Lecture 7- Pregnancy related Hormones Flashcards
Learning Outcomes female Reproduction & Pregnancy
*Describe the physiological change to the endometrium during the menstrual cycle relating these to:
*Endocrine regulation
*Ovarian cycle
*Explain the endocrine control Oogenesis including
*Ovarian and hypothalamo-pituitary hormones
*Feedback variation across the cycle
*Show an understanding of whole body effects of menopause, linking to hormonal effects on other organs
*Draw links between hormonal preparation of sexual organs for pregnancy and their ability initiate development during pregnancy.
Preparation of the Endometrium
In terms of what we remember from this week’s content, is that the endometrium,
which is identified down here at the bottom of the slide, changes during the menstruation cycle.
And what you can see is that there is sloughing off of the cells at the end, and therefore also stretching into the beginning of this cycle.
And that as this then moves past about day four,
there is a proliferative phase where there’s an increase in the size of the cells that form the endometrium and that there’s an increase in blood supply to this. And then what happens is this growth is supported by Oestrogens. And meanwhile in the ovaries we’re having growth of the follicle which then upon ovulation releases an oocyte.
And this oocyte release marks the establishment of the secretory phase that you can see in the endometrium
This is a point where we get really ready to receive this oocyte just in case it is fertilised.
And this presents us to the point where we are ready to talk about pregnancy.
Fertilisation
So what happens during fertilisation is that the spermatozoa that we talked about earlier on in the week that you can see at the top of this figure,
are able to approach, and eventually one of these usually is able to penetrate the surface of this oocyte.
That has been released. Remember, it’s a secondary oocyte.
And what we see is that the secondary oocyte has had the outer surface,
this zona pellucid, round the outside, has been degraded by the enzymes that are present in the head of the spermatozoa in the acrosome
And so this enables, uh, there to be, um, fertilisation.
And that this provides the additional chromosomes.
Because remember, both the sperm and the secondary oocyte have got a haploid number of chromosomes.
So they have N and together they make 2n amount of chromosomes okay.
So in this particular case the fertilisation has happened.
*Most likely to occur when sex takes place between 2 days before ovulation to 1 day after.
*Usually in the fallopian tube 12-24h after ovulation
*Capacitation of sperm occurs several hours after sex:
*Tail beats more vigorously
*Removal of cholesterol, proteins & glycoproteins from the head →easy release of enzymes in the acrosome upon penetration
And so when we look at what the important things to remember about fertilisation are that, um,
sex needs to take place between two days before ovulation and a day after because these spermatozoa,
remember, can live for around 48 hours after sex has taken place.
This usually, happens in the fallopian tube, but it depends on how long after fertilisation this has happened.
Uh, usually this happens about 12 to 24 hours after ovulation.
To start with, the sperm sort of tap their tails and bang them against the surface of the of the secondary oocyte.
And this tail beating more vigorously is known as capacitation.
And it does this for several hours. Uh, this helps in the removal of cholesterol proteins and glycoproteins from the head.
So it’s part of the release of those enzymes that are in the acrosome.
and this causes a nice easy, um, way for the spermatozoa to penetrate the secondary oocyte.
Movements of a fertilised embryo
So when we just quickly look at our ovaries here.
and our uterus is here in the middle. We’ve taken part of that in this picture over here on the right hand side.
And what you can see in our ovary where ovulation has happened.
as he egg moves across the Professor Zoidberg like looking projections,
we’ve then got fertilisation happening here within 12 to 24 hours following the release of the secondary oocyte into the fallopian tube.
And then what happens is, is we get the formation of two and then four cells.
That happens usually about between 25 and 30 hours after fertilisation.
Of course, then there is further division of the um of the fertilised cell,
which then leaves to bastocyst formation around four and a half to five days after fertilisation.
And this blastocyst is then able to implant itself in that nice, thick,
fluffy and comfortable and endometrium that’s been generated by the oestrogen and progesterone.
And it’s got plenty of resources. It’s got plenty of glucose, um, and glycogen that are here waiting for it to arrive so that it,
it’s got plenty of nutrients to allow it to continue its growth.
Now. This is then when the placenta is generated.
Okay. And we’re going to talk about that next.
Movements of a fertilised embryo
So what happens here is that over here on the right hand side, we’ve got the endometrium and we’ve got a gland here.
This is where we get the generation of some of those nutrients we mentioned earlier.
We’ve also got here our blastocyst
And what you’ll notice is that we’ve got it’s now become a trophoblast, and it’s got layers of cells around the outside.
I don’t need you to remember what they’re called,
but I do need you to remember that this is sustained almost completely by those nutrients that are generated by this endometrial gland up here.
So placentation is the process of forming the placenta.
And this is the site of exchange of the nutrients and the wastes between the foetus and the mother.
And remember that the placenta also produces some hormones, which we’ll talk about in a minute.
Um, and these keep the pregnancy viable. Okay.
If this doesn’t happen and there is not enough hormone here, this will end up in a termination of the pregnancy.
The placenta has got, um, some foetal and some maternal tissue.
the chorion, a membrane that surrounds an embryo and is a key component of the placenta,
is, generated by the foetus, whereas the amnion is what’s generated by the, mother.
Hormones that support pregnancy
So in terms of what hormones are generated, you can see this is the placenta up here on the top.
And the thing that happens immediately when there is um.
Implantation of that blastocyst is that human chorionic gonadotropin, or hCG, is produced up to 1-5 days after fertilisation. and up to around 12 weeks after which the hCG starts to drop.
And this is because the placenta starts to generate its own other hormones- progesterone and oestrogen that will take over the maintenance of the pregnancy.
The HCG, is what’s detected in a pregnancy test.
It’s also a G protein with two subunits.
The alpha subunit is very, very similar to LH, MSH and TSC all of which we’ve talked about in the last two weeks.
But the beta subunit is, um, different from other pituitary glycoprotein hormones, so that’s what makes it unique.
This, HCG is able to rescue the corpus luteum from degradation because if you remember, without the presence of progesterone.
The corpus luteum will degenerate.
And so therefore this hCG keeps the corpus luteum from breaking down whilst it is expressed for upto 12 weeks
and it does this to ensure that the corpus luteum will generate progesterone and oestrogen until this is taken over by the placenta.
That’s around three months after conception or around 12 weeks.
And it’s after this point when the oestrogens in the placenta and progesterone have started to be generated,
um, that we lose the corpus luteum and there is no more need for hCG.
This hormone and progesterone and oestrogens here in pregnancy work not only to keep the endometrium,
nice and well supplied with blood and nutrients, but it also ensures that there’s preparation of mammary glands for lactation in the breasts.
And it also ensures that the mother’s body is ready for birth in a number of different ways.
Now, what you’ll notice here is that the corpus luteum.
It’s got an arrow going to relaxin, which suggests that the corpus luteum is also continuing to generate relaxin.
This again is taken over by the placenta when the corpus luteum disappears.
But the relaxin is really, needed.
Very much later on in pregnancy. Um, although it builds up over time, it increases the flexibility of the, pubic symphysis,
which is, uh, what keeps the pelvis in a female, um, nice and close together.
So what it allows is a bit more stretch in those tendons to allow for the head of the foetus to move between them,
and it also helps to dilate the cervix very, very late on, uh, close to labour.
Human chorionic somatomammotropin (hCS) is also produced by the placenta, and it helps mammary glands prepare for lactation.
So this is coming later. Um, but it’s enabling the mammary glands to develop and
increase in size so that they are ready for generation of milk once the foetus is born.
This enhances also the rate of protein synthesis because if you’ve noticed.
somatotrophin is ofcourse growth hormone.
And so with the use of growth hormone we’d expect for an increase in protein synthesis,
which is exactly the same thing that we see with human chorionic somatomammotropin.
So matter a trophy. There’s a decrease in glucose use.
Um, as. This process continues.
And there is, uh, the generation of ATP from fatty acids instead, as the glands in the endometrium, uh, stop producing glucose.
And then finally, the last hormone that is important during pregnancy is corticotropin-releasing hormone, or CRH.
And this establishes the timing of birth. And we’ll look at that in a little more, um, uh, detail in a minute.
But this increases secretion of cortisol
Hormones that support pregnancy and what happens when
In terms of what happens when, um, you can see on this graph where there’s an increase in time by weeks,
uh, the duration of a pregnancy is usually about 40 weeks, and this is the blood levels of that hormone.
So here you can see the hCG peaks about 8 to 10 weeks into pregnancy.
And then it will decrease significantly after 12 weeks.
Um almost to undetectable levels at the bottom.
And progesterone and oestrogen slowly increase throughout the pregnancy; before it overlaps and becomes higher than the hCG levels it’s by the corpus luteum.
And then after when it starts to massively increase.
This is where the placenta starts to take over.
The high levels of relaxin that are produced by the placenta.
Of course, make all ligaments, not just those in the pelvis, um, soften.
Um, and this can sometimes lead to, uh, what’s perceived as, uh, pregnancy clumsiness,
where you fall over and things like that and tend to get, um, sore wrists.
And it seems that people are more susceptible to repetitive strain injuries.
And the reason for this is because of those ligaments being really soft,
which is why you get told not to exercise too, vigorously during pregnancy.
These effects can persist for about a year post-birth.
The high oestrogen and progesterone, um, are seen really quite, um.
Obviously in the final trimester, and this can lead to lots of ups and downs of mood.
Um, and in addition to that, that they are of course,
making breast um development and therefore growth, which can also make those particularly sensitive. And it’s preparing the body for lactation, which is controlled by prolactin and oxytocin
*High levels of relaxinproduced by the placenta causes ligaments to soften, which can lead to stability issues.
*These effects can persist for up to 1 year post birth.
*High Oestrogen and progesterone particularly in the 3rdtrimester of pregnancy.
*These control breast development and preparation for lactation (controlled by prolactin and oxytocin)
Control of Labour
in terms of labour.
This is the one example of positive feedback that we have that’s very obviously positive.
And progesterone inhibits contractions during most of pregnancy but the placenta secretes corticotropin releasing hormone,
which stimulates the anterior pituitary gland, um, of the foetus to secrete adrenocorticotropic releasing hormone, or Acta.
And that leads to generation of something that can be converted into oestrogen,
which causes a very sharp rise at the top of that point on the on the graph previously.
Um, and that overcomes, uh, the effects of progesterone.
And so with that increase in oestrogen, there’s an increase in oxytocin receptor concentration on the cell surfaces of the uterine muscle cells.
And this means that oxytocin release will then cause uterine contractions and relaxin release, which of course will induce birth
Hormonal control of Lactation
So in terms of hormonal control. Of lactation this is again controlled by the anterior pituitary and what you will notice is that, uh, prolactin here, um,
is released by the anterior pituitary along with a whole bunch of other, um, hormones that you’ve done already.
Uh, with Professor Johnson and myself this week. Last, uh, two weeks.
So what happens is in lactation is that the baby suckling the nipple leads to an increase in, um, and touch sensations.
And so therefore, those touch sensitive, uh, neurones in the nipple send, um, information up to the hypothalamus
Um, and this leads to stimulation of the posterior pituitary.
So the other side to what we’ve been talking about earlier this week.
Okay. And in the posterior pituitary,
it is nerve conduction rather than the release of something from the hypothalamus that goes through the portal circulation.
So there’s neuronal input into that posterior pituitary.
The posterior pituitary then releases oxytocin into the blood, which then leads to the activation of myoepithelial cells in the mammary glands.
And that leads to the milk ejection response. And it is quite like ejection.
It’s almost like somebody pulling a water gun at you.
Okay. And so this continues around, um, in a positive feedback so that.
The more the child sucks, the more milk that is ejected.
Okay. And the milk availability. Encourages the continued suckling if the child.
And so those touch sensation um receptors those are going to feed back and increase the release.
And there’s increase in the release of oxytocin as well, where you’ve got skin to skin contact with the child.
So then the only way to break this cycle is the interruption by the baby ceasing to suckle,
which will then prevent this from happening and prevent further release of milk.
Production and ejection of milk from mammary glandsPrimarily controlled by Prolactin:
*Prolactin-inhibiting hormone (PIH) from hypothalamus inhibits prolactin secretion (anterior pituitary).
*PIHinhibited by stretch receptor signals from nipples when sucking occurs
*During pregnancy inhibited by Oestrogen and progesteroneOxytocin →release of milk into mammary ducts via the milk ejection reflex:
*Secreted by posterior pituitary, inhibits PIH, in response to touch receptors
summary of hormonal control of lactation
In terms of hormonal control of this process.
Ejection of milk from the mammary glands, um, is called lactation,
and the primary hormone that is controlling this process is prolactin that we’ve mentioned already.
There is a prolactin inhibiting hormone that is released in the hypothalamus.
And this can prevent the release of, um prolactin secretion.
Um, and it acts on the anterior pituitary.
Prolactin inhibiting hormone is prevented um from its release by activation of the stretch receptors um in the nipples when suckling occurs.
And so during pregnancy um,
this process is inhibited because oestrogen and progesterone can have a negative feedback
on the release of prolactin from the anterior and from the posterior pituitary.
So in terms of obviously oxytocin, uh, there is release of milk into the mammary ducks via the milk injection reflex, which we touched on earlier.
And this is secreted by the posterior pituitary and inhibits prolactin inhibiting hormone in response to touch receptors.
