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Lecture 22

Lecture #22

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University of Toronto St. George
Jennifer Harris

Thursday, April 2, 2009 - Last day we talked about multicellular development, talking about how cells can change their architecture to create specific tissues, to create tissue structure and that is morphogenesis. - To day we wi l ltalk ab o ut cel l dif ferentiation which is important for telling cells what to do. How do those cells know to undergo those morphogenetic mechanisms, how does a cell know to turn into a liver cell or a heart cell? There have to be mechanisms to distinguishing one cell from another. - This is what we re talkin g ab o ut here, dist in ishin g one cel l from another. Then once theyre distinguished, they can then have different properties in terms of cell proliferation, cell-cell interaction, cell movement or cell shape changes to create tissue form. - Cel l dif efrentit i n oc urs in 2 main ways. There are intrinsic mechanisms and there are extrinsic mechanisms. - This is very important to understand, so the asymmetric division is one way you can have cell differentiation through an intrinsic mechanism. Here you start off with one cell, now there is some asymmetry within this cell that when the cell divides, one daughter will inherit certain cytoplasmic material & the other daughter inherits another type of cytoplasmic material. The proteins inherited by the bottom daughter will give the cell different properties than the other daughter cell above it. This is intrinsic, it is happening just from the redistribution of proteins within the cell. - The other me chanism is extrinsic, an d so this can occur during the development of the embryo, these cell will still be dividing so you still need to grow all the cells making up the body but in this case they would divide symmetrically, they would be the same based on division. One of the daughter cells would then be influenced by outside signals that the other one isnt influenced by and the outside signals then change this cell and convert it to a different cell from the other daughter cell. - Cells can differentiate from either internal or external signals. - An example of intrinsic cell differentiation comes from asymmetric cell division in C. elegans, the nematode worm, shown at the bottom of the slide. This has become a very valuable model system for developmental biology. - One interest in g strength that it has is that every single cell division starting from the fertilized egg has been mapped until you form this final worm. People know exactly starting from that egg, what those 2 cells turn into then following that division see what those cells turn into all the way down the line until you form this full worm. You create a lineage of cells that you can follow until you form the different parts of the worm, the gut, the vasculature, the nervous system and so on. - You can see at the top, the fertilized egg, its first division forms two lineages on the left and right, these lineages are very different from one another so this one on the left is making the nervous system and the other linage is pretty much making everything else, musculature and the gut. - So is this differentiation of these 2 cell lineages, is this from an extrinsic mechanism or an intrinsic mechanism? What people found when people looked at these eggs was they found proteins on one side of the egg & not on the other side implicating this was an intrinsic mechanism not extrinsic, an asymmetric cell division & it is described further in the next slide. Partitions cell fate determinants Polarity Spindle alignment - So in these pictures right here, we can see the individual fertilized egg, this one nucleus in the middle so its a single cell. We are looking at the division that is about to occur here. In this picture were looking at the same cell but we are looking at a fluorescently marked protein in the cell and this fluorescently marked protein is specifically on this half of the single cell so it is specifically partitioned to one side of the cell. So this and other cell determinants, a set of cell-fate determinants will be on one side of the cell and a whole other set of cell-fate determinants proteins that affect cell fate will be found on the other side. - We talked ab o u t this a bi t durin g discus sio n of cel l divisio n, if the spindle then aligns horizontally to split the DNA, then cytokinesis splits the cell vertically, now these cell fate determinants will be specifically partitioned into the right daughter cell & not the other daughter cell. So this asymmetric cell division partitions these cell fate determinants and this defines the germ line in other tissues. - This req u ires 2 ty es of asym m etry, one major asym m etry is this cortical polarity, the cortex is just under the PM so you have this polarity there that needs to be asymmetric and then the cell needs to properly align the mitotic spindle in the direction of that asymmetry so that only one of the daughters will gain these proteins and the other one wont. - This is illustrated up here as well, here is the fertilized egg, when this divides, this cell will be different from this cell and you can follow these divisions going down throughthe first stages of embryogenesis. You can see some, then theres another division plane marked by the black line, this is also an asymmetric division, another with the purple and orange, etc. There is one that is symmetrical as well, both daughters are the same (the green ones) & they continue to be the same throughout many further divisions. You can see asymmetric cell division can distinguish some of the cell types in the embryo but there is also these symmetric cell divisions going on. These cell types have to be distinguished by external mechanisms so these extrinsic signals which will then make this cell different from the other. - Now he willdiscuss a few examples of this. For the rest of the lecture well be talking about external signals determining cell fate and differentiation. What were looking at here is the creation of patterns in the embryo, so we have extrinsic signals signalling between cells creating patterns across the embryo and then that pattern of cell differentiation then leads to patterns of morphogenesis. You have an arm & a head forming in a proper place & so on in an embryo. Isolated differentiated cells - The first example he wants to talk about is direct lateral inhibition. The pattern that this signalling will create is a pattern of isolated differentiated cells. - The final pattern is shown in the right slide where each of these green hexagons are cells and you can see the dark green one has differentiated, it is different from the surrounding ones, it is separated from the other differentiating cells. - So this is a very nice way to say separate neurons throughou t the body so that theyre not just clustered in one area in the tissue & actually spread out throughout the tissue. That is one way that this type of patterning is used. - Now the way this arises is first of all, the cells all begin equal, the light green on the left means none are differentiated, none of the cells are differentiated & these little red Ts here, this means that all of these cells are inhibiting one another. They are all inhibiting one another from differentiating so if there is no differentiation & all the cells are inhibiting one another, stopping each other from differentiating. - But then just randomly, some of these cells will gain an advantage, they will start to differentiate & gain the ability to inhibit their neighbours more strongly. They wont be affected by inhibition from surrounding cells very much because these ones become weakened, the darker green becomes stronger & inhibits the surrounidng cells more strong ly. This inhibition eventually turns off all of the signals for inhibition in the surrounding ones & then youre left with this one cell in the middle that is now differentiated & fully inhibiting neighbouring cells from differentiating. This is where you can have this pattern of separated individual differentia
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