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Lecture 4

BIO270 Lecture 4.doc

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David Lovejoy

Wednesday, November 18, 2009 BIO270 Lecture 4 **Continuation from Lecture 3 Recording** - Okay chapter 14. - The whole chapter is basically on it – this covers the whole area. - We’re going to be talking about this for the next 20 minutes as well as probably a good chunk of the lecture, not next week but the week after. - He already talked about a number of these things. Obviously there are a number of very elaborate processes from the time that a cell develops to the point where a multi-cellular organism continues. There are several different stages in life history & depending upon the type of organism, reproduction will affect the organism differently at different stages. - It’s involved at all stages, so we have essentially fertilization & that occurs in a number of different ways. - There is a guy he knew who wrote this book called Sperm Wars. This guy, he was at the last university where he was at & he wrote this book & it was a best seller – it sold so well that he had a career as a writer after that but it was a highly controversial book. One of the reasons why it was controversial was because the way he conducted his research – he never published it in per- reviewed journals. Instead what he did was that he put an ad out in Cosmopolitan that said “Ladies, is your husband being unfaithful to you?”, something along these lines, “send me a sample of his sperm & I will analyze it.” You can imagine all these samples coming from all over the world – some of them have been fixed, others have not, & he looks at a handful of these things & created a grandiose theory based on this & was saying you know when there are multiple partners that occurs in a short period of time that sperm have the ability to out-compete other sperm. Well this occurs with a number of animal species but it has never been shown for humans. He wrote this book called Sperm Wars that you know even if you can go & meet some girl before the next guy does, you can’t necessarily guarantee that your sperm will carry the job out for you. He wrote that book & it sold very well & it was so controversial that he was asked to keep a low profile at the university. He wrote another book about a professor who was being marginalized by all these other colleagues – don’t know where he got that idea from. Anyways you might see the book out there – it’s still in print, it’s seen by the prof periodically but there aren’t many scientific factors to support what he’s saying. - We’re not going to be talking about the development of reproduction in this course, but you should be aware that there is a variety of different developmental stages. We have a number of developmental courses in CSB since that is one of our strengths in that department so if you’re interested then take some of those courses. - & there are some very interesting aspects in terms of development of the reproduction system. Reproductive systems that will occur in early life history, then juvenile & then adult & then eventually you get to death. - We’re showing a crab here but the reality is that it’s the same basic stages for any type of organism. - So there are a couple of things to think about. Growth & differentiation, reproductive traits that are established during embryonic development – how those systems are turned on during embryonic development can have an effect on what it will do as an adult. - A number of these hormones that are responsible for development can change very much the efficacy of the reproductive system & the primary sex characteristics, the functional aspects of reproduction are the gonads, though there are all the secondary sex characteristics & organs which are responsible to improve the efficiency of procreation. - Okay now we’re mostly going to be talking about sexual reproduction in this course, he may talk about asexual reproduction. - Sexual reproduction is the production of offspring from 2 parents that contribute nearly equal amounts of genetic material. We say nearly b/c in the mammalian system, for example, most of the mitochondrial DNA is contributed by the mother, not the father. There is a small amount by the father, but it’s almost insignificant amount. - There are a couple of components of sexual reproduction that we will consider: listed in slide. - Mostly we’re going to focus on how the circuits all fit together but there are a number of components that need to be considered. - Now why do you have sexual reproduction? Well we can spend the entire course talking about that – there have been great tomes that have been written about why sexual reproduction is so common. People continue to argue about that so he won’t get into that debate because that will be the end of the course, we just wouldn’t be able to complete the rest of the course. - However, a simple way of looking at it is that it generates genomic variation. As long as the genes are mixing, there is lots of genomic interaction & variation & that allows for multiple phenotypes & allows for greater adaptive ability as the organisms go forth & come across different types of environments. - So there are haploid gametes formed from a diploid parent & this is referred to spermatogenesis in males where testes produce the small gametes (sperm) & what’s called oogenesis in females & the ovaries produces the large gametes (ova). - It’s been argued that when you look at the development of gametes, if you take any type of system, it widely would have a large one & a small one. Why not just have 2 gametes of roughly the same size? What they find is that when they look at various models, the most stable situation is where one is small & one is large – small one is much more mobile & can use all the energy for motility whereas the larger one is much more stable, it has nutrition & has a greater survivability & again people love to write a lot of thesis about these sorts of things. Essentially when you look at it, it always gets divided into small & large – large is what we associate with females & small is with males. From a biological point of view, it’s almost arbitrary. - The recombination creates various hybrid chromosomes & we get a number of different genetic combinations. This allows very large number of genotypes which help the fitness of the population. - With gametogenesis, it’s slightly different. - In the female, with oogenesis, we start off with the primary cell here this is the very primitive cell – we call the oogonium which is laid down in the embryo. That will lead to a diploid cell which is the primary oocyte, okay so this will continue to grow. It will then divide where we have the secondary oocyte here & then this will degrade & here we’ll have the mature ovum & then a polar body here. - So one cell here, interestingly in mammals, the oogonium, like he mentioned, were in the female are all laid down in embryo so the full complement of eggs that a female will have are already there at birth & there will not be more after that. - The male situation is somewhat different, well quite different actually. It produces sperm for the rest of their lives, starts off as a spermatogonium & then leads to a primary spermatocyte here & then it divides here with our meiotic phase II into a secondary spermatocyte & then divides again & now we have our four haploid cells here & then these spermatids then develop into mature sperm & really, it’s just a nucleus within a little missile more than anything else, just a carrier of the genetic information & very highly specialized to carry that information. - Reproductive hormones are responsible for all of these things. - So development to ensure that the reproductive system is all in place – to make sure that all the tubes are in place, the gonads are all in place, the support tissues are all in place. - Sexual maturation: not only do all the systems have to be in place but there has to be the required behaviour, musculature in order to use that. - Then gametogenesis obviously. - Then the mating & the behaviour & the parental behaviours as well which is required that is also under the actions of some of the reproduction hormones. - We’ve already talked a little bit about this – common themes of reproductive hormones. There are both positive & negative feedback controls that’s pretty straightforward in the male but gets a little complicated in the female, we’ll go into that in detail. - He thinks that these are all straightforward. Target tissues can alter the number of receptors. At different stages of the reproductive cycle there is more sensitivity or less sensitivity to the reproductive hormones. Some of the organ systems will undergo growth & degradation period depending upon the animal & the tissue. - Males & females of the same species may use the same hormones although they’ll have slightly different functions & we’ll talk about that when we get to the system. One of the things most people don’t realize is that both males & females produce androgens first & then it gets converted to estrogens. The same pathway occurs in both males & females – the difference is in the females, the androgens are produced & then they get converted to estrogens but then they get vastly converted to estrogen so there is a very large amount of these estrogens, but less of these androgens. In the males, the pathway is still there but the pathway drives towards the androgens & only a tiny amount of estrogens are produced in the male, but they are there & they are required, they just do different things. - Okay steroid hormones, we talked about this. They are derived from cholesterol & they are liganded transcription factors so receptors, they bind a nuclear hormone receptor in target cells. - In vertebrates, the hormones are produced in the gonads primarily & androgens, testosterone, dihydroxytestosterone are examples of androgens & estrogens, the book gives us an example of this hormone. Estradiol is probably a better example because it’s the most potent of those hormones but there are 3 different estrogens: estriol, estradiol & estrone – estradiol is the most potent one & that’s the one we’re going to be referring to but no doubt about it, estrone is present there it’s just not very potent. - Okay this is what they look like. Again you don’t need to memorize this structure but you should know the pathway. - So progesterone will be converted to androstenedione which is an androgen. Okay androstenedione can be converted to either estrone or testosterone so we’ve got our androgen it can be converted to a weak estrogen or a fairly potent androgen. - Testosterone can be converted into dihydroxytestosterone which is the androgen that is associated with a number of the secondary sex characteristics or in fishes, a subspecies, it produces another variant of testosterone called 11- ketotestosterone – for most fishes this is the active form of testosterone, this ketotestosterone. - You’ll note if you take a look at androstenedione or testosterone with the enzymes cytochrome B450 aromatase – this is a very important enzyme so you should know it, you should be aware that this enzyme causes the aromatase reaction, this is the aromatase reaction formation of the benzene ring here & there & it can convert in a one step reaction, androgens to estrogens. This is actually reversible but this enzyme will drive it so testosterone can be converted directly to estradiol & androstenedione can be converted directly to estrone. - These are the very potent ones, these are the less potent but they bind the same receptors. - Gonadotropins – these are nonsteroidal hormones, they are peptide hormones by the anterior pituitary. We already talked about LH & FSH & these are the ones that control the steroid hormone synthesis in the gonads. - There are 3 types of gonadotropins. There are the 2 pituitary gonadotropins: follicle stimulating hormone (FSH) & luteinizing hormone (LH) & another one produced by the placenta in primates called chorionic gonadotropin & when we talk about the female reproductive system we’ll talk about that one, but they’re all gonadotropins, they all do the same set of mechanisms. - Okay the release of the gonadotropins is controlled by the gonadotropin releasing hormone (GnRH) from the hypothalamus. GnRH controls both LH & FSH in a very interesting pattern which we probably won’t go into this year but if we ever get involved in that it’s really quite a neat mechanism. - Here is the overview. GnRH is produced in the hypothalamus, it sends its axons down to the median eminence, the portal vessels & then GnRH percolates through these blood vessels all the way into the anterior pituitary & that causes the release of the gonadotropins & they will act both on the ovaries & testes depending on the sex, obviously. They produce the steroids, testes, primarily testosterone & ovaries, primarily estradiol & progesterone. - And then that will feedback. Now our book shows that it’s a negative feedback because statistically it is a negative feedback but there are examples where this circuit will have a positive feedback on it. **SKIP** - He’s just going to say this in passing because this is an animal course. There is a series of steroids in invertebrates & these are these ecdysteroids. They are associated with reproductive events & development there. - There is a wonderful course in 3 year on insect reproduction which is really quite neat. They discuss it sometimes in fourth year as well but we should know that there is another group in insects that are steroids that play this role. - He’s telling us to forget the rest of the stuff here, & he’s ending today. **Lecture 3 Recording Ends Here** **Lecture 4 Recording Starts Here** - So we’ve already had a little bit of an introduction to a number of different hormone systems. The principal reproductive hormones that we’re going to be dealing with fall into 2 classes for the sake of this course & those are a series of peptides, which you’ve already had a bit of an introduction to these things in your labs, & then the other group are steroids so we’re looking at 2 very different types of systems. - Now while we think of reproductive hormones as being associated with the day to day operation of the reproductive system, in fact they’re involved with far more other things so they play a role with the entire development of the reproductive system & it’s the same hormones, there’s a number of others that get thrown in there, but essentially it’s the same system & it’s actually interesting just how early this system is turned on in embryogenesis. - Well once all the associated reproductive tissues are in place, then as the organism gets a little bit older, then these hormones also play a role with sexual maturation & then obviously the gametogenesis, the development of the sperm & ova (we talked a little bit of that, at least the proliferation aspects of it) & then mating, the sexual behaviours, reproductive behaviours, that are required to attract a mate play a role. - & it doesn’t really matter if whether you’re a mosquito or a human, they’re still a powerful set of hormones that control these behaviours & we as humans are not exempt from these hormones either – they have a huge behavioural effect on us. - Now we’ve talked a little bit already about the various positive & negative feedback loops – it gets to be a little bit complicated in the reproductive tissues, although I’ll try to keep this a little simpler, but I think this summarizes the key elements that we’re going to be talking about. - Negative & positive feedback loops – we’re going to see that in both the male & the female. The interesting thing about the female is it’s a far more complex system & what starts off as a positive feedback loop then turns into a negative feedback loop whereas in males, being a little bit simpler in that respect, is consistently a negative feedback loop, at least as far as the neuroendocrine component goes. - Now one of the things we’re going to think about is the hormone levels will be controlled by synthesis & degradation of the number of the hormones & also the cells as well. We’ll use the reproductive system as really a mechanism to understand how these things all come in together & although the same basic mechanism applies for virtually every other hormone system. - Now the target tissues alter the number of receptors. We’ve already talked about that, but this plays a particularly strong role in reproductive tissues. - One of the things to think about is that males & females may use the same hormones, although they have slightly different effects & I think I mentioned in the last class, both males & females produce androgens & estrogens, it’s just a question of how much they produce. - & although we’re only going to be talking about 5 or 6, maybe 7 different hormones, there are probably about 20 or 30 of various hormones that affect reproduction & there’s a number of other classes too that play a role, but we’re not going to talk about that, at least not in this class. - Okay, we’ve already talked about steroids. - The 2 principal steroids that we’re going to be looking at: the androgens & the estrogens. There’s a number of different androgens & there’s a number of estrogens & they play different roles. - Estradiol, as I mentioned, is going to be the key hormone that we talk about in the females; testosterone is the principal hormone but we cannot lose sight of other ones, such DHT dihydrotestosterone. - Remember these are steroids & **this pathway I mentioned you should know – you don’t have to memorize the structures, just know the generalized structure here.** - Progesterone will be converted to androstenedione, to testosterone & dihydrotestosterone which I just mentioned through one enzyme, a particularly important enzyme, cytochrome P450 aromatase (**you should know this**), this causes conversion of these 2 androgens in a one step conversion to the estrogens, which is a very interesting situation. - In fishes, the key hormone is 11-ketotestosterone instead of testosterone & this is the key pathway here. - **So you should know this basic pathway – as long as you know the names, can spell the names correctly, you’ll be okay** - Now gonadotropins, these are peptide hormones that are produced by the anterior pituitary gland – we’ve already talked about that. - Two principal ones we will focus on: follicle stimulating hormone (FSH) & luteinizing hormone (LH) – it’s the same in both males & females. - & there’s another one that’s produced in the placenta in primates called chorionic gonadotropin which plays the same kind of gonadotropin role during the beginning of pregnancy. - At the apex of this is a hormone called gonadotropin-releasing hormone. This is a peptide hormone, it’s one of the releasing factors of the hypothalamus & this hormone is responsible for the entire regulation of the reproductive system. - If this hormone is not there, there is no reproductive ability. If the concentration of GnRH is too high, ironically it shuts down the reproductive system. If the concentration is too low, it shuts down the reproductive system. So it’s very precisely regulated in this respect & the reason for this is that b/c of the importance & the energetic investment that goes into the development of progeny, you find that the reproductive system is very exquisitely regulated. So if there’s a physiological problem, one of the first things that happens is it gets shut down. - This is the same basic scheme. - We’ve got GnRH in the hypothalamus – it regulates the gonadotropins from the anterior pituitary, causes the release of FSH & LH & they have the effects on the ovary & the testes & the steroids will feedback in a negative or in some cases, positive mechanism back to GnRH to change the regulation of GnRH. **SKIP** **I’ve got a series of slides here b/c we covered this last year, but we’re not going to talk about the invertebrate sex hormones so you can ignore it – ignore the writing, the readings & the slides – will not be on the exam** **SKIP** - Although it’s not regulated by hormones per se, there’s just a couple of different things you should be aware of. - There’s 2 types of sex determination here. The XY, XX – that’s us where the heterogametic sex is male & homogametic sex is female. However there is a secondary system which is referred to as the ZW, ZZ system & you find here that the female is the heterogametic sex & the male is the homogametic sex so don’t get the idea this is the same for everything. - In fact, a number of taxa will have both – for example frogs, some species of frogs use XX, other species of frogs use ZW & through evolution, these change back & forth – an XY, XX system can change to a ZW, ZZ system. There’s an evolutionary reason for this – I’m just going off on a side b/c I think it’s interesting, but I’m not going to test you on it. If you’ve got a heterogametic sex so you’ve got the Y chromosome here, that means that Y chromosome is inherited by every single male member so if you go back in evolution far enough, there’s going to be one male that’s going to have that Y chromosome. It’s not associated with recombination so one of the concerns is over a period of time, the Y chromosome will degrade. If you look at it in history, you find actually these things go back & forth so after a period of time, then you can find the ZW, ZZ in which case you’ve got the female now as the heterogametic sex & the W chromosome is not recombinant & then you have the same problem. Now having said that, the XX, XY system does dominant as far as we know. So that was just a little bit about sex determination – that’s all I’m really going to say about that. - Now with our way of life, we’re associated, we think primarily in terms of sexual reproduction. However, there are a number of species, reasonably advanced species, that don’t utilize sexual reproduction - In some cases, with simpler organisms, we have clones (we’re not going to talk about that) – a little bit of budding here & there. - But what is a little more common to what we will see is parthenogenesis & there’s a number of species that have various forms of parthenogenesis & the fact that they’re that finding more & more, just recently they just found that hammer head sharks & komodo dragons can become parthenogenesis & there’s this theory out there actually that you can get what’s called facultative parthenogenesis – so if an individual, a female, for example, is isolated from her *con specifics*, from her male associates for a long period of time, in some cases, then what can happen is that a pathenogeneic mechanism gets turned on & the individual then has progeny as well. And this is starting to become more & more common & we know that it occurs b/c in some cases, well in all cases that we’ve seen it, it’s happened in zoos, & with the records that they have that can go back years & years & say well geez, this lizard has never been exposed to a male & then of course when they look at the progeny, they find out that they can be clones. - & in parthenogenesis, there is no male gamete that’s required & what happens is you get essentially a cleavage of the female, of the ovum & then that develops into an entirely new individual. **SKIP** - There’s a number of different types of parthenogenesis. - This is for your interest if you’re really interested in this sort of thing. I’m not going to talk about this too much in the course, it’s there for your own interest – I think it’s cool. - It does play a role about the evolution of a species. - I think it’s kind of neat, but we’ll just skip over that. **SKIP** - I’m just going to mention this in passing b/c we cover this to a th great amount of detail in my 4 year course in terms of the endocrinology of it. - With hermaphroditism, it’s the capacity to produce both eggs & sperm. - There’s 2 types: simultaneous hermaphrodites where they can produce eggs & sperm at the same time – now there’s no known examples as far as I know of vertebrates that do this, but there are a number of invertebrate species that are capable of doing this – earthworms for example & mollusks are capable of doing this. If you’ve got a fish tank, you’ll probably note that if you’ve got one snail in your aquarium, within a few months, you’ve got hundreds of snails in there. - What is more common amongst the vertebrates are the serial hermaphrodites – this is very common in fishes & in some cases, some amphibians, but very, very common in fishes & these animals will change sex in response to environmental cues & the environmental cue could simply be gee, there’s not enough males or there isn’t enough females out there so I’m going to change my sex to balance it. But in some cases, they’ve linked it to high estradiol that’s produced by corals very interestingly & some species of corals will produce a large amount of estradiol & then it goes into the area around the reefs & this is where you find a lot of sex changing fishes actually. - So there’s 2 types: protogynous species – this is first female, where they’re born female & then at some point become males. & then the proandrous where they’re born male & then at some point later, they become females under the appropriate cue. - This is quite common in reptiles, there’s a little bit in amphibians & some people have argued even birds have rudimentary abilities to do this & some studies seem to support that. - & what happens is depending upon the incubation temperature, then the embryo develops as either a male or a female. - I think this has been particularly well studied in turtles. - One of the things that it does is if you have a particularly cold year, then you have a predominance of one sex; particularly warm year, you have predominance of another sex. - Interestingly, this concept has been used to explain why the dinosaurs perhaps died out b/c it has been argued that they also used this temperature dependent sex determination & so assuming that the asteroid hit the earth & caused this winter, lowering of the temperatures & it was a skewing towards a males for example & of course males can’t do much in terms of reproduction, they need a female, females can at least become parthenogenetic & so that skewing of the population is one theory as to why they died out afterward. - There’s a number of different reproductive strategies. In light of the sex determination, we can add another layer of complexity to that. - In one case, you can have ovipary & this is the development of an external egg where the ova is expelled from the body & development occurs externally in an egg & fertilization can be external, as in fish – there are a lot of fish that use internal fertilization so that’s okay, that’s what we’re going to use for the sake of this (don’t you just love all of these different rules & regulations & how every one can be broken & biology is just best for changing things), & internal birds & reptiles. There’s a couple of mammals that do this too actually. - Then there’s vivipary – life giving where the young develop within the female body & fertilization is internal, & mammals & well there are a number of species, number of fishes, large number of sharks actually, vivipary in sharks is really well developed. - Another term, it’s losing favour now amongst reproductive biologists, & this is ovovivipary & this is a kind of transition b/w ovipary & vivipary where the young develop within the female, but derive nutrition from the yolk. So think of it as an internal egg & an egg is actually formed, & you get this yolk sac & everything, but the female develops it within inside her. It’s been losing favour b/c it’s been very difficult to say exactly when ovipary ends & vivipary starts & so they use this term ovovivipary to use that entire group in the middle. However, the reality is either the animal develops inside or the animal develops outside & there’s a number of different ways it can develop outside & inside as well so you tend to see this term being used less & less. - And as I’ve already alluded to, the reproductive strategies can vary considerably within taxonomic groups & some sharks are viviparous, some are ovoviviparous & some are completely oviparous as well – sharks are very interesting that way. - Now you’ve had a little bit of discussion in the class already, I think we’ve kind of discussed this a little bit & so we’re going to be looking primarily at the vertebrate follicle – we will not be looking at the invertebrate follicle. - Here’s the general structure here. We’ve got the ovum here & a series of follicle cells all around it & this is actually theca lining, that’s a series of cells around it as well, theca cells & then there’s a clear matrix around it, sort of colloidal suspension called zona pellucida. - So what’s going to happen as you go through reproduction, as you go through reproductive development is that that ovum will start to become quite elaborate. So we have the ovum here & we can think of all of these cells that are surrounding it as nurse cells & they do a number of different things – this is going to be our primary steroid synthesizing component of the ovary. The theca cells will actually produce androgens & they then will put their androgens, they put them out & they get picked up by these follicle cells, these granulosa cells & then those androgens will get converted to estrogens & then that gets released & goes into the bloodstream. - And what will happen during the course of the maturation is that these granulosa & theca cell layers will expand quite considerably & as they expand, they’ll produce a large amount of estradiol & then later progesterone which will then act to regulate the GnRH & the rest of neuroendocrine cycle. - But the other thing that they’re doing as they proliferate, they are producing all kinds of various growth factors & a number of nutrients which will get shunted to the egg. You can’t see it in this picture because well, it’s just a drawing but there are all kinds of cytoplasmic bridges that occur – it’s really neat when you look at this under a high powered microscope & you can see places where material actually gets transferred. - One of the students in my lab made a really interesting discovery. We were working with a hormone that we thought played a role with stress & it was primarily stress & then he started looking in the reproductive tissue & found that it was really highly expressed in the granulosa cells, but he found that it was really highly expressed also in the oocyte so we couldn’t figure that out & then we were looking at high resolution images & we could see all of these cytoplasmic bridges, then when we looked to find out where the hormone bound, we found that it bound specifically in the oocyte – it was being produced here in the granulosa cells, the follicle cells, & then it was getting moved from here into the oocyte where it was bound & we think what it is it’s a growth factor, a kind of repair factor, that helps stabilize & protects the cells, but really exciting finding when we were just trying to finish that study. - Vertebrate oocytes grow by accepting material from the follicle cells through those cytoplasmic bridges that I was telling you about. Now once they grow to particular sizes, the follicle will rupture & the ovum escapes the ovary & moves into the coelom. - If you look at the structure of the oviduct, you find it that as organisms became more complex, that oviduct started to surround the ovary to increase the probability that those ruptured eggs, those eggs ready to be fertilized, go into the oviduct so this increases the efficiency. - Now there have been a couple of cases where through variations in the structure that ovum has actually instead of going into the oviduct, it’s gone into the *celic* cavity of the individual & this has been shown in humans – this is the so called ectopic birth & what happens is instead of implanting in the uterus, the fertilized cell, then forms a placenta with the mesentery & these babies have been born, obviously by Cesarean section, & they’re healthy, they’re a little smaller than normal, but they’re healthy – the interaction with the placenta & the mesentery is sufficient to cause a normal development, which is very interesting. - It’s called vitellogenesis b/c it’s named after the proteins & lipids so the vitellogenins & it’s a variety of proteins & lipids & they get produced & this is a sign of the developing reproductive system, developing ovarian system. - Interestingly, the formation of the vitellogenins, which is distinctly female, can occur in some male fishes – there is a number of these various types of industrial waste that have estrogenic-like material & when they’ve looked at male fish, they found that they were partially sex reversed & they could measure the vitellogenins being produced & there’s a number of these things that are being produced & it’s a very sensitive measure for that. There’s a river in Northern England when they sampled the fish up there, 100% (that’s pretty scary when you think about it) of the males showed some signs of sex reversal b/c of these industrial wastes. - If we just take a look at what’s happening here. **Don’t worry about the invertebrates here – we’ll just focus on the vertebrates.** - There’s going to be some sort of external cue – now the external cues can be the formation of another mate, it can play a role in temperature, light, in the case of the goldfish, all you have to do is throw something green in the tank & they get excited & so it’s a series of external cues & it’s usually many external cues that are all integrated as well & we talked a little bit about the integration mechanisms in an earlier class. So we get this central signal, this integration occurs & then we release of GnRH & then from the pituitary, we get the release of FSH which acts on the gonads, in this case, it’s going to be the ovary, & then this causes the release of the estrogens that acts on the liver to produce vitellogenins & then it’s transported through the bloodstream back to the ovary & then you get final processing here. So you can measure this directly in the bloodstream, which I had just mentioned in the case of fishes. **SKIP** - Let’s forget about insects. - Obviously the structure is going to be different in aquatic & terrestrial animals. - In aquatic animals, the egg clearly needs to be utilized to a lot of the nutrients that are already in the water. Water is not in short supply, however, for the first animals that came & evolved out of fishes & crawled up onto the beach, many, many million years ago, they had to produce some way of producing those eggs so the animal could still live – water was now a scarcity & so there is a very different type of structure obviously for the terrestrial animals so there is no dehydration. So what we see is the eggs typically in reptiles & birds have hardened shells & some primitive mammals, for example, the akidna & platypus also lay eggs, but it’s not a calcium carbonate based egg – it’s more of a proteinaceous based egg, they have sort of a leathery type look to them. - The metatherian, that’s marsupials: kangaroos, koalas, opossums so these are pouched animals & then the prototherians, the ones I just mentioned previously, the akidna & the platypus, they do have an egg. Amnion: a membrane building the amniotic sac that surrounds and protects an embryo. - There are some fundamental differences b/w males & females
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