1 - Reproductive Physiology Flashcards
What are the two main functions of the male reproductive system?
Provision of Androgens
- primarily testosterone
- to initiate and sustain the necessary male phenotype
Production of Mature Sperm
Outline the anatomy of the male reproductive system
Outline the anatomy of the testes and how this contributes to their function
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
What is the epididymis and its function?
EPIDIDYMIS
One in each scrotal sac
Sperm are released from the testis and stored here prior to ejaculation
What happens to sperm at ejaculation?
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
Outline the main regulatory pathways within the adult human male reproductive system
How many sperm are produced by the testes per day?
Overall production from the testes = 200 million sperm per day
When does the process of spermatogenesis occur?
Starts at puberty
Normally continues for the rest of a human male’s adult life
What happens to the process spermatogenesis over time?
Spermatogenesis continues to occur throughout the male’s lifetime. However, the quantity and quality of sperm produced may decline with increasing age.
What occurs in the seminiferous tubules?
What human female organ parallels the function of the testes in males?
The ovary parallels the function of the testis
They both produce gametes and steroids needed for reproduction
What do ovaries produce?
Gametes = Oocytes
Steroids needed for reproduction = Progesterone and Oestrogens
Outline the anatomy of the female reproductive system
What is the function of the fallopian tube?
Fallopian Tube (Oviduct)
The route by which oocytes reach the uterus
What is the function of the uterus?
Provides an appropriate environment to sustain either an oocyte or a conceptus (fertilised conceptus)
The conceptus (normally) implants in the uterus
Outline the major components of the human ovary
Outline the changes in follicular structure and size as they develop
NOTE: Follicles may not always develop all the way to ovulation, as atresia is common
What cells in developing follicles produce estrogens?
Thecal cells of developing follicles
What cells of follicles produce estrogens and progesterone during the second half of the ovarian cycle?
Granulosa-luteal cells produce estrogens and progesterone during the second half of the ovarian cycle
How long do the ovarian and endometrial cycles last?
They both last for approximately one month
How many days does the menstrual cycle last?
The menstrual cycle lasts approximately 28 days.
This varies between individuals
Normal Range: 21-35 days
How long can menstrual cycles last in younger tennagers?
The length may be up to 45 days
What tends to happen to the length of a woman’s menstrual cycle as she approaches menopause?
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.
Summarise the main events of the menstrual cycle
Outline the regulation of hormones during the human menstrual cycle
Note: change in regulation of the hypothalamic-pituitary axis, changing from negative feedback to positive feedback to cause LH surge and ovulation
What changes occur to tissue structures during the normal human menstrual cycle?
Outline the hormonal changes in the human menstrual cycle
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.
Outline folliculogenesis in the human ovary
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’.
What follicles are in the human ovaries at any one time?
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.
How long is there between the release of secondary follicles from the ovaries?
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.
How many primordial follicles are present in human ovaries?
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
Does oogenesis occur during meiosis?
Meiosis during human oogenesis is an intermittent process. Both the first meiotic division and the second meiotic division are paused during follicle development.
When does Meiosis I start?
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.
What process causes completion of meiosis in a human oocyte?
Only fertilisation can cause the completion of meiosis in a human oocyte.
How many primordial follicles do the ovaries contain?
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.
How many sperm do the testes produce per day?
Produce about 100 million sperm per day, from puberty onwards.
What does sexual reproduction produce?
Produces offspring that differ genetically from both parents
What is sexual intercourse required for?
Required for sexual reproduction
Sexual activity
Sexual pleasure
Human bonding
Outline the link between sex and the brain
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.
What systems regulate penile erection?
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.
What changes occur during erection?
- Initiated by: increased parasympathetic activity to smooth muscle of pudendal artery
- Increases the activity of Nitric Oxide Synthase (NOS), and hence nitric oxide (NO)
- NO increases production of cyclic GMP which induces dilatation of arterial smooth muscle.
- Counteracts sympathetic-maintained myogenic tone
- Increases blood flow in corpus cavernosum
- Which compresses the dorsal vein, restricting the outflow of blood
- The urethra is protected from increased pressure by surrounding corpus spongiosum (less turgid)
What reverses the changes that lead to penile erection?
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.
What is the female equivalent of male erectile tissue?
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)
What is the process and main events of fertilisation?
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.
Outline what occurs when a sperm has entered the oocyte
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.
Outline what occurs after fertilisation with regards to the cortical reaction
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.
