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[Music]
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hi my name is Jason peler and I want to
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welcome you to online developmental
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biology this is the first in a series of
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video lectures on what I think is one of
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the most exciting and important fields
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in all of science now I know that's a
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strong statement and uh admittedly I'm a
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little bit biased because I've been
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researching and recently teaching
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developmental biology for well over 10
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years now so it's definitely a subject
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that's very important to me personally
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but I do feel pretty confident that as
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you watch these videos uh you're going
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to agree with me that this is just a
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fascinating subject it's really really
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amazing to learn about how what starts
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out is just a single cell undergoes a
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whole series of changes to form an
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increasingly complex embryo which will
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then go on to uh develop various tissues
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and organs
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and ultimately give rise to a complete
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new individual with all the incredible
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anatomical features that we find in
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multicellular organisms so I think it's
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a really exciting field it's also a very
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important field and one that has a great
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deal of relevance to biomedical research
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especially in areas like stem cell
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biology where recent breakthroughs are
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beginning to change the way that we
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study and even treat a variety of
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different diseases so in that respect I
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think the field has already had and will
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almost undoubtedly continue to have a
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very big impact on all of us so to sum
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up I think it's a really fun subject
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it's a very important subject and I hope
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that you're going to enjoy learning
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about it as much as I enjoy teaching
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it okay so what I want to do in this
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initial lecture is first to spend a few
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minutes discussing the history of this
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field
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and then additionally I also want to
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introduce some of the central Concepts
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in modern day developmental biology and
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that will also serve as an overview of
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the major topics that I want to focus on
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in this course so in terms of the
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history of the field uh I thought I'd
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start out with an idea that today seems
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really really strange but it's one that
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was actually fairly popular during the
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16 and
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1700s and this idea was the concept of
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the
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homunculus so this term homunculus
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refers to a belief that was shared by a
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number of early developmental biologists
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who thought that the reproductive cells
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and I should add here uh there was some
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debate about whether this was true for
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the sperm or the Egg but the general
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idea was that the reproductive cells
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contain miniature adult individuals so
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if we were talking about human
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reproduction for example the notion for
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at least some biologists at the time was
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literally that there were miniature
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humans present within the sperm or the
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Egg and these were thought to have all
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the same body parts same anatomical
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features and everything that you'd find
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in fully grown humans just much much
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smaller so according to this idea
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development is really simply a matter of
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growth of this pre-existing individual
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that's already present within the
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reproductive
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cells now there were a variety of
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different terms that biologists used to
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refer to these hypothetical individuals
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and one of these was homunculus which is
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actually Latin for little man
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and this is a famous picture showing
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what one of these miniature humans was
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imagined to have looked like uh this was
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drawn by an early Dutch microscopist who
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was one of the first to observe sperm
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cells under the microscope and it shows
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what he imagined to be present within
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the head of the sperm now this was from
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the late 1600s and at the time light
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microscopes didn't really offer a high
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enough resolution that he would have
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been able to see something like that
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even had a been there so it's just a
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depiction of what he imagined to be
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present but I do think it's a really
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nice illustration uh of the thinking
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amongst a number of biologists at the
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time about how development might work
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now of course we know today that from a
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scientific standpoint uh this was not a
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particularly good idea to say the least
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but I think it definitely gets points
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for creativity and uh it's just a really
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fascinating chapter in the history of
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the field so I thought it would be a fun
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way to start this lecture
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one of the cool things about teaching
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online is that you can do things that
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you wouldn't normally do in a typical
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classroom type setting uh like for
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example changing your shirt right in the
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middle of a lecture and I really
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couldn't resist doing that here because
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I've got the world's perfect t-shirt for
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talking about the homunculus so I'm from
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the state of New Hampshire and we have a
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great micro Brewery here called smarty
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nose Brewing Company and interestingly
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enough one of the beers they make is
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called homunculus so this is the shirt
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they have to go with a beer and I love
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the clever slogan on the front as you
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can imagine it's a hoppy beer and then
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the back's got this really great picture
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of a bunch of sperms swimming toward an
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egg and if you take a closer look here
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the egg is actually the bottom of a beer
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bottle and then all the sperms swimming
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toward it have little homunculi in them
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which are actually the faces of the
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Brewers and other staff members so
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anyway I thought it was a great shirt
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and uh I figured I'd never have a more
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appropriate occasion than this to show
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it
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off okay there's another term I'd like
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to introduce here before moving on and
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that is preformationism which is
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essentially just the belief in a
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homunculus or something analogous to it
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and just to reiterate the key idea with
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this concept is that the adult
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individual is assumed to be pre-formed
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or already present within either the
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sperm or the Egg prior to fertilization
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so according to this line of thinking uh
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development is really simply a
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matter of growth of of this
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pre-existing
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individual now there were probably a
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couple of different reasons why this was
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a popular idea during much of the 16 and
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1700s for starters if you're trying to
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understand uh how organisms develop it's
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a lot easier to think about growth of a
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pre-formed uh organism than it is to try
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and come up with an explanation for how
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a complete new individual is going to
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arise to noo or form a new uh with each
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passing generation so maybe uh your task
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as a scientist is a little bit easier if
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you're just trying to think about how a
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pre-formed individual would grow
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additionally preformationism fit pretty
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nicely with some uh creationist ideas
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about the origins of human life and
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there were some people who took the
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homunculus concept to the extreme and uh
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postulated that because it was thought
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to be an adult organism uh that the
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homunculus would contain its own set of
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reproductive cells which would have
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homunculi within them and then those
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would have still another generation of
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reproductive cells in homunculi and so
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on and so on so this is uh kind of
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analogous to the traditional Russian
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stacking doll where you have one that
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fits inside of another and then another
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smaller doll that fits inside of that uh
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etc etc so with this kind of model A
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Divine Creator is assumed to have
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created all the humans that are ever
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going to be present on Earth in the form
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of something like a family of human
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stacking dolls made up of multiple
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generations of Uli there were certainly
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biologists at the time who raised
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objections to those types of models uh
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for example if you're going to accept
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that that type of model then by
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extension you're going to be forced to
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accept one of two Alternatives neither
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of which seems to make a whole lot of
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sense either you'd have to assume that
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there's going to be a finite and
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ultimately very limited number of total
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human Generations that will ever occur
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or you would have to somehow account for
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the fact that at some point there's
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going to have to be a humulus that would
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be uh presumably smaller than the
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smallest unit of matter in order to fit
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into uh the homunculus from the previous
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generation so neither of those uh
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scenarios seems particularly logical and
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for those types of reasons uh even at
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the time when when preformationism and
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and these types of creationist ideas
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were proposed there were certainly
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biologists who raised objections uh and
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favored alternative models now the other
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idea that had been proposed to explain
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how development works is referred to as
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epigenesis and this theory states that
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individuals arise and acquire their
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ultimate form through a series of
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progressive changes interestingly this
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theory of epigenesis actually predates
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preformationism by a couple thousand
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years this goes all the way back to the
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time of Aristotle and you may recall
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that Aristotle was a famous Greek
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philosopher who studied all sorts of
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different fields including developmental
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biology uh he was actually one of the
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earliest developmental biologists at
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least in recorded history and Aristotle
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spent some time observing developing
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chicken embryos he would crack open
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fertilized eggs at different stages and
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observe the developing embryos and based
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on those observations he formulated a
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number of different ideas about how
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development occurs including proposing
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this theory of
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epigenesis so of course we know today
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that Aristotle was absolutely right
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about this and you probably take the
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theory of epigenesis for granted but I
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think it's important to remember that
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Aristotle really had no knowledge of
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cells he didn't really understand
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anything about fertilization or how that
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works so uh given his lack of knowledge
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about how development begins I think it
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was actually pretty impressive that he
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had the inside to realize this is in
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fact how development occurs anyway fast
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forward a couple thousand years and you
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have the preformationist at least
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temporarily taking us a pretty big step
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backward But ultimately the scientist
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who favored the theory of epigenesis
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prevailed in the debate over epigenesis
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versus
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preformationism and that was primarily
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for two key reasons both of which had to
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do with the development of better light
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microscopes so first of all scientists
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were continuing to observe uh tissues
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from a variety of different species
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under the microscope and they recognized
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that all living organisms are composed
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of cells so that's a key part of the
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cell theory right that cells are the
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fundamental unit of life additionally
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developmental biologists are looking at
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sperm and looking at eggs under the
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microscope and they recognize that those
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are both individual uh cells they're
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very specialized cells but individual
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cells nonetheless and so taken together
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uh these two observations are very
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problematic for the
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preformationist because the hula should
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be composed of cells but that would in
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turn mean that either the sperm or the
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Egg should contain many smaller cells
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and of course that's not what was
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observed so basically there's a growing
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disconnect here between some of the
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predictions of
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preformationism and what scientists are
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actually observing and together with
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some other experimental results that
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I'll tell you about in future lectures
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uh this eventually leads to widespread
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acceptance of epigenesis and rejection
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of
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preformationism and this was a really
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important point in the history of the
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field because as we head into the 1800s
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that really changes the way that
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biologists uh approach the problem of
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development so at this point you can
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start looking at it as follows uh
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development begins with the fusion of
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two individual cells an egg and a sperm
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that are going to come together at
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fertilization and then that will lead to
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a whole uh series of progressive changes
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that we can collectively refer to as
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epigenesis
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and that will culminate in the formation
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of a new individual so let's say we're
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talking about human development here I'm
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a really lousy artist but if you give me
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a second I'll try and diagram a newborn
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human
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here okay like I said I'm a a pretty
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lousy artist so that's supposed to be a
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newborn infant and not some kind of
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weird looking space alien or something
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but anyway you get a point right uh in
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the 1800s developmental biologists are
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starting to to think about the problem
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of development in terms of what are the
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various processes that uh underly
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epigenesis in other words what are the
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the series of events that are going to
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underly this really incredible
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transformation so in a second here I'm
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going to ask you to hit pause on your
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video player and grab a pen and paper or
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open a word processing document and take
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a couple minutes to jot down a few of
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your thoughts about uh some of the big
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picture type of biological phenomena
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that you can think of that are are going
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to happen here and you can probably come
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up with some of those on your own so
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when you're ready hit pause and take a
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take a few minutes to do that and then
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you can hit play again and and compare
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your answers to
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mine okay what' you come up with uh
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before I give you my answers let let me
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just emphasize that you may have written
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some things down that don't show up on
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my list and of course that by no means
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indicates that they're not important
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parts of development as you can imagine
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there's all kinds of things that are
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going on here and for starters I'm just
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going to look at this from a very big
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picture type of
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perspective okay so the first thing I
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wrote down is the generation of the
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reproductive cells of course that has to
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happen before we can even get to the
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point of fertilization so this process
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of uh of the reproductive cells is
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referred to as gam
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Genesis and the root word here gamet uh
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refers to the reproductive cells so gam
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Genesis is the process by which the
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reproductive cells are generated that
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encompasses meiosis which is something
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you may have uh have written down on
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your list it also depends on Specialized
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stem cells called germline stem cells
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and there's been a lot of really
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interesting research in that area over
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the past several years that could have
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significant relevance to human biology
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so for starters uh several recent
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Studies have challenged the
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long-standing assumption that women are
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born with all of the oyes or eggs that
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they'll ever produce now that work is
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pretty controversial but I think it does
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raise some very interesting questions
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about the basic science of human
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reproduction it was also recently shown
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that functional sperm and oyes can be
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generated from stem cells called IPS
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cells that are derived from skin cells
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in mice and so these types of studies uh
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could have really important implications
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for our understanding of human
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reproduction and they could potentially
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even lead to new approaches for treating
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infertility as well okay so we'll go
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into some detail on the biology of comma
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Genesis in an upcoming lecture once the
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reproductive cells are formed they can
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then come together and fuse at
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fertilization
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and as I've diagrammed the process up
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here uh it looks very simple and
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straightforward you have these two cells
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that come together and fuse and you're
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often running in reality it's a lot more
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complex than that uh the sperm often has
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to travel great distances to reach the
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egg that's of course the case in human
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development um once the two cells are in
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the right vicinity the sperm and the egg
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need to uh find and recognize one
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another and then there needs to be a
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physical attachment between the two
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cells the sperm gains entry into the egg
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and then uh that all culminates in the
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combination of the genetic material from
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the two so there's a whole lot of things
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going on here and it's a lot more
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complex than I I've diagrammed it here
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we'll go into a lot more detail uh on
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fertilization and all these different
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aspects of that process a little bit
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later in the
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course okay the next thing that I have
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on my list is um I think intuitively one
00:16:28
of the more obvious aspects of
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development once fertilization happens
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uh you have a single cell that that's
00:16:35
formed from fertilization but then in
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the case of a newborn human for example
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you're talking about an organism that's
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made up of trillions of cells so clearly
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there's got to be a major increase in uh
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overall cell number here and that's
00:16:49
associated with growth of the developing
00:16:52
embryo so both of these things involve
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repeated rounds of cell division or
00:16:57
mitosis now I'm not going to talk uh
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about the cell biology of mitosis in
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this course but we will look at some
00:17:03
really interesting aspects of what are
00:17:06
known as the embryonic
00:17:14
cleavages and the embryonic
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cleavages are the cell divisions that
00:17:19
happen uh in the early embryo so that's
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what gets us from one cell to two cells
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from two cells to four four to eight Etc
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and there are some really interesting
00:17:28
differences in terms of how those early
00:17:30
embryonic divisions occur from one
00:17:33
species to the next so we'll look at uh
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some of those differences and why they
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arise now not only is there an increase
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in overall cell number during
00:17:42
development there's also an increase in
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the number of types of cells that are
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present within the embryo so if you
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think about uh newborn human for
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instance as you know there's going to be
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all sorts of different types of cells
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that are present within the human
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anatomy so you know you'll have uh skin
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cell and neurons and muscle cells and so
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on and all those different types of
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cells have ultimately descended from the
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one cell embryo that was formed at
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fertilization so what that reflects or
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indicates is that uh embryos have
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mechanisms for generating diverse cell
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types from this common starting point so
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how does that work this is actually a
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two-step
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process and in the first step uh cells
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within the embryo are going to receive
00:18:27
instructions that specify a particular
00:18:30
developmental Fate In other words cells
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need to receive instructions about what
00:18:35
they're supposed to do and what they're
00:18:36
supposed to become during the course of
00:18:39
development so this is referred to as
00:18:44
Sate
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specification and with Sate
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specification uh the cells are receiving
00:18:54
those instructions that tell them what
00:18:56
they're supposed to do what they're
00:18:57
supposed to become sometimes those
00:18:59
instructions come from within the cell
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and other times uh there are signals
00:19:03
from outside of the cell that instruct a
00:19:05
cell what fate it's supposed to adopt
00:19:08
but either way they need to get those
00:19:09
instructions and that happens through
00:19:11
cell fate specification once they get
00:19:13
those instructions they can then move on
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to the second step which is called
00:19:22
differentiation and with
00:19:24
differentiation cells are physically
00:19:27
acquiring specialized traits that are
00:19:29
appropriate for their given
00:19:31
developmental fate so think about uh
00:19:34
neurons versus red blood cells for
00:19:36
example those are very different types
00:19:38
of cells they they each have a highly
00:19:40
specialized structure a highly
00:19:42
specialized function and differentiation
00:19:44
is the process where those uh unique
00:19:47
attributes or characteristics are
00:19:49
physically acquired during the course of
00:19:52
development now the intermediary between
00:19:55
Sate specification and differentiation
00:19:58
is altered gene expression so cells will
00:20:02
respond to signals that specify
00:20:04
particular Fates by turning some sets of
00:20:06
genes on and turning other sets of genes
00:20:08
off and that's what allows them to
00:20:10
specialize or
00:20:12
differentiate now both of these
00:20:14
processes are uh really foundational
00:20:16
topics within uh modern-day
00:20:18
developmental biology and so accordingly
00:20:21
we're going to spend a lot of time in
00:20:22
this course uh focusing on cellular and
00:20:25
molecular mechanisms uh that underly
00:20:27
these processes
00:20:29
all right moving on uh number five on my
00:20:32
list is tissue organization or
00:20:34
patterning so as the various types of uh
00:20:37
specialized cells are being formed
00:20:38
through differentiation they are going
00:20:41
to need to be organized into higher
00:20:43
order three-dimensional structures in
00:20:45
order to give rise to all the different
00:20:47
body parts that you're going to find in
00:20:49
you know a human or a frog or whatever
00:20:51
else we're dealing with and so uh
00:20:53
embryonic patterning or developmental
00:20:55
patterning is a term that refers to the
00:20:58
various processes that make that level
00:21:00
of organization possible so this
00:21:02
encompasses multiple different things um
00:21:05
one key aspect of patterning that we'll
00:21:06
talk about is Axis
00:21:13
specification and this is where the
00:21:15
various different body axes are being
00:21:17
laid down within the developing embryo
00:21:19
so the embryo will need to distinguish
00:21:21
its head from its tail and its back from
00:21:23
its stomach so when we get to this part
00:21:25
of the course uh I'll I'll review the
00:21:28
various body axes and we'll uh look at
00:21:30
how those are
00:21:31
established another key aspect of
00:21:34
patterning is referred to as
00:21:38
gastation and gastation is this really
00:21:41
remarkable
00:21:43
reorganization of all the cells that are
00:21:45
present within the early embryo to form
00:21:47
uh three major tissue layers so you
00:21:50
start out with what's basically a ball
00:21:52
or a disc of cells and then gastrulation
00:21:54
forms uh an outer layer of cells a
00:21:57
middle layer of cells and an inner layer
00:21:59
of cells from that pretty simple
00:22:01
starting point gastrulation then sets
00:22:04
the stage for uh
00:22:10
organogenesis and this is where a little
00:22:12
bit later in development uh specific
00:22:15
organs are going to start forming in
00:22:17
particular regions of the embryo as you
00:22:20
might imagine uh the events involved in
00:22:21
organogenesis are going to differ from
00:22:23
one organ to the next uh but there are
00:22:26
some common themes here and we'll
00:22:27
explore a few of those
00:22:29
uh looking at a couple different types
00:22:31
of organs as
00:22:33
examples uh last but not least the final
00:22:35
topic on my list is postembryonic
00:22:38
development so for humans and many other
00:22:40
species as well uh there are lots of
00:22:43
really interesting developmental
00:22:44
processes that continue to occur long
00:22:46
after we reach the conclusion of
00:22:48
embryonic development so with that in
00:22:50
mind uh in one of the final units of the
00:22:52
course I want to focus on a few
00:22:54
different topics uh within this general
00:22:56
area of postembryonic development so one
00:22:59
thing that we'll cover here is
00:23:04
metamorphosis as you may remember uh
00:23:07
there are some pretty amazing uh changes
00:23:10
that take place in insects and
00:23:12
amphibians and some other organisms as
00:23:14
part of their post embrionic life cycle
00:23:16
so we'll talk a little bit about the
00:23:18
biology of metamorphosis in this unit
00:23:21
another thing I want to focus on in this
00:23:23
unit is
00:23:27
regeneration so so regeneration is a
00:23:30
subject um that's particularly
00:23:32
interesting to me because uh my research
00:23:34
students and I are studying mechanisms
00:23:36
of tissue regeneration uh in my research
00:23:39
lab so I'll tell you about the biology
00:23:42
of some naturally occurring regenerative
00:23:43
processes such as limb regeneration in
00:23:46
salamanders which is Illustrated in the
00:23:49
really beautiful series of pictures
00:23:51
shown here and we'll also explore a
00:23:55
really exciting New Field called
00:23:56
regenerative medicine where there's been
00:23:58
some amazing breakthroughs over the past
00:24:01
several years that are really starting
00:24:03
to change the way that we're approaching
00:24:05
the search for treatments for all kinds
00:24:07
of different diseases so we'll talk
00:24:09
about uh regenerative medicine in this
00:24:11
unit as well and then finally I also
00:24:14
want to cover
00:24:17
aging so why is it that organisms tend
00:24:21
to exhibit a general decline in
00:24:23
anatomical Form and Function as they
00:24:25
grow older I think that's a really
00:24:27
interesting question and here again
00:24:29
there's been some pretty remarkable
00:24:30
advances over the last several years and
00:24:34
I'll tell you about some of those
00:24:35
breakthroughs as we uh conclude this
00:24:37
unit with a discussion of Aging so
00:24:39
here's a summary of the key
00:24:41
developmental processes that I just
00:24:43
reviewed and I do want to reiterate here
00:24:45
that this list is by no means
00:24:47
allinclusive there are certainly a
00:24:48
number of other topics and you might
00:24:50
have come up with some of those uh that
00:24:52
I didn't get to here but that are
00:24:54
important aspects of Developmental
00:24:56
biology and that we'll be exploring in
00:24:58
some future lectures but before I
00:25:00
conclude the current lecture I want to
00:25:02
return for just a minute to the history
00:25:04
of the field and consider how it got to
00:25:07
where it is today initially this field
00:25:10
was known as embryology and that
00:25:12
reflects the fact that uh as we've
00:25:14
already discussed the primary focus uh
00:25:17
initially was on uh determining how
00:25:20
development originates and also on
00:25:22
characterizing the events that occur
00:25:24
during early stages of embryonic
00:25:26
development so research working in this
00:25:29
field in the 18th and 19th centuries
00:25:31
were really spending a lot of time
00:25:33
sitting in front of their microscopes
00:25:35
and simply observing the developing
00:25:37
embryos of various different species
00:25:39
like for example the Frog embryo shown
00:25:41
here so uh with a few very important
00:25:44
exceptions that I'm going to come back
00:25:45
to a little bit later in the course for
00:25:47
the most part at this point the field
00:25:50
was uh very
00:25:52
descriptive in nature so in other words
00:25:56
uh the emphasis was really simply on
00:25:58
describing what was happening during uh
00:26:01
development and I think it's a little
00:26:03
bit unfortunate uh this term descriptive
00:26:05
has a rather negative connotation in uh
00:26:08
scientific context today but it's really
00:26:11
important to remember I think especially
00:26:13
for younger biologists who might just be
00:26:15
getting uh started with conducting
00:26:16
scientific research that you can really
00:26:19
learn a great deal about development and
00:26:21
other areas of biology uh simply by
00:26:24
making really careful and thorough
00:26:26
observations of whatever whatever it is
00:26:28
that you're studying and then uh
00:26:30
carefully and and clearly describing
00:26:33
those observations to other scientists
00:26:35
so that can provide a really important
00:26:37
level of insight and that was absolutely
00:26:39
the case with classic embryology the
00:26:42
really painstaking observations of the
00:26:44
early researchers in this field even
00:26:46
though they were they were mostly
00:26:47
descriptive uh they really laid a very
00:26:50
strong foundation for for a future
00:26:53
research now as we move into uh the
00:26:55
middle part of the 1900s the field
00:26:58
underwent some pretty significant
00:26:59
changes and that was primarily due to
00:27:01
the molecular biology Revolution so uh
00:27:04
you have the discovery that DNA is the
00:27:06
hereditary material and solution of the
00:27:09
structure of DNA and cracking of the
00:27:11
genetic code all these these major
00:27:13
breakthroughs in molecular biology had a
00:27:16
pretty profound impact on a lot of
00:27:18
different fields in biology including
00:27:20
the study of development so with
00:27:23
developmental biology at this point uh
00:27:25
we move from primarily a descriptive uh
00:27:27
Focus to more of a
00:27:30
mechanistic
00:27:33
emphasis so now uh the focus for
00:27:36
developmental biologist becomes trying
00:27:38
to understand how various developmental
00:27:40
processes are occurring so what are the
00:27:42
underlying cellular and molecular uh
00:27:45
mechanisms and the key tool that they've
00:27:47
used and and continue to use is
00:27:51
genetics so genetics uh has provided a
00:27:55
really powerful way of trying to gain
00:27:58
mechanistic insight into developmental
00:28:00
phenomena and so over the past several
00:28:02
decades uh using increasingly
00:28:04
sophisticated genetic tools we've really
00:28:07
learned a great deal about how various
00:28:09
developmental processes occur so uh
00:28:12
those range from things like access
00:28:13
specification to differentiation and
00:28:16
gastr relation all the things that we
00:28:18
talked about uh just a few minutes ago
00:28:20
uh we now have a great deal of insight
00:28:22
into how those processes occur through
00:28:25
application of genetic tools and you'll
00:28:27
get an appreciation for that level of
00:28:29
mechanistic insight as we go through
00:28:31
some of the material in future
00:28:33
lectures okay so in conclusion uh I
00:28:36
think clearly we've come a long way from
00:28:38
the days of the homunculus uh there's
00:28:41
certainly still a lot of really exciting
00:28:43
and important unanswered questions for
00:28:46
future developmental biologists to
00:28:47
address but we've come a very long way
00:28:50
from where we started and we now have a
00:28:52
great deal of insight uh into not only
00:28:54
what happens during development but also
00:28:56
into how it occurs
00:28:58
and so uh I look forward to telling you
00:29:01
more about that really remarkable body
00:29:03
of knowledge as we uh move forward with
00:29:05
future
00:29:27
lectures e
