1 - Reproductive Physiology

Question 1

What are the two main functions of the male reproductive system?

Answer 1

Provision of Androgens

• primarily testosterone
• to initiate and sustain the necessary male phenotype

Production of Mature Sperm

Question 2

Outline the anatomy of the male reproductive system

Answer 2

Question 3

Outline the anatomy of the testes and how this contributes to their function

Answer 3

TESTES

Seminiferous Tubules

• produce sperm

Leydig (Interstitial) Cells

• produce testosterone
• produce other androgens

Testosterone is released into the circulation, from which it can affect the whole body

Question 4

What is the epididymis and its function?

Answer 4

EPIDIDYMIS

One in each scrotal sac

Sperm are released from the testis and stored here prior to ejaculation

Question 5

What happens to sperm at ejaculation?

Answer 5

Leave the epididymis

Pass through the Vas Deferens (which are contractile) and are mixed with fluid from the seminal vesicles

The fluid then leaves the ejaculatory duct

Passes into the urethra, where it mixes with secretions from the prostate gland

Question 6

Outline the main regulatory pathways within the adult human male reproductive system

Answer 6

Question 7

How many sperm are produced by the testes per day?

Answer 7

Overall production from the testes = 200 million sperm per day

Question 8

When does the process of spermatogenesis occur?

Answer 8

Starts at puberty

Normally continues for the rest of a human male’s adult life

Question 9

What happens to the process spermatogenesis over time?

Answer 9

Spermatogenesis continues to occur throughout the male’s lifetime. However, the quantity and quality of sperm produced may decline with increasing age.

Question 10

What occurs in the seminiferous tubules?

Answer 10

Question 11

What human female organ parallels the function of the testes in males?

Answer 11

The ovary parallels the function of the testis

They both produce gametes and steroids needed for reproduction

Question 12

What do ovaries produce?

Answer 12

Gametes = Oocytes

Steroids needed for reproduction = Progesterone and Oestrogens

Question 13

Outline the anatomy of the female reproductive system

Answer 13

Question 14

What is the function of the fallopian tube?

Answer 14

Fallopian Tube (Oviduct)

The route by which oocytes reach the uterus

Question 15

What is the function of the uterus?

Answer 15

Provides an appropriate environment to sustain either an oocyte or a conceptus (fertilised conceptus)

The conceptus (normally) implants in the uterus

Question 16

Outline the major components of the human ovary

Answer 16

Question 17

Outline the changes in follicular structure and size as they develop

Answer 17

NOTE: Follicles may not always develop all the way to ovulation, as atresia is common

Question 18

What cells in developing follicles produce estrogens?

Answer 18

Thecal cells of developing follicles

Question 19

What cells of follicles produce estrogens and progesterone during the second half of the ovarian cycle?

Answer 19

Granulosa-luteal cells produce estrogens and progesterone during the second half of the ovarian cycle

Question 20

How long do the ovarian and endometrial cycles last?

Answer 20

They both last for approximately one month

Question 21

How many days does the menstrual cycle last?

Answer 21

The menstrual cycle lasts approximately 28 days.

This varies between individuals

Normal Range: 21-35 days

Question 22

How long can menstrual cycles last in younger tennagers?

Answer 22

The length may be up to 45 days

Question 23

What tends to happen to the length of a woman’s menstrual cycle as she approaches menopause?

Answer 23

When a woman is approaching the menopause, her menstrual cycle is likely to become irregular in length (shorter or longer), or maybe skip 1/2 months before resuming.

Question 24

Summarise the main events of the menstrual cycle

Answer 24

Question 25

Outline the regulation of hormones during the human menstrual cycle

Answer 25

Note: change in regulation of the hypothalamic-pituitary axis, changing from negative feedback to positive feedback to cause LH surge and ovulation

Question 26

What changes occur to tissue structures during the normal human menstrual cycle?

Question 27

Outline the hormonal changes in the human menstrual cycle

Answer 27

The key changes in the four main hormones are summarised, showing the time-frames of the changes. Note that the basal body temperature rises slightly (about 0.5°C) around the time of ovulation.

Question 28

Outline folliculogenesis in the human ovary

Answer 28

Note that the first meiotic division is completed during the formation of the Secondary follicle, linked to the LH surge. Meiosis II follows immediately after this, but pauses in metaphase II as shown.

Two important points need to be noted from this figure. Firstly, that the time taken for a primordial follicle to develop into a secondary (mature) oocyte is more than one month, so Figure 2.10 does NOT reflect ovarian function fully. This also applies to Figures 2.5 and 2.9, which imply a relatively rapid change from a primordial follicle to a ‘growing follicle’.

Question 29

What follicles are in the human ovaries at any one time?

Answer 29

The reality is the human ovaries contain multiple follicles at all stages of development at any time, with one dominant follicle entering the later stages to form a secondary (Graafian) follicle.

Question 30

How long is there between the release of secondary follicles from the ovaries?

Answer 30

The ovaries usually alternate the release of the follicle, so each ovary has approximately 56 days (2 Months) between the release of one secondary follicle, and the release of the next one.

Question 31

How many primordial follicles are present in human ovaries?

Answer 31

The human ovaries contain about 2 million primordial follicles at the time of birth

Only about 400 of them will be released at ovulation during a reproductive lifetime

12 per year for 30-35 years on average

Question 32

Does oogenesis occur during meiosis?

Answer 32

Meiosis during human oogenesis is an intermittent process. Both the first meiotic division and the second meiotic division are paused during follicle development.

Question 33

When does Meiosis I start?

Answer 33

Meiosis I starts during embryonic development

It halts at the diplotene stage of prophase I (primary follicle)

This persists until puberty when meiosis resumes as secondary follicles develop

Only a sub-population of follicles become secondary follicles, and these undergo another pause in meiosis at the metaphase stage of Meiosis II.

Question 34

What process causes completion of meiosis in a human oocyte?

Answer 34

Only fertilisation can cause the completion of meiosis in a human oocyte.

Question 35

How many primordial follicles do the ovaries contain?

Answer 35

Contain about 6 million primordial follicles at about 20 weeks of development

By delivery of the infant, this has fallen to ~1 million per ovary.

About 400 follicles will be ovulated during the reproductive life-time.

Question 36

How many sperm do the testes produce per day?

Answer 36

Produce about 100 million sperm per day, from puberty onwards.

Question 37

What does sexual reproduction produce?

Answer 37

Produces offspring that differ genetically from both parents

Question 38

What is sexual intercourse required for?

Answer 38

Required for sexual reproduction

Sexual activity

Sexual pleasure

Human bonding

Question 39

Outline the link between sex and the brain

Answer 39

Activation of pleasure pathways therefore encourages intercourse, thus continuation of the human race. These brain functions complement those discussed earlier in this chapter, in which the role of the brain in controlling the production of gametes (hypothalamic-pituitary-gonadal axes) was considered.

Question 40

What systems regulate penile erection?

Answer 40

it is clear that the regulation of penile erection is also partly under the control of the brain, via the spinal cord and efferent nervous system.

This is not the complete picture, as tactile stimulus of the penis can activate the afferent system (pudendal nerve) as shown, involving a more direct interaction between the spinal cord and penis.

Question 41

What changes occur during erection?

Answer 41

1. Initiated by: increased parasympathetic activity to smooth muscle of pudendal artery
2. Increases the activity of Nitric Oxide Synthase (NOS), and hence nitric oxide (NO)
3. NO increases production of cyclic GMP which induces dilatation of arterial smooth muscle.
4. Counteracts sympathetic-maintained myogenic tone
5. Increases blood flow in corpus cavernosum
6. Which compresses the dorsal vein, restricting the outflow of blood
7. The urethra is protected from increased pressure by surrounding corpus spongiosum (less turgid)

Question 42

What reverses the changes that lead to penile erection?

Answer 42

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 43

What is the female equivalent of male erectile tissue?

Answer 43

The female equivalent of the penis is the clitoris

The clitoris, like the penis, increases in size as a result of an increased blood flow into the tissue

The mechanism is the same (i.e. release of NO)

Question 44

What is the process and main events of fertilisation?

Answer 44

Deposition of sperm within female system following sexual intercourse.

Sperm are deposited near cervix

Cervical mucus is normally hostile to sperm

This forms a physical barrier to sperm

Cervical mucus changes at mid-cycle

This change permits sperm to enter uterus

Passage of sperm through uterus

Passage of sperm into Fallopian tube

Swim from there to Fallopian tube ampulla

~30 µm/sec average speed; 2 mm/min; 12 cm/hour

Survival of the fittest (a few days)

Capacitation – takes time within uterus

Capacitation is essential preparation before the sperm meet the oocyte

Meeting of egg with sperm

Fusion of egg and one sperm (within 24 hours post ovulation)

Acrosome reaction: penetration of Zona Pellucida (& Coronal cells)

Calcium flux

Resumption of meiosis, release of 2nd polar body

Alignment of maternal and paternal chromosomes to generate zygote.

Change in Zona Pellucida to prevent additional sperm fusing with zygote.

Initiation of mitotic (cleavage) divisions in embryo.

Question 45

Outline what occurs when a sperm has entered the oocyte

Answer 45

Fusion of the sperm with the egg is a complex process, requiring interaction with the zona pellucida so that the latter is digested by the enzymes released during sperm capacitation.

Question 46

Outline what occurs after fertilisation with regards to the cortical reaction

Answer 46

The cortical reaction is initiated

• hardening of the zona pellucida
• exclusion of other sperm.

By this stage, Meiosis II in the maternal chromosomes is completed, leading to the formation of polar body II, and the female pronucleus. The head of the sperm is undergoing decondensation, with the previously tightly packed paternal chromosomes forming a much looser structure. Both pronuclei are haploid at this stage of fertilisation.

Both pronuclei are now visible; note that by this stage duplication of the DNA in both pronuclei has been duplicated, making 2n chromosomes of maternal and of paternal origin. The next stage is development is a mitotic division, giving 2 daughter cells, which must have the same chromosomal complement, so duplication is required at this stage. Maternal and paternal chromosomes mix for the first time, as the metaphase plate of the first cleavage division is formed.

Separation of the male (blue) and female (red) chromatids can be seen; each cell should receive one paternal copy and one maternal copy of each chromosome, so the 2 cells shown in (F) have an identical chromosomal complement.

The source of this data is not given – for reasons that will become clear!

While this figure 2.18 summarises the events that take place during fertilisation, it is far too simplistic, and omits many of the key events that take place during fertilisation. In addition, it is in error in suggesting that there is an ‘egg nucleus’ – that should be an egg (oocyte for preference) pronucleus; also the ‘nuclei’ do not ‘fuse’.

This is included to remind you that information may be incomplete or wrong, even though it has been published.