1 - Reproductive Physiology Flashcards

Question

Outline the regulation of hormones during the human menstrual cycle

Answer 1

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

Answer 2

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.

Answer 3

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’.

Answer 4

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.

Answer 5

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.

Answer 6

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

Answer 7

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

Answer 8

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.

Answer 9

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

Answer 10

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.

Answer 11

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

Answer 12

Produces offspring that differ genetically from both parents

Answer 13

Required for sexual reproduction Sexual activity Sexual pleasure Human bonding

Answer 14

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.

Answer 15

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.

Answer 16

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)

Answer 17

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.

Answer 18

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)

Answer 19

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.

Answer 20

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.

Answer 21

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.

1 - Reproductive Physiology Flashcards

(46 cards)