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okay so those are the three modes of specification again conditional if you move cells from one spot to another and they will respond differently to a different area based upon the signals the morphogen gradients and the cell the cell signaling that goes on in that particular area now there comes a point where the cells become determined remember when they're determined when you move them to another place they no longer respond to various signals that are already established in their fate so in that regard conditional specification creates these patterns within the embryo but only to a certain point and then the cells are are gonna for the most part become specified autonomously because they're already specified in their fate or determine I should say all right so let's talk about induction and competence because this is a important part of conditional specification okay so mostly what we're going to talk about right now is conditional specification and some of the factors behind it since most of what we study is is for the benefit of humankind and therefore we want to look at the method of our development so we can understand stem-cell therapy and other things a little bit better for us so induction and competence let's define these words first and then give you some examples of how this works when something is induced it ultimately produces a signal or the induction process is kind of the morphogen gradients ultimately these cells produce a signal a protein that can change the behavior of adjacent cells so the inducer are the cells that are producing the protein the morphogen so induction is just the process of one cell telling another cell what to do based upon its secretion of its proteins or I will show sometimes just it doesn't actually secrete the proteins it just kind of has some membrane proteins that will touch the other cell and still tell it what to do now just because one set of cells are sending out a signal does not mean that the other cells have the cap to respond to that that capacity is what we call competence so competence is the ability for the cells to be able to respond to the inducting signal hey early on most cells are competent but as the cells become determined as they become determined in what fate they're going to become their competency goes to almost zero meaning they don't respond to any of the adjacent cells they're already determined in what fate they're going to become so really induction and competence are in the earliest stages of specification when one cells giving a signal and the other cell can respond to it now there's always an inducer and a responding tissue so cell induction requires two two types of tissues one that is secreting the protein and the other that is typically responding there are situations where it's it's own inducer meaning some cells can release something and that just Auto regulates its own cells but most of the time it's one tissue or one group of cells that are secreting a protein that causes another group of proteins to respond now it matters what type of gradient is being sent or type of morphogen for example here we have this is these are some samples or examples that you'll see in your book here we have just basic ectoderm or the epidermal tissue that will become the ectoderm r2 derm components of in this case looking probably at that bird over here we have different types of inducing tissues this tissue comes from the mesoderm where the wing forms this tissue comes from the mesoderm where the leg forms and this tissue comes from the meezerman where the foot forms so this is the same tissue that is receiving the signal but each one of these miso terms are expressing a different morphogen gradient so what happens is due to the fact that each gradient is different is maybe a different protein the same tissue will respond differently based upon what the inducing tissue is giving for example if you put we visa Durham under this it will form into wing feathers if you put thymio durham under here it'll form into thighs which are different and then of course but Phillies are derm under here it'll form into the claw in this regard so even though this tissue it is fully competent to become any component of the ectoderm of the organism it matters what the inducing tissue is okay so this is the factors the reason why each one of these is different because they're expressing some different myriad of proteins or combinations of proteins that do that now in some cases you have what's called reciprocal induction where one induces one and then the other one in turn induces the other so you have this crosstalk between the tissues it's not always just ones the inducer and once a responder sometimes this tells us what to do and then in turn this one comes around and tell us the other one okay now you do this okay and that kind of falls into line with a concept called sequential inductive events which just means tissue as it develops sometimes goes through this stepwise process that does matter in terms of its overall development meaning you can't skip steps you have to first give it this protein and it does something and then you give it another protein and it does something and so on and so forth it can't respond to the second protein until the first ones that its job so sequential induction events that's really how most things developing you and I it's never just one type of induction it's this and then it's this and then it's this and that ultimately what caused these tissues to go down their particular fate is multiple inductive events that develop the system overall it's a very complex process now here's an interesting thing a lot of the morphogen gradients that we're going to talk about today as well as their subsequent signal transduction pathways are almost universal in some organisms meaning we and other vertebrates and even some invertebrates use some of the same proteins to pattern the brain or to pattern the the ectoderm or whatnot the difference is the genetic capacity of those tissues to respond to that now what am I talking about well here's an example where they did a transplantation experiment they took a newt and a frog which are similar or closely related to one another and their genetics and they took the tissue from the Frog and put it where the newt set or they did the the opposite they took it from the newt and put it in this area where the Frog is that well what ended up happening was the frog tissue tells this region is supposed to become well in this case it's supposed to become the suckers that are on a frog tadpole well in this regard the the newt responds in kind to whatever the Frog is inducing well over here the Frog has these newt balancers because that's what the tissue knows to be able to respond to so even though they both have the same inductive tissue there's only a limited capacity on what the the tissue is supposed to be able to do for example here in the newt normally under this signal the tissue would respond and become balancers but it becomes only what it knows what it can become and in this case is the Frog suckers from that tissue so the longer the short of this experiment is showing you that you may have the same inductive tissue say that this Mesa derm tells the ectoderm to become a feather but if we don't make feathers we're not going to make feathers we're gonna make skin you know under those same conditions there's a limited capacity to what this it's not going to say hey you're supposed to become feathers so make feathers there's no genetic capacity in those cells to do that but they will respond in whatever capacity they know how to respond so we can't really generate feathers in our skin because we don't have the genetics to do so but we would respond to the same signal and instead of producing a feather we would produce hair or something of that nature okay that's kind of the idea behind here is that the inductive signals are the same between species but the tissue has a limited capacity depending upon the species on how its going to respond all right it'll respond as it knows how to in its genetics okay last types of interactions before we move on instructive interaction is ultimately this is where a lot of the conditional specification comes into play was that you have to have the cells are periodically telling these other cells what to do okay they they can't respond autonomously they don't develop autonomously in order for the genes to be turned on it has to have received this signal from the inducing tissue so instructive interaction is one of those things where the morphogen gradient must be present in order for the cells to be able to respond however there comes a point where the tissue starts going down this process of specification and it has everything it needs it's almost autonomous it just needs the right environment to finish off so permissive interaction is one of those things where the tissue has already gone through those stages of induction and it's almost ready to be finished off it doesn't need any more instruction from any other tissues but you still have to keep it in that environment because there are other factors maybe there's a extracellular matrix that the cells need to be able to maintain that maybe they need to be in contact with other cells they don't necessarily need an inducer they just need the appropriate environment which typically means that they're touching other cells and they've got they have the same conditions that they need in order for that so it's kind of splitting hairs but these are kind of different levels of specification instructive is the earliest stages permissive is the late stages of specification where the cells are almost committed almost determined to be what they are and they just need to be maintained in that environment and then ready to go this is just to give you some basic concepts of the complexity of of induction of conditional specification we're cells you can see it's not just a throw some morphogen gradients there's a lot of things as you induce me I induce you I can respond this way you go through the sequential that's really all I'm after here is that you understand some of the complexity here so let's go into some of the things regarding cell signal let's get into the actual morphogen gradients the signaling and what role they play in development okay and you're gonna see these signal transduction pathways throughout the entire semester you're gonna see sonic hedgehog we talked about competence okay remember competence is actually the the more competent a cell is typically the more pluripotent it is meaning it has the ability to bind to and respond to the various tissues now if you look at the human body and you look at all of our tissues each tissue has a very limited level of competence what am I talking about well we release hormones all the time but the hormones will only affect those tissues that are actually expressing the receptor for that hormone so if we have a growth hormone only the cell with a lot of the cells in our body respond to growth hormone but only the cells that have that particular receptor will respond in that regard so you can just this is one of the things about development is you can send all these proteins out into the ether so to speak or into the area and only those cells that have the receptor to respond to that will respond and that's where competence comes that comes into play and that's built up over time over the earliest stages of induction that start telling them to express certain receptors let me show you how this works so let's say this is the inducer right here it throws out this triangle-shaped protein and notice this cell doesn't have a receptor that will bind that properly so guess what it doesn't respond this is what we would call non competent so non competence sells ourselves that cannot respond to a morphogen gradient to some particular factor whereas competent cells our cells that can respond so competency is relative if we say a cell is competent you have to assay what is it competent - okay so competency is relative to the actual inducing signal this cell right here is the inducer these are the ones that respond to it the responder okay so here what happens and this we're going to go over for the rest of today's class is when the protein targets a particular receptor on the surface of the of the membrane then it starts this cascade of events in the cytoplasm that typically turned transcription on that's the ultimate goal here is that it causes new genes to be transcribed and translated and that in turn causes this cell to start acting differently creates new receptors that then might respond to a different signal you know so that's really what this is all about is that in the earliest stages the cells will start turning on various receptors and certain cells are competent to certain inductive signals others are not competent to other inductive signals here's an example of how this works I formation very complex process but typically there is a tissue that is necessary to cause the lens to start forming the induction of the lens by the optic vesicle now if you take that tissue and Transplant it to say this lower region of the embryo where the trunk is at due to the history of this tissue it doesn't have the receptors for that morphogen gradient over here if you remove the actual inducer notice this tissue right here it won't actually form a lens there's no signal being produced and so the responder can't do anything again conditional specification this tissue must be present for this to become the lens otherwise there's no signal for it to be induced to have that if you take some other tissue other than the optic vesicle and put it in there it shows that because it doesn't have the same morphogen gradient it still won't induce it to become lens so they did this because they wanted to see well what if there's just tissue there let's just replace it with a different tissue and the mirror nature of having tissue there doesn't cause that to become the lens it has to have a very specific protein or proteins that are being secreted by this tissue for that to develop into the lens of the eye we've talked a lot about transcription factors and that's what we're going to focus on here is