okay so today we're going to talk a little bit about basic embryology and one of the reasons that i like to do this is to give you an idea of how all the various structural components that is the cellular and tissue components arise embryologically and how we look at each system in terms of development so there's prenatal development and postnatal development now there should be a handout that you can essentially follow along with in part with this lecture on embryology so you want to make sure that you download and print that and uh it also has some figures on it there'll be some handouts for that uh related to that and i'll have that under the announcements link as well so let's talk a little bit about prenatal development and what that really refers to so we just finished discussing cells and organelles and where all that comes from and we said that according to the cell theory that all cells come from pre-existing cells and that is the first cell that actually is derived in the body through this process we call fertilization the fusion of egg and sperm and so what we want to be able to do is sort of follow this along and understanding the development of things prenatally gives us an indication and sort of a a guide post or series of guide posts to sort of understanding uh how things function normally in terms of their chronology of development that is the timing of development and how they develop in in sequence with one another along with the idea that if something goes wrong early on uh in development it has much much greater effect so the earlier that something goes wrong or is read out incorrectly the more problems that they're going to be later on some of these show up early on and some of them that is at birth and some don't show up until much later in in life so in our handout we talked a little bit about the idea here uh in terms of the formation of a single fertilized cell so i'm gonna let's just start this off with our introduction here and again this is going to be our basic embryology lecture so we start off with a fertilized cell and this cell is called the zygote we're going to go through these processes here and show you the actual sequence where this this happens but what i want to do is emphasize this idea that that cell is what we call total potential in other words it has the ability to become every single cell that happen that occurs in our body it develops that right we talked a little bit about differentiation from the point of the zygote on and so most of that occurs most of that occurs through the process of what we call mitosis and so mitosis is basically the duplication of cells sometimes they're called sister cells because they have the same genetic material so these cells contain the same genetic material so if you took a cell out of your liver and took a cell out of your your brain and went out of your skin they would all show the same chromosomes same genes all those things different genes get turned on and other ones get turned off at different times and that allows for this differentiation to to occur so again i just want to emphasize this idea of differentiation things become different and again we're going to follow that that sequence here in just a few moments so when we get uh the cells beginning to divide as i said occasionally there are errors in duplication of these cells of our our chromosomes our genes uh and different kinds of alleles that are read out and that can cause different kinds of what we call birth defects and so about ten percent of babies are born with some form of genetic birth defect and they might be very very minor but only two to three percent of these are serious enough to really require medical intervention right so the heart doesn't develop properly and to go in and surgically repair that that heart uh presumably so a lot of these diseases though sort of show up oftentimes in adolescent years or they show up in teen years and sometimes even much much farther along in sort of middle and old age where those occur so different kinds of things happening we have this huge range of genetic variation that that occurs throughout the the human genome itself so let's start with some basic anatomy and we're going to start with the basic male reproductive anatomy because what we really want to do is just talk about the players that are involved here in in all of this so we're going to just look at uh the the things that are actually involved in in this process uh as well so uh on your handout sheet again there are you know good pictures of this or at least decent pictures of this and again in your textbook and at the very end of the course we're going to talk in depth about the reproductive anatomy of of males and females so you want to look at your handout for for these to kind of follow along so we can start off here with the testes and the word testicle is a common what we call a colloquial term so this is the anatomical term and that's what we're going to use and again singular for this is testis so this would be singular here for these and of course these are paired and they will say they're paired and they're found within the scrotum housed within the scrotum itself the little sack that divides these two so we have two a set of these and the testes produce what are called spermatozoa spermatozoa and these are just commonly known as our sperm cells so these are our sperm cells when the these are produced under the influence i should mention here also of a male hormone called testosterone and there are different kinds of testosterones that are found in the body in different kinds of these we call androgens so but primarily under this and again we'll talk a little bit about this at the very end of the of the course so testosterone helps uh with the sort of development and also the rate of development of of these and sperm cells are once a male reaches adolescents will continue to produce sperm throughout their entire life no matter how old they are they may not be producing as much as in their 20s and 30s when they're in their 80s and 90s and that kind of thing but they're still producing sperm the other thing that happens though in prenatal development is that this testosterone helps also with the development of the whole reproductive system and so it matures at that reproductive system at least to a certain point and continues to have that influence but it isn't until uh we actually have um you know adolescence occurring and the male begins to produce viable sperm that we see that that beginning to happen and that of course is postnatal in terms of that so once this is formed here in the testes we have a series of ducts so these ducks that are here are going to transport the sperm and i'm just going to use the term sperm here to talk about spermatozoa or sperm cells and so sperm is very specifically just the the cells themselves here that are part of this it's going to transport these sperm cells i'll use that term sperm cells from the testes and that's going to go to another set of ducts called the epididymis to the epididymis and here sperm cells mature in the epididymis so they're going to mature in the epididymis itself so this is where they go they're kind of like um you know not quite mature enough yet until they get into the epididymis and again we'll discuss this uh near the end of the of the course how that that works now we also here are going to let me switch this out we're going to have glands that will produce a variety of we'll say various nutritive and also protective secretions so we just talked about the process of secretion in our cell lecture so a bunch of these these secretions that occur and join the sperm cell pool okay and this then this material the secretions and the sperm cells it's called semen so again we'll get into some of the um various specific lands a little bit later on and excuse me and we'll get into some of the components of that seminal fluid that occurs as well a little bit so semen is undergoes this process called ejaculation so is oops should really say is ejaculated sorry here so sp semen is ejaculated through the process of ejaculation um through this tube we call the urethra and the males it's very long it's actually three different components that are there and it goes through the urethra and through the penis and in fact the penile urethra is the tube that allows that to to occur and that will be then transported so this semen is transported under normal conditions to the vaginal canal during intercourse okay so that's where it's introduced so now we want to see how it actually makes its way up through the female reproductive system and again your handout you should be able to to see this so we're going to look at the basic female reproductive anatomy and the important players here with regard to to this process all right so let's begin in this case remove this here let's begin with the ovaries and the ovaries are also paired those are right and left on these when we look at these they produce what are commonly called the ova and ovum is singular again and in fact they're not really truly over we sort of call them that eggs they're really what are known as secondary oocytes some of you may know that or not it's not that important but we're going to just use the term egg or ovum with regard to that and that occurs during a process known as ovulation which typically happens in females once a month so typically in mid-cycle so again this ovum here is going to be released during this process of ovulation and again we'll talk about the details when we get to the end of the course okay so the uterine tube and this uterine tube is sometimes known as the fallopian tube or the oviduct or other names for it so oviduct or also known as the fallopian tubes but we're using the anatomical terminology here now that's used is the uterine tube so the uterine tube of the uterus this nice large pear-shaped muscular structure picks up and transports the ovum and it's going towards the uterine cavity in the uterine cavity so we're going in that in that direction now this is if we're going to have a meeting of the egg and sperm uh here so we're going to progress as though that's going to happen so sperm cells these spermatozoa enter the uterus me they enter the the uterus uh in the basically what we call distal end just getting those directions down and the and this is known as the cervix the term cervix as like cervical vertebrae means neck so this is a narrow portion of the of the uterus and so there's an opening that occurs here this is known as the os os means mouth ass of the cervix and there's a little canal there uh in which the sperm begin to swim up through the cervical um canal and towards the uterine body okay so we're going in that that direction now fusion of ovum and sperm cells should typically occur so this again we're looking at the sort of typical situation typically occur in the uterine tube and the fusion process here is known as fertilization so a fertilized ovum occurs basically the head of the sperm is going to go into the ovum itself not the whole part of the sperm uh and loses his tail on the outside there and then we're going to have eventually fusion of the nuclei of the egg and the sperm that is the ovum and the sperm now what happens next here with regard to this is we're going to have the zygote remember that's our fertilized ovum the zygote is going to mitotically divide so it begins that that process and eventually undergoes this process called implantation and this occurs on the inner wall which is known as the endometrium so i'm going to put that here as well endometrium is the inner wall the inner lining of the uterus okay so that's going to occur on that inner wall of the uterus and the embryo basically develops in this region and this is often called this area in here when the uterus is doing that it's often known as the womb okay so the womb where that's taking place all right so that kind of gives us a little bit of where all this meeting is going to to occur and what we want to do then is talk a little bit about the prenatal development periods and it turns out that there are three distinctive periods of time where this occurs this is well defined by different developmental features that take place in different kinds of events that have been examined by embryologists and other scientists for a long time okay the first is called the germinal period the germinal period term germinal refers to germ means sort of the the point at which things begin to to happen to germinate uh if you will um and like when seeds germinate that kind of thing this occurs between time zero and two weeks typically 12 to 14 days period so zero to 12 14 days so if we look at uh the point of fusion that occurs here some people call that conception the moment of conception where the fusion of egg and sperm occur we call that time zero it takes somewhere between 12 to 14 days for this to occur one of the things that uh you should really know about the chronology of all this is that the timing is really elegant in all of this directed again by various kinds of mental genes and so the timing and the growth of that happens within all these stages is really really well known and we can usually get within easily a single day of development understanding the timing as well as the growth patterns that occur in each of these the second period that occurs is known as the embryonic period and this is where we're calling this an embryo at this this point and this occurs approximately between three and eight weeks and you'll see that there are some very specific developmental stages here and things that are happening within that those time periods that that demarcate this so after 12 to 14 days we begin this period here at the end of the eighth week we go into our third period which is called the fetal period and this is where the embryo now becomes a fetus and this occurs approximately between 9 and 38 weeks sometimes also defined as 40 weeks after uh lmp or last menstrual period because that's a two week time frame so this is sort of the normal typical development that we expect to see uh within this and so i'm gonna put in kind of like that's pretty close to to this and again these the timing is pretty pretty narrow so um not unusual at all for for that to occur okay so think about this i want you to sort of just concentrate on that for just a moment and think about the germinal period the embryonic period and the fetal period all right so now again embryonic embryo fetus okay and once the fetus is delivered we then call it a child okay so that's how that sort of is defined uh from a scientific and embryological point of view for all of this all right so now let's take a look at our germinal period stages so we're going to go through this a little more in detail so our germinal period stage okay and what happens and we'll actually call these stages because there are several different events that are taking place here in fact four specific events that are taking place okay the first of these as we mentioned is this process of fertilization so again some people use the term more loosely as the moment of conception uh where fertilization occurs so we don't know exactly when that that happens we can see it you know outside of the of the individual if we do this artificially but again this is really where we're going to have the essential fusion of ovum and sperm cell only one sperm cell can actually fertilize an ovum you can actually have what is often known as polyspermia or polysperming to to occur for for this okay now again i want to emphasize this typically occurs in the uterine tube and if it happens too late oftentimes implantation will not occur or it'll implant in the wrong place if it happens too late sometimes again it fails and you can get what we call a tubal pregnancy as well so what we're going to do is form we have that fusion this forms what are known as the pro nuclei because remember i mentioned that when the sperm goes into the cell it's actually just the head of the sperm that goes in and which is the nucleus of the sperm cell and they kind of sit there for several hours before there's actual true fusion and so we form we see these pro nuclei occur and i think there's some figures of that in your web lecture as well for you to uh to look at then once we have the fusion itself the actual fusion portion of that we now call that the zygote so we're going from essentially two half groups of chromosomes into one group of chromosomes we call that the diploid state and so this is the number of chromosomes you have that's 23 pairs or 46 chromosomes and so for the human genome that's the number of chromosomes that we have so every cell in your body with the exception of your sperm cells or your ovum or ovary cells here are going to have essentially 46 chromosomes so sperm cell will have 23 and an ovum 23 excuse me and when they fuse they will form the full complement the diploid state itself for that so now we have our first cell and we're going to sort of undergo uh that mitosis okay so let's take a look at our next stage within the germinal period and that is known as cell cleavage cell cleavage and this process known as blastulation blastulation all right so this occurs through the process of again mitosis so this is just dividing dividing dividing and every time you divide we're going to have these duplicated cells that occur so the embryonic mass forms first so this is a two-celled stage which was to a two-celled stage now one thing that i should mention uh here about this i'm going to sort of show you here let's say is the zygote and let's say we have a fusion of the egg and sperm a true fusion you'll notice in some of the figures here that there is a little membrane that occurs around this kind of a clear little membrane this is called the zona pellucida here and this prevents other sperm from actually getting in at this this point the reason this is called cell cleavage and not just cell division is because when this occurs what we're going to do is we're going to divide this cell into two cells that look the same but they're going to still have this membrane around them like this so they still have the same membrane so they're just actually having the size as opposed to getting larger and so this now is called the embryonic mass and the embryonic mass typically takes around close to 30 hours to complete so this is happening in the uterine tube after that fertilization and it's beginning to move down the uterine tube towards the uterine body and as it does that it begins to again undergo this division eventually we get to this stage known as the morula and in latin the word morula means mulberry like a little mulberry if you've ever had those or seen those and again there's some nice pictures here for you to look at on the web lecture and also in your book as well and here this is a solid ball of cells so a solid ball of cells typically this is at the 16 to 32 whoops cell stage so 16 to 32 cells here and this is a solid ball of cells that form that and again there's sort of what we call the early moreland the late morela doesn't really matter too much for this what's interesting though is that at this stage if we took the modulo apart particularly at the early stage you could actually have 16 identical or clonal types of cells they would be exactly the same they'd all develop exactly into the same type of individual so at this stage there's no real differentiation occurring uh and at the very very late late umorial stage just before the next stage there's a little bit of differentiation that's beginning to occur so that next stage is called blastulation so blastulation so what happens here in blastulations we're going to form what is known as the blastocyst okay so blasto means basically formation of cyst is a fluid-filled sac okay so whenever you see the word cyst it means fluid-filled sac so this is going to be formed here very very early on what happens is is fluid moves out of cells so these 16 to 32 cell stage and into the interstitium so remember the interstitium is between cells that to form oops a fluid filled structure and that's what we call the blastocyst when that when that happens so that forms that blastocyst so what does this look like and again you can see this in your figure here i'm going to do this in a different color so we're going to form cells that so we have this sort of hollow ball of cells uh that that occur and i'm going to have i'm going to differentiate that using a different color uh in this case get a different color for this and i'm going to have a massive cells here okay so the blastocyst and i will just abbreviate that as b cyst here we can look at the components of these so there it turns out that there are three components for for this so three different specialized components of this the first one is known as the embryoblast and also known as the inner cell mass so the inner cell mass or icm for this or the embryoblast so this tells us this is where the embryo is actually going to form okay so that is going to be the embryo blast so i'll do this again in a different color here so show that here's our embryo blast the second thing that forms here is we said it's fluid filled so we're going to have this structure called the blastoceal and whenever you see the word seal like this that refers to a fluid-filled sac basically like cyst as well and so that is going to be this area here so this is the blastoceal part of that and then the third layer is known as the trophoblast so again the term blast means formation of tropho means nutrition that usually refers to nutrition and growth as well uh in all of that okay so that will be these this outer ring of cells this is our trophoblast so three different components here we have the embryo blast the trophoblast and this fluid filled cavity that occurs in here which is the blastocell so that that cavity now it's the trophoblast i should just say say fluid filled cavity and this is going to attach to the endometrium remember the endometrium is the inner lining that's the inner lining of the uterus okay so the inner lining of of the uterus and that is the the endometrium okay okay so that's the blastulation up process that occurs following that and as it develops as well and again these are all sort of happening somewhat simultaneously or continuously as we go along through this we're going to have our third event and that is implantation so implantation so implantation begins about six sometimes five seven days after fertilization so six to seven days after fertilization is going to to occur now again that occurs on the endometrial lining right that's where that's going to take place and implantation is completed in about five days so typically by 12 days after fertilization so about 12 days after fertilization occurred so it takes about five days so it's a process that is ongoing for that period of time all right so now let's take a look at what else is going on so what we need to do is we need to look at the walls or the wall i should say of the endometrium and this is a really specialized area uh in the in the female uterus here this this is really pretty pretty amazing so there are actually three different layers and we look at these layers the first layer here is known as the decidua so deciduous like a deciduous tree means that things slough off they fall off like leaves would here this is the decidua basalis okay so this is the portion deep to the blastocyst so it's going to be sort of underneath the blastocyst itself so this is where the blastocyst is sort of moving into this this area of the endometrial wall itself then we also have the decidua capsularis so again the latin word for for this and this is the portion of the endometrium that covers the blastocyst it's going to cover the blastocyst here so it sort of surrounds the other portion of this and then we have the decidua parietalis so you're going to see the word parietal a number of times usually means wall or outer wall outer portion and so this is the portions of the endometrium so of course these are all related to the endometrium here we're talking about portions of the of the endometrium not involved with the blastocyst and there's another portion of the endometrium that never gets left off so it's not called the the decidua so that's a part that we'll talk about again uh down the line uh later on uh in this okay so the blastocyst is also maturing as it implants things begin to change so we want to sort of go through that that process and look at the structure of what is known as the late blastocyst so what i mean by late is the last part of its development so the blastocyst is maturing itself so it's actually changing as well and so we want to look at what's going on okay so we have a number of different structures first one is what we call the amniotic cavity and this is a portion that the embryo eventually and fetus will live within itself so this portion here is above that is more superior if you will above the embryo blast or icm and the trophoblast becomes more fluid felt this can become more fluid felt and as we're doing this we're actually sort of destroying the blastoceal that's being replaced uh with this as well so we've all heard about the amniotic cavity this is the one where the water breaks right and it's actually amniotic fluid that is protective of the growing embryo and fetus for this we also have what is called the primary yolk sac okay the primary yolk sac and this is going to be whoops this occurs below or deep two the embryo blast or icm and again we're going to have some fluid filling as well the third structure here is our really important one this is called the embryonic disc so the embryonic disc which eventually forms the actual embryo okay this is where the embryo is going to to develop okay and that's that embryoblast region that we're talking about so a little bit of the trophoblast is involved but we're not going to worry about the details of that now let's take a look at the trophoblast trophoblast actually differentiates into two different kinds of structures or two different groups of cells if you will so we're going to have what is called the cyto trophoblast and as the name implies it's going to be basically very cellular itself and that's going to be on the outside portion of this and that's where it actually attaches to and is interacting with the cells of the endometrium and then we also have what is called the syncytio trophoblast how's that name for a name so let's talk about what that is we know what the trophoblast is sin syn means coming together cyto again or scissio refers to cells so what ends up happening is that some of these cells begin to lose their plasma membranes and they become one big sort of mushy area uh in in all of this and so we'll see what happens with this institutio trophoblast which becomes very very important in all of this so we're sort of encapsulating the embryo and fetus with our cytotrophoblast and this is since issue trophoblast here is going to differentiate into some other structures so let's talk about those and so our go through this here okay the placenta and what is called chorion formation so placenta and corian formation so we want to talk about what that actually is for for this all right so the placenta so let's talk about the structural features there's two of them so there is the maternal portion the maternal portion is the endometrium this is the inner wall of the mother here and then we have the embryonic portion what is called the embryonic portion and this is known as the corion so that's known as the chorion so what are the functions for for these okay what are the functions for for all of these all right so i think i might have screwed up my outline here a little bit but i think we're good yes okay so um so functions this supplies the embryo and the fetus with nutrients so this is the connection here that allows the mother to supply these nutrients to the growing embryo and developing fetus for this it also removes waste products so think about this cells are going to produce waste right there's going to be garbage here so we need to get rid of that stuff so it removes the waste products from the embryo and fetus also okay so we're getting rid of of those as part of all that so that's how this begins to look now when we look at the corion formation itself i'll talk a little bit about this that is the embryonic portion okay so what's going on here with with regard to that remember we just talked about this syncio trophoblast here so this institution blast here this invades the endometrial wall what's happening here is that these cells begin to produce an enzyme this enzyme literally eats away the wall so that the implantation process gets deeper and deeper it starts dissolving the walls around the endometrium at that point and it sinks down into the endometrial tissue itself so this is now called the chorion by definition that's the embryonic portion of this the chorion develops these really interesting finger-like projections and these are known as the chorionic villi so here again this is plural of this villus is singular so i just want to differentiate a couple things you'll see this a little bit later on as well a villus is usually a long finger-like projection so this is what a villus looks like more or less and we'll also see these little bumps that come out this is called a papilla so they're both imaginations but one is really short and bumpy kind of like an egg crate if you turn the egg holder over you see this this little papilla uh whereas the villus is more like a large sort of finger like projection that that comes out and these themselves will also begin to develop little side finger like like structures and i'll show you that here in a minute and again you can see this in your in your hand i think fairly well so what are the the functions of the chorion so first of all this is going to anchor the embryo and fetus because remember it's going to get really big it's going to get heavy and it's going to you know sort of want to pull out excuse me this is an s right there uh for for that it also increases the surface area to volume ratio and remember we're supplying nutrients and taking away waste so the more surface area we have relative to the volume the more efficient this process is going to be it also secretes a hormone and this hormone is known as hcg hcg stands for human chorionic gonadotropin so human chorionic obviously produced from the chorion gonadotropin tells us that it is going to affect one of the gonads of course the female gonads are the ovaries and tropotrophic means growth or nutrients and so it's going to affect that process so let's talk a little bit about what actually happens here and for early pregnancy tests this is the hormone that's actually detected it only occurs if implantation occurs so it takes like you know five to seven days after fertilization to be able to detect this so we want to sort of think about how that works okay so let's talk a little bit about our hcg and how this cycle actually works okay so here we're going to have our chorionic villi and chorionic villi it's going to produce this hormone hcg that's going to go through the bloodstream right it's going to travel through the bloodstream here to the ovary now again we're going to look at the ovary in detail at the end of the course so don't worry too much about this but there's an area in here that we'll we'll talk about a little bit later on that affects this within the ovary itself and that is going to release two different hormones one of the hormones is called estrogen and the other one is called progesterone that's going to travel again through the blood back to the chorionic villi so what we're doing is releasing three different hormones here so we're releasing hcg we're releasing estrogen and progesterone in all this so what's happening is we're going to have again this going back now this happens for the first trimester first three months of development and estrogen and progesterone has the influence on growing right so we're going to say here that this is going to grow and maintain the placenta because the placenta fails there's no connection between the mother and the fetus and then it won't survive so remember that's what what this is doing so we have this really interesting release of a hormone here which affects that after the first trimester the chorionic villi actually produces its own estrogen and progesterone so it doesn't need to go through this process here okay right so that is the germinal period all the different events that that take place again think about this fertilization cell cleavage and blastulation implantation and placentation occurring all right so now let's talk about our next stage okay so hopefully you followed all of that okay so now let's go to the embryonic period stages and again i hope you appreciate the idea that this is sort of a continuous process it's not they're not necessarily discrete they're sort of occurring almost simultaneously and um sort of continuously with with um each other as this goes along so this process is sometimes also known as embryo genesis so embryo embryogenesis all right so again there are four uh important stages in this the first is we call the germ cell formation germ cell formation so again what we mean by germ cells are cells that actually give rise to other cells right just like the germinal period here it's giving rise to the embryonic period in in all of this okay so the embryoblast initially forms two layers so what we see is sort of a differentiation in that embryoblast that little inner cell mass that that occurs we're going to have these two distinct layers the first layer here we'll say let's do it this way the first layer is called the epi blast and you recall that epi that prefix means on top of sort of sitting on top of so on top of that so it's this sort of upper layer that occurs closest to the implantation side of of all this and eventually this is going to form a germ layer we're going to be talking about here in just a little bit called the ectoderm and the term ecto this prefix means outside derm refers to sort of skin sort of like a thick you know thick layer of of cells here forming a sort of skin type layer it's not actually the skin per se although we'll talk about that as well as we go along and then we have the hypoblast and hype of course means beneath or below in this case and this is going to form this germ layer called the endoderm so this is the inside portion so outside inside so those are our two distinct layers so the way we want to sort of think about this here is we're going from essentially you know i think i did it in green so i'm going to do this green here again so we're going to go from a mass of cells like that this is going to be our e-blast all right this is our e-blast here and we're going to go into some differentiated cells here so that we're going to have two distinct layers and what i'm going to do is sort of show this in by using two different kinds of of colors if you will so it just makes a little bit easier so we're going to have a group of cells here that goes like this this is going to be our epiblast and then we're going to have this other group of cells and this is going to be our hypoblast two different groups of cells and i'll just tell you that this ectoderm endoderm this forms the outer portion of our bodies and this forms sort of the inner layer this is actually your gut region in here so you can sort of see how that that will eventually work so that's where we're sort of headed uh in in all of this and then we have this really interesting process that that takes place so remember we talked about blastulation here and that's where we're forming the blastocyst and then these layers begin to differentiate now we're getting into what is called gastrulation gastrulation forming what is sometimes just called the gastrula as opposed to the blastula part of this so it turns out that there is this mass movement mass movement of ectodermal cells so we're going to have this mass movement of ectodermal cells and that's going to occur at the interface between the epi and hypoblast right so it's going to occur right in through here and what's going to happen in all this uh which is really really cool is that we're going to have a mass of these sort of squeezing in in that direction in between those those two layers so we're going to squeeze that in i'll draw that over again here in just just a moment as part of that now what this does is that this forms a third layer and this layer is called the mesoderm or mesoderm and meso means middle right so measle means middle so we've got an outside and inside and now we're squeezing in to the middle right between them as part of that all right so now let's take a look at how that that works and what's going on and the important thing about this also is that the mesoderm forms most of the body's structures so you can almost bet in the most in most cases that it's made out of mesoderm and we'll see that as we talk about tissues next time as as well with regard to that okay so turns out that the mesoderm is found in in two different forms so the first form is what is known as misen time i love introducing all these terms to you at this point so mizin is the same as miso means middle chyme is sort of a slushy mushy kind of material so it's real liquidy with lots of cells that are not really connected very strongly together and this is going to form most not all of course most of our connective tissue structures and again we're going to talk about these uh structures or these uh tissues next time in our next lecture as you'll see in where that that comes into play so connective tissue is like all over the place and what's great about the word connective is that it connects things it connects things to each other and connective tissues to connective tissues and to other kinds of tissues as well the other thing that happens here is we form what are called somites somites these are basically paired blocks of mesoderm whereas the mesenchyme is sort of slushy mushy these are pretty solid these are cells that are really packed kind of together here for that and they actually form three different kinds of structures now before i do that i want to again kind of go back here and just sort of show you that process that we were looking at before and i'm going to sort of blow this up a little bit more so let's say we have a group of these epiblast cells here which are forming the ectoderm and we're using our hypoblast cells here i'm going to make these a little bit smaller and they tend to be a little bit smaller as well not quite as abundant and what's happening then is we're going to have this movement of cells i guess i should have i done this in a different color sorry should i have this in red here you can use whatever colors you like but there we go so now what's going to happen again is we're going to have this movement oops we're going to have this movement of these cells squishing in between i just want to make that a little bit clearer so it's kind of these cells are going to migrate and squeeze in between to form our third layer so what this is going to look like uh in this case here is we're going to have our oops this big thick layer in here and i'm going to use a um a distinctive color just black here for this to really contrast this and then we're going to have our okay so what we end up with here is our ectoderm i just want to make sure that this is clear for you this is our mesoderm and here is our endoderm so this forms a little sort of flat plate if you will on on this and the way this kind of looks here if we saw this kind of in three dimensions is it's kind of a weird sort of flat plate kind of goes kind of like this so we're going to have this group of cells here in kind of three-dimensionalize this that's going to be this group of cells right there and then we're going to have another little plate here of these cells that's going to be this group and then we're going to have this plate of cells here and that's going to be b cells here so that's our three different layers that occurring so at this point the embryo is flat okay kind of like three sort of odd sort of shape coins kind of stack stacked together kind of stretched out in one direction with kind of a sort of a head-like region and a kind of a tail-like region and um essentially what's going to happen you might read about like primitive streaks and all that kind of stuff and that's kind of like right here it's going to happen kind of along the center portion of this what's important also about this uh is that as this forms these paired blacks of blocks of of mesoderm here what's going to happen is we're they're going to occur right along the midline here on either side so they're going to be on either side of of what's actually taking place uh in in all of it all right so now we're going to have as i said the somites here are paired blocks of tissue we're going to have layers and they're all going to differentiate into different things we're going to have what is called the scleratone so sclerotome sclero means hard and this is going to form bone bone is hard we're going to have the myotome and that's going to form as you might guess muscle and then we're going to have the dermatome and that's going to form the dermis of the skin so if you think about this bone inside muscle around it and skin around that right so these three three things here in in that direction the way that this uh this looks okay so when we look at at these blocks and the waves sort of look at at how these these sort of form you can kind of get a sense of uh where this is this is coming from these three different areas and we'll refer to these as we go along and how they actually develop into these different tissues of bone muscle and and skin itself okay so the third event that's taking place here is what is called neuralation and again these are things that are happening more or less continuously and kind of in sequence but also on top of each other to a certain degree so this is the formation of the initially the central nervous system so the central nervous system and of course that is comprised of the brain and the spinal cord and it's also involved in the formation of we'll call them associated structures these are sort of special senses structures uh that we have and these are the eyes ears and nose okay so really important detecting structures right that are part of our nervous system and if you think about those now there are two types of tissues and these tissues here formed or will say derived from the ectoderm now what's really easy to remember about these is that all nervous tissue every bit of nervous tissue doesn't matter what it is comes from ectoderm that's where it's derived from so we're going to have this stuff called neuroectoderm or neuro ectoderm and this forms specifically brain and spinal cord but then we also have these really interesting little cells called the neural crest cells the neural crest cells these are amazing because they go out and they form all kinds of amazing things and these will form for example um spinal nerves they also form cranial nerves oops you can see that's an eye there cranial nerves and things we're going to talk about in the integumentary system called melanocytes and specialized cells of the called the adrenal medulla they also help form teeth all kinds of stuff and this is just a partial list of all the things that that they do so in the process of neurulation i kind of want to draw how this works and again there's some nice views of this in the web lectures here if we look at our um the top of our our ectodermal region here we're going to start off with a sequence that kind of looks like this group of cells i'm just going to keep it kind of simple here rather than drawing multiple layers of this so this is what this this looks like and then what ends up happening is we start getting a little bit of a fold in here kind of like this do that very well it looks like that so now what we're doing is we're actually forming a couple of different structures so one of these structures here is this is called the neural groove okay so this is the neural groove here and this little area right here is called the neural fold so what's happening here is that we're getting cells to kind of migrate up in this direction and start forming this little groove that that occurs here just like that okay now what's going to happen is that eventually we're going to have that fold all the way over see if i can do this form it like this okay so now we've actually created a third structure here this neural groove is now formed what we call the neural tube and that neural tube is going to form the brain and the spinal cord so we're going to have that and that's going to separate now at the same time that that's going on we're going to form these cells let me use this here and i'm going to just make this well in fact let me do it this way so that'll be a little bit easier to kind of follow eventually we're going to have again this plate over like that this is our narrower term and maybe what i should do is just label that as neuractoderm okay and now we've got this neural tube like that and on either side of this on either side of this i'm going to draw these two kind of like eyelashes looking kind of things one kind of like this kind of like that and these here are the neural crest cells and this is going to occur all the way along through here and then these cells are migratory they're going to migrate out into all these different areas here let me raise that up a little bit so you can see it so they're going to migrate out into all these different areas that's pretty cool uh in terms of all the things that they do and where they they go so we'll talk about a number of these things where they where they pop up as i've mentioned before so there are 31 pairs of spinal nerves there are 12 pairs of cranial nerves there are tons of melanocytes these the ones that are pigmenting your skin producing melanin um adrenal medulla is modified cells that are involved in in hormone production and things like that so all kinds of of really interesting stuff okay so the last um thing that that's happening here in terms of this formation or events is the thing called organogenesis and as the name implies this is going to start forming all the major organ systems so this begins typically around the third week of development so pretty early on some things are some systems are delayed a little bit in this third fifth week third and sixth week they begin to develop uh in in all of that so all the major systems the respiratory system cardiovascular system urinary system all these different things here and they continue to develop you know throughout this period they're maturing different things are beginning to happen and as we go along we'll talk about some of these different systems and where they're they're actually taking taking place okay so that's the four stages of the embryonic period so that will get us to our last period and this is our fetal period this is where from the ninth week on we're calling this the fetus so typically this begins about 60 days after fertilization so again the timing of this is really pretty amazing and of course measuring this through various uh various means like ultrasound and so forth and now they have three-dimensional ultrasound which is really pretty cool we can three-dimensionalize the growing embryo and fetus and we can see this they can measure uh this and the measuring tells us exactly where we're at with usually within a millimeter and this is done by millimeters from sort of rump to to head to crown and and they're called the carnegie stages and you can look those up you'll see some of that uh in the um in the web lectures as well and uh on the internet if you want to look those up it's really pretty incredible all the things that that happen here so this uh is basically organs and organ systems continue to develop and mature and so this will carry on post-medically as well obviously because we grow and our muscles get bigger and stronger and our bones grow and all these different things so everything has to continue to change but this is all the prenatal period material and what we're going to do next time is put along with cells and their organelles and so forth together with this development and talk about tissues and how this relates to tissues in these three germ layers that form these and so again when things begin to happen within those germ layer regions uh things can can happen developmentally the later this goes on uh without any impact uh the more normal the development will be the earlier it happens the more widespread the pathology or pathophysiology can actually be so this is uh this is pretty uh pretty important here so you know let me just mention here that uh just to give you some some stats here um a 60 day old fetus typically is around three centimeters in length okay so typically around three centimeters in length okay so one centimeter um you know too well 2.54 centimeters is one inch so this is about one and a third inches in length right so really really pretty pretty small and it also weighs about two and a half grams right so talking about over you know 400 some grams is a pound so this is really really tiny at this point at full term and again these are averages here so at full term we typically have fetus at about 50 centimeters right say 18 19 inches something like that here and about 3 500 grams right so we have a growth pattern of 15 times a little bit more than that and a weight pickup here of about 1400 times so that's really huge so this is all that rapid development that's basically taking place during that time okay so we'll talk about tissues next time