Online Developmental Biology: Introduction to C. elegans

Oh this video provides a brief introduction to a very popular model organism in developmental biology the round worm Caenorhabditis elegans or C elegans for short so let me start out with a little bit of background information C elegans are microscopic round worms this one in the video clip appears a little bit flat but they're actually round and you can see that in this really beautiful scanning electron micrograph to give you an idea of scale here an adult animal is about a millimeter in length but there are only about 50 microns or one twentieth of a millimeter wide so depending on how good your eyesight is that's just barely visible to the naked eye C elegans are part of the phylum nematode ax the nematodes are more commonly known as the round worms and this is a group of well over 20,000 different species found all over the world in turn the nematodes are grouped with several other phyla as part of the FDIC's OA the largest group within the animal kingdom so the aect Isis Owens are characterized by having an external protective covering called a cuticle that they periodically shed and this includes not just the nematodes but also the arthropods such as the other most widely studied invertebrate model organism Drosophila now there are parasitic nematode species but C elegans are free-living or non parasitic in the wild they live in the soil where they feed on bacteria that grow on rotting vegetable matter so if you do any gardening for example and you have a compost heap it's very likely that you have lots of C elegans and or related nematode species crawling around in there so that's where they live in the wild but you can also grow them very easily in the lab I'll go into more detail on that in just a few minutes and under those conditions they make a really great model organism for studying various aspects of both embryonic and post embryonic development so we'll go into some detail on that in just a bit but before I get to that I want to tell you a really interesting story about how C elegans was first established as a model organism this is in my view one of the most remarkable biologists in the history of science I'm his name is Sydney Brenner he was born in 1927 in South Africa and what makes him so remarkable is that he's made multiple really fundamentally important contributions to biology over the course of his career across multiple different disciplines ranging from molecular biology to developmental biology to genomics and I want to take just a minute here to highlight a few of his key contributions to science so first of all Brenner was one of the researchers who demonstrated the existence of messenger RNA so if you remember the central dogma of molecular biology that says that DNA the hereditary material is transcribed to make messenger RNA or mRNA for short and then mRNA in turn is translated by the ribosome to make protein so this is how genes are expressed right this is how you get from the hereditary material in the form of DNA to biological form and function the key structural and functional components of cells in the form of proteins now this is pretty basic biology you'll find this of course in just about any high school biology textbook that you look at so you might kind of take this for granted but you have to remember that it took a lot of really painstaking and and very clever experimentation to work all this out so if you go back to the late 1950s at that time we knew based on the work of Watson and Crick with the help of Maurice Wilkins and rosalind Franklin we knew the structure of DNA we also knew based on previous experiments that DNA is the hereditary material so we had a pretty good understanding of DNA at that time we also knew based on the work of classic biochemists that proteins as I said are key structural and functional components of cells so we had a pretty good understanding of the two end points here but what wasn't clear is how you get from here to here so how is the information that's stored in the in the DNA a blueprint how does that get read and converted into a biological output in the form of proteins and so that's what Brenner helped figure out he helped to show that messenger RNA is the key intermediate here that transmits this information so together with his colleagues they published this work in this really landmark paper and molecular biology this is a classic in the field that they published in 1961 ok so this was a really critical breakthrough but even more impressively Brenner then went on working with Francis Crick and others to start cracking the genetic code so at this point on they knew that messenger RNA was the key intermediate here but then the critical question became how is the ribosome decoding the nucleotide sequence within the messenger RNA to generate the appropriate corresponding amino acid sequence within the polypeptide chain that's coded for by that particular mRNA so what Brenner and Crick and their colleagues demonstrated is that the code is a non-overlapping triplet code so let's say that we have an mRNA sequence of Aug CGA and so on well this will be read three non-overlapping nucleotides at a time so Aug codes for methionine that's a start codon then CGA codes for arginine so we'll get methane in position number one arginine in position number two and so on so that's what what Brenner and Crick and their colleagues demonstrated is that the code is a non-overlapping triplet code and in fact Brenner is the one who came up with a term codon which is defined as three consecutive nucleotides that are coding for a given amino acid so this was really really impressive work foundational work in molecular biology the logic that they used to figure all this out was just incredibly elegant and I think most biologists would agree that this paper is one of the all time classics and biology it's a really really beautiful and well written paper and a great read now Brenda can certainly have chosen to hang up his lab coat at this point and call it a career and I think by anyone's point of view I would have been considered a highly successful career at that but he wasn't done yet he decided that he was going to move on to another project so at this point he realized that there'd be more and more people getting involved in research on the central dogma and that there would be lots of people to figure out all the details of how DNA replication and transcription and translation work at a molecular level and so the problem that he chose to work on was the development of the nervous system so how do neurons form during development and how do they all get wired together in the right way to create a functional nervous system now he chose to take a model organism approach and so that man his first order of business was deciding what model organism he was going to work with so he gave that a lot of thought and after careful consideration he decided on C elegans now he chose to work with C elegans for this project for two major reasons so first of all C elegans has a very simple nervous system the adult animals have exactly 302 neurons furthermore those neurons are wired together in exactly the same way from one organism to the next and you can see that in this really nice animation of the nervous system what you're looking at here is a map showing the positions of all 302 neurons as well as all the connections between them the wiring diagram for the C elegans nervous system was generated through the work of a number of individuals including Brenner himself and Brenner was able to map the position of numerous neurons through electron microscopy okay so this was key advantage number one small and simple nervous system compared what I just told you about the C elegans nervous system - for example the nervous system of a mouse so a mouse contains about a hundred million neurons and there's a lot of variability from animal in terms of how all those neurons are connected so you can see how if you want to understand how a nervous system is put together during development it's going to be a lot easier to work with an animal like C elegans where there's a lot less complexity in terms of the size and structure of the nervous system okay so that's key advantage number one the second key advantage to working with C elegans for this project is that Brenner was able to show early on that C elegans is highly amenable to genetic analysis so remember we've talked about how genetics is a key tool that developmental biologists use to study the mechanisms of development and so Brenner wanted to exploit that approach for this project and he was able to show that he could induce a variety of mutations with various different mutant phenotypes including abnormal movement so for example here's a comparison of a wild-type animal on the left and an animal with a mutation in a gene that's required for neuronal function on the right and you can see there's an obvious difference in movement between the two so by identifying and characterizing these types of mutants Brenner was able to start teasing apart how the nervous system developed and function and he published that work and yet another landmark paper this article really laid the entire foundation for the whole C elegans field by showing that C elegans is highly amenable to genetic analysis alright so Brenner initially decides to study C elegans because he's interested in the nervous system but it quickly becomes apparent that it's just a really great all-around model organism for studying essentially all aspects of development and so ultimately in recognition of the importance of Brenner's work in establishing the entire C elegans field he's awarded the 2002 Nobel Prize in Physiology or medicine along with two other early C elegans researchers will tell you about in upcoming lectures now Brenner went on to have another important chapter in his career he established another model organism namely the pufferfish fugu as a model for research in genomics but I'm not going to say anything more about that here in the interest of time okay so I have a lot of admiration for Sydney Brenner given his really long list of impressive scientific accomplishments but I also think he's just a fascinating and really charismatic individual as well so I had the chance to go to dinner with Sydney Brenner when I was in graduate school I did my PhD research and Geraldine say news lab at Johns Hopkins University and thus a new lab studies embryonic development in C elegans so my second year in graduate school dr. Brenner came to Johns Hopkins to give a guest lecture and that was already a pretty exciting occasion for me to get to have a chance to hear a talk by a renowned scientist like Brenner who was of course a pioneer in the field that I was working in so I was already pretty excited but what was even more fun is that because I was working on C elegans at the time I was selected as one of a handful of graduate students that got to go to dinner with him after his seminar so we all went out to a nice restaurant and dr. Brenner Ward a few bottles of really good wine and so we sat around drinking wine and listening to him tell us some just amazing stories about his career and not just about his research he's got a great sense of humor and so he had us all practically falling out of our seats laughing with his stories about the practical jokes that he used to play on Watson and Crick and and that sort of stuff so it was a really memorable meal one that I won't soon forget and a pretty special experience for me as a young scientist anyway I thought you might enjoy hearing a little bit about Sydney Brenner and his work as part of this lecture and incidentally if you'd like to know more about him I highly recommend his autobiography my life in science in the last part of this lecture I want to tell you a little bit more about the development of C elegans with an emphasis is on some of the traits that have made it such a popular model organism in the field of developmental biology all right so first of all it has a very simple anatomy and you already got some sense for this when we talked about the anatomy of the nervous system but this really extends to the entire adult anatomy in general so most C elegans adults including a1 picture - here are were called hermaphrodites and hermaphrodites make both sperm and eggs so they make both sperm and eggs and they can self fertilize they can reproduce without the presence of a mate and the adults the elegance hermaphrodite has a total of exactly 959 somatic cells so the somatic cells are the non germline the non reproductive cells and each adult hermaphrodite has a total of exactly 959 somatic cells so just like with the nervous system that's a pretty small number of overall cells in a pretty simple Anatomy which makes it easier to track where and when and how cells form during the course of development now additionally the adult and the embryo are both transparent and you've got some sense of that in this video clip that we looked at before so this is an adult from Aphrodite here and note that you can see right through the skin and body wall and visualize the developing embryos and other structures so this is true for the embryo as well it's also transparent and I'll show you that in just a second and what that means is that you can watch cellular processes as they occur in vivo or in the living organism during the course of development so that's a pretty useful trait now C elegans also has what's known as an invariant cell lineage I'm going to do a separate lecture on the concept of cell lineage so I'm not going to go into detail on this right now but suffice it to say that what this means is all of the 959 somatic cells are formed in exactly the same way from one organism to the next in other words are the specific cell types that make up the adult Anatomy those are being formed at the same time in the same place in a very reproducible fashion from one individual to another now together with the simple anatomy the small number of overall cells that make up the adult anatomy and the transparency of the adults and the embryos that makes it really or I should say relatively easy to follow cellular processes in development as they unfold okay C elegans also has a short life cycle with a large brood size that means that they generate a lot of progeny so here's a summary of the life cycle as I just mentioned most adults are self-fertilizing hermaphrodites there's also males those are rare in the wild but they can be maintained by crossing in the lab so the hermaphrodites can cross fertilize like a female as well as self-fertilizing so each mafourdi generates around 300 progeny over the course of about 3 to 4 days now the embryos begin development in utero and then they're expelled into the environment through an opening in the body wall so they complete development externally the entire process of embryonic development takes about 12 hours so that takes us from fertilization all the way up to when the embryos hatch and when they hatch their small larvae called l1 larvae now these larvae grow and then they undergo a mole where they shed their cuticle so remember the cuticles the outer protective covering that I mentioned earlier that's observed in the aect Isis OA so this process is repeated three more times and we go we go on to l2 and then l3 and l4 larval stages so altogether the larval stages take about two days and then we get back to sexually mature adult hermaphrodite now under adverse conditions for example lack of food the animals can enter an alternative stage called the Dowler stage and they can survive for really long periods under harsh conditions for example again lack of food in that kind of state and then they can re-enter the normal life cycle once they re encounter favorable conditions ok so short life cycle of about three days and a large brood size of about 300 progeny perm aphrodite so these are both significant advantages in terms of a short life cycle the obvious benefit there is that you don't have to wait very long to be able to observe whatever process it is that you're studying during the course of development so that's important in terms of a large brood size that's beneficial for multiple reasons for starters it's really useful for conducting genetic analyses now you can mutagenize and screen really large numbers of animals so that's helpful and then additionally it's just nice to have a lot of animals to work with for your experiments you can just have a really large sample size so that's always helpful another important consideration here when working with C elegans is that you have a ready supply of easily accessible embryos so I told you just a second ago that the embryos begin development in utero but it's pretty easy to dissect the embryos out of the adult arm Aphrodite and when you do that you can place them on a microscope slide and put a cover slip over them and then you can watch embryonic development occurring under the microscope and you can also take pictures at set intervals under the microscope with a digital camera and then if you compress all those pictures into a short time frame you can create what's called a time-lapse movie so here's a movie where we're watching all of embryonic development all the way from the one cell stage up to the l1 larvae stage and remember that's about 12 hours in time but it's compressed into around 20 seconds here so we'll get into the details of all the biology in the next unit but you can already see how this makes the elegans a really nice model organism for studying embryonic development now remember C elegans in the wild lives in the soil where it feeds on bacteria that grows on rotting vegetable matter but it's also really easy and inexpensive to maintain very large numbers of C elegans in the lab and this is another important advantage of this model organism so this relates partly to their small size and rapid life cycle and large brood sizes which we've already talked about but also to the culture conditions that we use to maintain them in the lab so you can take a little auger plate like this just like you'd use in microbiology and you can spread the surface with some ecoli bacteria which C elegans will very happily eat in the lab and then if you let that grow for a couple of days you'll have an auger plate with a thin film of e.coli growing on the surface and then you can use a tiny metal wire call to pick to transfer some hermaphrodites to the dish they'll swim around the e.coli eating it and generating lots and lots of progeny through self fertilization so after a few days you'll have a plate full of hermaphrodites so this is what they look like when viewed under a dissecting microscope as they swim around on the surface of the auger plate in the e.coli you can see various larval stages here along with some adult hermaphrodites and some embryos as well now once the animals have eaten all the e.coli you just transfer some of them to a new plate and of course if you start lots of new plates you can expand your lab population very very quickly the final thing I want to emphasize in terms of advantages of working with C elegans as a model organism is that there are some really great experimental tools that have been established for studying C elegans development in the lab now I'm going to tell you a lot more about the details of these techniques a little later in this unit but for now I just want to point out quickly a few of the key tools that we use to study C elegans so first of all I already mentioned that C elegans is very amenable to genetic screens so screens like the one that Brenner did are really great ways for studying mechanisms of development so that's one important tool additionally C elegans has a sequenced genome and in fact it was the first multicellular organism to have its genome sequence that happened back in 1998 and the lessons that were learned from the C elegans genome project were really beneficial for the completion of the Human Genome Project so sequenced genome additionally not only can you conduct classic what are called forward genetic screens but you can also take what's called a reverse genetic approach and with reverse genetics you start out with a known gene and then if you wanted to determine the phenotype of that gene you essentially turn it off and you can do that in C elegans using an approach called RNA interference or RNAi AI for short and this is another Nobel prize-winning discovery it's a really important phenomenon and tool and so I'll tell you a lot more about that later in this unit okay lastly transgenics so you can express foreign genes or transgenes pretty easily in c elegans for example you could express a gene encoding a fluorescently labeled protein and then that allows you to track the distribution of that protein in vivo in the living organism so this embryo is expressing a fluorescent version of a protein called a jm1 it functions to hold cells together as the embryo elongate and so with a fluorescence microscope you can follow changes in the expression or distribution of labeled proteins like this in these really cool time-lapse movies now this adult term Aphrodite is expressing two different fluorescent proteins one of them is tagged with green fluorescent protein and one is tagged with red fluorescent protein so that means you can simultaneously follow the expression of both tagged proteins in the same animal so anyway these types of transgenic embryos and animals are really really great tools for studying C elegans development alright a conclusion I think Sydney Brenner made a really abstaining choice when he established C elegans as a model organism it's a really great experimental subject for studying developmental biology and in fact as we look ahead to future units I'll frequently be using C elegans as an example to illustrate the different developmental processes that we're going to be covering you

Channel: Jason Pellettieri

Views: 103,559

Keywords: Caenorhabditis Elegans (Organism Classification), Nematode (Organism Classification), Roundworm, Model Organism, Developmental Biology (Field Of Study), Embryology (Field Of Study), C. elegans, Biology (Media Genre)

Id: zc1P7lGSzdU

Length: 26min 5sec (1565 seconds)

Published: Fri Feb 21 2014