Embryonic Stages

hi everyone it’s mr. senti and today I have the pleasure to be discussing with you a little bit about pregnancy and it’s the very first part of pregnancy if you consider pregnancy being nine months that’s approximately correct to be more accurate it’s about nine and a half months or 40 weeks and so I want to talk to you a little bit about that and embryonic development and the first basically a discussion of the first trimester of your pregnancy but you know the thing is it’s so exciting about having a baby you know the one of the first things that you want to know when you have discovered that you are pregnant is when is that baby coming out and so the doctor will count 40 weeks and so you’re like wait a minute I thought it was nine months but it’s really like nine and a half but it’s it varies in individuals and so basically what a doctor is gonna do though it’s gonna start counting from the first day of your last period okay so this is what we know and so why would you do that though like why would you count like the first day of your last period well usually that’s known and and the mother will be able to speak to that and so technically you know as you may imagine you don’t become pregnant during your the period it really happens around ovulation time where you’re conceiving and so that’s around the second week of your of a normal cycle so when a doctor is telling you you’re four weeks pregnant when in fact you’ve been pregnant for just two weeks so that’s that so this embryonic stage of development or the first trimester is really kind of defined as really the time between when your fertilized in about eight weeks okay so then what would you call this after eight weeks eight weeks is around two months so after the after the second month of pregnancy then you’d start talking about fetal stages or fetal development and then when they obviously when the baby come that comes out or fetus comes out it’s a baby there knew more and so we’re talking about the first trimester and this is what an embryo looks like in five weeks right here so it’s kind of Awesome see the head and and the vertebrae are starting to form and here’s them some internal organs and so you can see here again the head and the body and you can make out various organs right here and some some of the the limbs are starting to form and that’s what’s interesting is the the feet and hands are basically solid and then the cells sort of die off and separate the fingers and toes of the phalanges and so during this first trimester during embryonic development all of the internal organs are gonna or major ones are going to start to develop and basically in the fetal stages there’s some cool stuff that happens but basically the it’s just going to enlarge and so what you can see here is that the embryo is floating in this membrane called the amnion which forms an amniotic sac with it and the baby is is floating basically suspended in this amniotic fluid which is cushioning and protecting the embryo during its development and what you’re gonna see notice here is the umbilical cord which is going to be connecting two lips connecting to the placenta which is a really cool organ which is a combination of blood vessels from the embryo and from the mother and that’s where gas exchange takes place and so this particular video will not be a discussion of the placenta but I will certainly attempt to make one specifically on the placenta because it’s just that good and so what I wanted to start off with sort of a transition if you’ve been following a previous video on fertilization and the cleavage period what’s happening is this zygote is basically cleaving into this first couple of cells called the Moriya and then it’s called the the blastula and it forms this fluid-filled structure called the blastocyst and it’s basically the blastocyst which is going to implant in the endometrium which is the uterine inner uterine lining of the uterus and so these outer cells of the blastocyst are referred to as trophoblast cells yeah and it’s these trophoblast cells which are some of the ones that are first to differentiate early in pregnancy and eventually become a membrane called the chorion which will

eventually differentiate into the fetal portion of the placenta and so I know there’s there’s a lot of terms involved in embryonic development don’t be discouraged only be encouraged because basically with the terms we’re trying to do sorry for preaching but what the terms are trying to do is clarify our understanding and that’s not to intimidate or it’s basically just to to be able to you know really focus on what’s happening specifically and so there are many new terms that will be introduced so the trophoblast cells are these outer cells of the blastocyst as I mentioned and this is actually showing the blastocyst implanting this is inside the uterus this is a scanning electron micrograph showing how the blastocyst will begin to implant inside the uterus and so basically here we have it and so here’s the you know when we talk about the outer cells we talk about these trophoblast cells okay and then there’s its fluid-filled inside and these cells in here these green ones are referred to as the inter inner cell mass or embryo blast cells now these are very important because it’s these inner cell mass of cells or I see em which are basically going to become the embryo so this develops into the actual embryo right here and so those are particularly important again a cool micrograph of that these are the outer trophoblast cells and these the inner cell mass cells which will eventually become the EM biryeo so these outer ones of the trophoblast cells so soon after fertilization the embryo develops into this hollow ball of cells called the blastocyst and then that’s what’s implanting into the uterus but it’s these inner cell mass cells that are really cool just a little side journey on these inner cell mass cells you might have known a known of them before these are quite famous the the ICM cells are sometimes known as embryonic stem cells and their stem because they could like a tree stem branch off into and differentiate into all of the different cell types of the embryo and ultimately into us so they can at first they’re all the same but then they can develop into neurons or adipose tissue or muscle cells or white blood cells so they’re they’re really really important because as it turns out in the late 1990s researchers were able to extract these ICM cells or embryonic cells and basically culture them in petri dishes and we found out that they grow best on fibroblast cells which are a type of connective tissue and so you we can actually grow these and so we can grow embryonic stem cells and why would we do that because they’re pluripotent as I was mentioning before they can develop and differentiate into other cell types in the body and if we’re able to manipulate these stem cells and grow them because of the fact that they’re very plastic plastic meaning that they can actually develop into all cell types even more so than our adult stem cells which you might be familiar with which are found in the mirror of our of our bone which can differentiate into different blood types these particular embryonic ones are really important because understanding how they do that it’s not clear exactly how they’re able to differentiate like that or what are the mechanisms that cause it and so it’s one of those areas where there’s degree of research and so they’re kind of there’s a lot of unsolved problems which makes it really interesting and so some of the basic research is trying to figure out how this works and there’s a there’s an urgency because you might know this this is kind of obvious there’s implication and importance for human health because you know the thing is we have damage to our body we can have we can be eight even if we’re healthy now we could be in a car accident something can happen we can have a heart attack which would would destroy some of the muscle cells in our heart we could have a stroke and cause some some neural damage in the brain and so as it turns out if we use these stem cells we might be able to insert them into damaged tissue and they can regenerate and become and replace heart tissue and maybe cure some some damaged tissue in the future so there’s a lot of hope there embryonic cells so getting back to the embryology one of the major events that happens to the blastocyst is

that there’s an invagination there’s like a folding inward of the embryo and this inner cell mass in here which is eventually you know found that the baby the baby forms this embryonic disc and there’s there’s three distinctive layers that form in this embryonic disc and so this structure is known as the gastrula so again sorry about all the terms so what happens is the blastula forms the gastrula and then it’s this embryonic disc which forms these three primary germ layers which are going to become all of the organs of the embryo and so just a little discussion of this and so here’s the blastocyst with the inner cell mass and then there’s this invagination forming the embryonic disc which is which is the gastrula and so these primary germ layers will then form all of the organs of the body and so they’re called the outer one is called ectoderm and the middle is mezzo durum and the inner cell mass is called endoderm so it’s ecto mezzo and endo and so you can really get into this if you’re studying embryonic development but I’m just basically going to give you just an outline of this so the ectoderm can eventually form the cells of our epidermis or of our skin it can form the neurons of our brain the some of these the germ cells over here on the side can form sperm and egg the endoderm can form portions of our lung some glands in the body like the thyroid pancreatic pancreas and the mezzo or mezzo in the middle can form muscle cells eventually some portions of the kidney red blood cells and again part of our digestive tract as well and so just to sort of continue this discussion of this at first you know again a sidebar at first it becomes like well do I need to memorize or should I need to know this you don’t mean it depends on what you want to to remember personally in my estimation I don’t know that you do need to recall this kind of what the ectoderm becomes because simply you could look it up and sorry I just bring it to your attention simply because of awareness but don’t feel obligated to recall all things I don’t think I don’t think that’s the the correct pedagogical approach to this I think when you can look something up if you’re overwhelmed like I can’t remember that the ectoderm produces portions of the nervous system part of the sensory organs or line the the mouth and anal cavity I can’t remember that you can look it up also the means of dirham becomes our message arm becomes muscle as I mentioned before it’s a bone blood comes from the mesoderm kidney and then endoderm produces the digestive tract respiratory tract so this is what we’re getting at and so what’s happening big picture is here we are in the uterus and so we’re implanting and so this the gastrula is is is forming here we have the embryonic disk right here which is made up of those three germ layers and then eventually what’s going to happen as the Coria that the trophoblast cells will eventually become something known as the chorionic membrane I’ll explain that as much as I can it’s the chorion will then evolved into Bhilai which are extensions into the the endometrium which will eventually unite with blood vessels from the mother to become the placenta and I know that that that’s somewhat ambiguous and so I’ll try to make a separate video on the placenta because it’s just an incredible thing but I also want to talk to you a little bit about you might be familiar with the fact that there’s an umbilical cord which connects the embryo to the placenta the forming placenta which basically has blood vessels at us two arteries in a vein that’s going to help provide oxygen and nutrients and and bring back carbon dioxide into the mother and so everything that the mother is eating and breathing is going into the baby via the umbilical cord now you might be surprised but we have a couple of other membranes I mentioned this amniotic membrane or amnion which forms the amniotic sac and this blue is the amniotic fluid and but there’s also a couple of other membrane tzer’s there’s the yolk sac and and something called the Atlantis that I want to talk about so here’s another picture showing the embryo in here and the various germ

layers over here and then it shows the chorion forming and eventually it will become the placenta and here’s the amnion which will then mentally surround the the embryo and cushion it with fluid okay and so close up of what we were talking about before it so I’m gonna run through some of these membranes during the this time of this membrane that I want to focus in on this and this amnion is particularly important because this amniotic membrane most people know the amnion because they’re familiar with the amniotic sack in this fluid because again during delivery one of the very first stages of delivering a baby is that you may have heard of this is that the bag of waters break and so that’s the amnion membrane rupturing and the amniotic fluid is leaking out the birth canal it’s one of the first signs of labor but not only that the bag of waters but that it surrounds and pushes the embryo as you can see here and it’s a protective environment one of the other reasons that I think most people know about the amniotic sac is that there’s a procedure known as amniocentesis now amniocentesis is sometimes referred to as amniotic fluid test or aft it’s a procedure in which the physician will use a syringe with a with a needle and actually go through the skin go through the muscular wall of the uterus and in to the amniotic sac and so some of the amniotic fluid is drawn into this syringe and so so what what would you do this well it’s in the fluid the amnion can’t contains some of the fetal cells that have Slough tauf in and so there’s some fetal cells that are floating around in the amniotic fluid and why would you do this because you could actually then take a look at that amniotic fluid and look for some sort of problems that might be occurring in the fetus like for example you can examine it for genetic abnormalities like an abnormal chromosome number and so to ensure that there’s no damage being done to the to the fetus when a needle is entering into the into the uterus I mean this is kind of a blind procedure if you think about it here’s shoving this pretty long needle into the into a thick uterus you don’t want to actually penetrate the fetus and so this is accompanied by ultrasound and so when you have an ultrasound image of what’s going on you could be more assured that you’re drawing out just the amniotic fluid and some of the fetal cells and again you take those cells and then spin them down and then you can culture them and and look at chromosomes and so just went to get second and take a look at what I’m talking about just to make sure that everyone’s clear on this this is a typical ultrasound so while the ultrasound is is being performed this is when the physician is able to insert the needle into the body through the uterine wall and draw out some of this amniotic fluid which is where I’m pointing right here so all this dark area is the amniotic fluid which is surrounding the baby just to make sure that you’re not gonna gonna hurt the baby in any way and so let’s take a look here so again when I was mentioning is that when you draw the cells out you could then take a look at those cells and find some that are mitotic in other words there’s chromosomes being formed you can take a picture of those chromosomes which is called a karyotype and then you can align those homologous chromosomes together and you could actually if you will you could if you know your your genetics you’ll know that if there’s two very large chromosomes 45 and 46 you’re gonna have a girl so you can determine sex by doing an amniocentesis but but really what your what your want to do is look for chromosomal abnormalities to make sure that the baby actually has 46 chromosomes and not like 47 like an example of that would be if you count them up look at this this baby baby girl because she doesn’t have a Y chromosome to access this baby girl has 47 chromosomes and she has three of the 21st pair and you’re supposed to have a mother father mother father in this case there’s three and so this particular genetic syndrome is known as Down syndrome and it’s it’s one of the most common genetic disorders and one of the reasons that it’s common is that get back to this is that these particular

chromosomes are the very small and it’s sort of like socks in a dryer they’re the ones that always get lost and when I meant get lost I mean during egg formation and sperm formation one of one of the processes to create those reproductive gametes is meiosis and so there there needs to be the chromosomes need to separate during anaphase one of meiosis and so sometimes that doesn’t happen properly and there’s a nondisjunction Allah vent resulting in an abnormal chromosome number in the gametes or aneuploidy and so this particular abnormal chromosome number is one extra one or trisomy it’s supposed to be diploid or two so this is trisomy 21 which is Down syndrome and so when do you perform amniocentesis it’s usually done between the 16th and 22nd week of pregnancy and so you know this is past the embryonic stage so this is like fetal stages we’re talking about now but women will be encouraged by their gynecologist and obstetric physician or OB GYN if they pose a particular risk in other words if if the female the mother is of a higher age because as it turns out there’s a correlation between the mother’s age and the the incidence of Down syndrome and so again this is a good picture showing here here’s the bladder here’s the here’s the the uterus here’s the placenta over here bilkul courts the idea is to draw out some of the amniotic fluid using ultrasound to make sure that you don’t hurt the baby here’s the cervix right here and then the birth canal and so I thought I’d throw out a few statistics on this and of Down syndrome as you can see here there’s a real correlation and mother’s age when mother gets up a little bit older not to say that you’re you’re old when you’re 45 years old but you’re old in terms of one you’d want to start your family and so the incidence of Down syndrome as is a little high at that age and so doctors gonna want you to or suggest that you get an amniocentesis if you’re in this particular area to make sure everything’s going well with the baby and so I mentioned a couple these other membranes and this Koree on the chorion starts this membrane right over here starts to develop villi or the extensions finger-like extensions and ultimately what this membranes going to be doing is helping exchange oxygen and carbon dioxide with the mother and the baby and that’s going to eventually form into the placental villi and so here’s something that’s kind of interesting I mentioned this before that there’s another membrane called the yolk sac now you might be thinking a yolk I thought only eggs of a chicken had a yolk but now we have one as well it doesn’t remain all during during the development of the fetus it happens early on in as it’s shown here and what it does is that it provides a nutrient-rich supply of food for the for the embryo but if disintegrates because the food and nutrients are going to be provided by the placenta and so let me come back here and so another function of the yolk sac is that it’s a primary site of red blood cell production and that is you might imagine that because normally in an adult red blood cells are produced in the bone marrow and so obviously there’s no bone in the embryo and so red blood cells are produced there so here’s a cool picture of the embryo here’s the placenta in the umbilical cord as you can see here here’s the amniotic sac and the amniotic fluid and so the yolk sac I was mentioning before are forms very early on and it’s the site of red blood cell production that’s pretty important and it also gives rise to some of the white blood cells as well and so the Atlantis another membrane forms during the third week that helps to to join along with this connecting stalk that will eventually give the umbilical cord which is made up of the the two arteries in a vein and so a better picture of that here’s the allantois right here and so what does this do it’s basically sort of like a little bag that is the primary structure involved in nutrition and excretion early on before the placenta is totally formed and it’s to collect waste from the embryo and so it functions in those

capacities and so I think one of the things that is most interesting that people want to know about embryonic development is like well what’s going on with the heart went one when can I start to hear the heart well the fourth week pretty early the heart begins beating the fourth week of of your pregnancy and so that the head the jaws begin to form the end as mentioned before in the previous picture your limb buds start to form and eventually this chorionic membrane becomes very bill I like and it’s going to eventually form into the placenta and again it’s a separate video on that chorionic villi becoming that placenta and so actual size of the embryo is really tiny okay like a grain of sand or grain of rice but ultimately it’s looking uh looking pretty advanced if you ask me and so here you have it embryonic stages it’s your embryo and your amniotic fluid right there in your chorion and ural Antos and your yolk sac and then eventually and I hope this gets your curiosity going like check this out this is going to be the placenta and so we want to we want to know all about how the mother’s oxygen is going to come into the baby and the baby’s carbon dioxide it’s going to come out and the mother’s food’s gonna come in and etc so that’s going to be a cool discussion of the placenta so I hope you enjoyed this video on embryonic stages of development thanks for watching