oestrogen and progesterone levels affect the mucosa of the cervix?

Answer 16

When just oestrogen, no progesterone:
- Abundant clear, non viscous mucus

When both progesterone and oestrogen:
- Thick, sticky mucous plug.

Question 17

Where is the site of fertilization?

Answer 17

The ampulla of the fallopian tube.

Question 18

How does sperm change once deposited in the vagina?

Answer 18

Capacitation of sperm

Further maturation of sperm in female reproductive tract (6-8 hours)
Sperm cell membrane changes to allow fusion with oocyte cell surface
Tail movement changes from beat to whip-like action (3mm/hour)

Question 19

Tell me a little about the amount of sperm involved in coitus.

Answer 19

200-300 million per ejaculation.
300 reach fertilisation site
1 sperm required for fertilization but
299+ needed to disperse the zona pellucida

Question 20

What is the survival time of fertile gametes in female reproductive tract.

Answer 20

Spermatozoa - 48-72 hr (?)
Oocytes - 6-24 hr (max)

Fertile period:
- Sperm deposition up to 3 days prior to ovulation or day
of ovulation

Question 21

How is an oocyte transported?

Answer 21

Beating cilia

Peristalsis of fallopian tube

Question 22

Give an overview of fertilization.

Answer 22

Occurs at ampulla of Fallopian tubes
Requires loss of sperm outer shell (acrosome)
Penetration of ovum membrane, fusion of ova and sperm membrane
Eventual formation and fusion of pronuclei

Question 23

Describe fertilization in some detail.

Answer 23

Sperm pushes through granulosa cells
Protein on sperm binds to ZP3 proteins of zona pellucida

Binding triggers acrosome reaction:
- Acrosome enzymes exposed to zona pellucida
- Hydrolysing enzymes digest through ZP
- One sperm penetrates: fusion of plasma membranes
(egg and sperm)
- Sperm moves into cytoplasm: zygote
- Polyspermy blocked (cortical reaction) (stops zona
pellucida from allowing any more sperm in)

Egg completes meiosis II
Pronuclei (2 sets of chromosomes)
Pronuclei fusion (diploid zygote)
Mitosis

Question 24

What is cleavage in reference to fertilization?

Answer 24

Series of metabolic changes and rapid mitotic division (cleavage)
Increased number of cells 16 - 32 without growth
Totipotency

Question 25

Describe conceptus in the uterine tube.

Answer 25

3 days, awaiting rise in progesterone (smooth muscle relaxation)
period of cell division to blastocyst stage

Blastocyst

• Loss of Totipotency
• Outer layer (trophobast surrounds embryo)
• Inner cell mass (becomes surrounds embryo)
• Fluid filled cavity

Question 26

When and where does the blastocyst implant?

Answer 26

6 days

Endometrium of uterus

Question 27

What is an ectopic pregnancy?

Answer 27

When there is a failure to transport the egg
Embeds in uterine tube, ovary or abdomen
Embryo dies
Severe risk of maternal haemorrhage

Question 28

Describe what happens to the conceptus in the uterus

Answer 28

Zygote to blastocyst: days 14 to 21 of uterine cycle
Progesterone priming of endometrium
Conceptus nourished in intrauterine fluid, 3 days floating
Sticky trophoblasts over inner cell mass, adheres to endometrium (hCG)
Implantation commences (6 days after ovulation)