having talked about the process of fertilization as well as the female reproductive cycles it's a good time now to talk about the first week of human embryonic development and that is what you're going to look at in this particular lecture so welcome to the virtual class let's look at the events that take place during the first week of human embryonic development just as a matter of introduction we agreed that the prenatal developmental periods can be divided into two the things that happened before conception would include gametogenesis and fertilization process itself the things that happen after conception that period can be grouped into three we have the pre-embryonic period which focuses on the first and the second week of development and that is a subject matter the first week of development then we have the embryonic period which traverses from the third to the eighth week after conception and we have the fetal period which focuses on the ninth week until when the baby is born so in this lecture we are focusing on this one first week of human embryonic development this is what we are going to cover we want to explain the events that occur during the first week of human embryonic development as we do that we are also going to look at the morphological changes which take place during the first week of human embryonic development and we're going to see that at the end of the first week we have a structure known as the blastocyst part of the objective of this lecture is also to look at the parts of the blastocyst let's begin with the first agenda there focusing on the events that take place during the first week of development remember that fertilization took place at the ampulla of the fallopian tube which means that the conceptus is still within the fallopian tube immediately after fertilization so the events of the first week will also largely take place within the fallopian tube i'm not saying all of them will take place within the fallopian tube there are some that will not take place within the philippine team but most of the events take place when they conceptus is still within the fallopian tube as a matter of fact maybe the first four to five days are actually spent within the fallopian tube and uh only the latter days of the first week will the conceptus be within the endometrial cavity usually the conceptors will reach the endometrial cavity perhaps around this fifth to the sixth day after conception and that tells you something therefore in terms of the events that are taking place one of the event is that the conceptors must move it will be moving from the site of fertilization which is the ampere of the fallopian tube to the site of implantation or let me say the appropriate site of implantation which is the endometrial cavity so the movement of the conceptors is one of the key events of the first week of development what facilitates the concept has to move we can mention two things one the fallopian tube is lined by simple columnar ciliated epithelium the ciliary activity of the fallopian tube propelled the conceptus from that site of fertilization to the site of implantation and if you're not i am using the term concept as i'm not saying zygote or whatever because that cycle is not going to last long it's going to change and so the term conceptus is a more umbrella term more appropriate to use in this particular context so the concept has moves and the movement is facilitated by the ciliary activity of the fallopian tube the other mechanism that would promote the movement of the conceptus is the parasitic activity of the fallopian tube now the peristaltic activity of the fallopian tube is basically the action of the smooth muscles of the wall of the philippine tube that movement is particularly important when the conceptus is passing through this region the ismic region of the fallopian tube which is ideally one of the narrowest parts of the fallopian tube and so as it passes through the ismic region of the fallopian tube it will require some penicillinic push so that it can move through there to the endometrial cavity okay apart from movement of the conceptus from the site of fertilization towards the site of implantation the other event that took place during the first week of development is that the conceptors will have to undergo cleavage now what's cleavage in the lecture of fertilization and its results we defined cleavage as a series of mitotic cell division that the zygote undergoes and remember we call it the zygote when it's just a single cell the moment it becomes more than one cell we no longer call it zygote but those daughter cells are generally called blastomeres both the zygote as well as the blastomeres undergo mitotic division so that repeated series of mitosis is what we call cleavage the conceptus undergoes cleavage process so it means that the zygote divides mitotically the blastomeres also divide mitotically and as division goes on it means therefore that the conceptuals will be growing in size and as it grows in size it also means that the conceptors will undergo some morphological changes as you can see even in this uh diagram we see a single cell there now it is added to two maybe four and the number keep on increasing so this growth including numerical increase as well as growth in size and so the morphology of the concept is also changing as the cleavage process is also taking place now that we've said that the cleavage process leads to morphological changes in the conceptus i want us to now merge this to agenda together and talk about then the morphological changes which take place during the first week of development as we do that we'll also be still talking about the events that take place during the first week of development let's do it this way consider that fertilization took place on the first day or we can say that the first day is considered to be the day of fertilization so this is what we have we have a cell here that has two pronuclear within it and it also has two polar bodies within it it tells you something this is basically the ordered stage of development remember from fertilization i told you that the autistic stage is that stage when we have a single cell with the two pro nuclei it tells you that this cell has already the the oocyte here the secondary outside has already undergone the second meiotic division and that is why you are seeing both polar bodies because the second polar body can only be formed if the second magnetic division is completed so the second metric division has been completed and eventually we also have two pronucle one from the sperm head malpra nucleus and the other one perhaps from the oocyte and that is the female pronucleus but they are not yet met so that time when the two prenuclear have not yet met is called the autistic stage the tuple nuclear will meet and when they meet maybe in that manner as we can see that and finally they have met we call that stage the zygote stage of development and so when development is starting on the first day we have the zygote remember some things here this is the zona pelucida the glycoprotein covering that surrounds the zygote is what you call the zona pellucida and you can also appreciate that there are some sperms which have been trapped within the zona pelicida during the process of fertilization remember that there is zona reaction which prevents further entry of sperms and so some of them are unlucky enough to be trapped within the zona as you can see here so this is zona pellucida and this is the zygote one of the results of fertilization was initiation of cleavage or basically mitotic division so the zygote undergoes mitotic mitosis the first mitotic event will lead to us having two cells the daughter cells are known as blastomeres and that mitotic process that is underwent by the zygote as well as the blastomas is what we've just called cleavage [Music] so the first cleavage event gives you two cells we have two blastomeres surrounded by the zona pelicida this stage when we have two cells is simply called the two cell stage this first mitotic event can take place within 24 hours after fertilization which means you can have it within day one or it may take place after 24 hours it may be completed after 24 hours and so that's why you can also have it on the second day after fertilization the two cells still undergo cleavage as well and let's assume a scenario where we have success rate in both of them so both undergo cleavage event when they both divide mitotically we are going to have a stage where we have four cells that time when we have four cells is simply called the fossil stage embryology is not difficult easy just follow so that's the fossil stage now generally this second cleavage event will take place is in less than 24 hours and so the four cell stage is present ideally on the second day after fertilization because this second one takes place in less than 24 hours that's why we have the four cell stage on the second day okay let's bring this one to the next page remember we're just on the second day we need to account for seven days and at this point we are at the four cell stage that's a question before i tell you about this rest of developmental process i've put for your question that i want you to think through so i'm giving you [Music] maybe 30 seconds to think through that and make a decision before we go right let's proceed now remember we reached the second day and at that stage we had a structure that has four cells so we call it the four cell stage these four cells will still continue to undergo cleavage maybe not necessarily now at the same rate let's be practical now so some cells will have divided some no and maybe even the rate of division is different and so i don't want you to narrow your mind on from four you must go to eight then 16 then 32 and there's another option open your mind now maybe three of them will have divided by after a particular period and one is lagging behind maybe out of those three then the daughter cells begin to divide immediately so let's just understand it this way that we can reach a time when you have a mass of cells which look like this we see such a mass of cells on the third to the fourth day and at that time we have about 12 to 32 cells in total that 12 to 32 cell stage is known as the moral stage of development so from the third to the fourth day we have a structure called the modula i want to notice something in these two images the four cell stage and the modular stage that they're both covered by the zona pelosi that means something it means that we don't have adequate space for expansion because the zona felicita is anatomically limiting so there's no room for expansion we are saying that cleavage is going on which means we are increasing the number of cells but we are also saying that the zona pellucid is intact which means that there is no space for expansion these many cells which are coming up don't have room to expand what are the implications of that implication of cleavage is that we have a numerical increase and so during the first week of development we will have numerical increase in the number of cells but because there is no enough space for expansion maybe some two things will have to take place here one is that the size of the daughter cells will have to be smaller with each cleavage event so we have reduction in the size of cells with each cleavage event you can look at that here so these are the four cells maybe if i take you back first let me take you back so that you see the previous slide sorry not that one that one look at the size of the zygote it's filling the whole of the shell of the zona pelican then the two cells are there we've increased in number but definitely the data cells are smaller than the zygote then look at the four cells we've tried to make the shell of the zona be relatively constant but we've increased the number of cells so the four cells each of the cells are smaller than the two cells now let's form that one to where we were so here we see that the four cells are within the shell and here those 12 to 32 cells are also within the shell of the zonophilicity each individual cell is smaller than these ones so you agree with me that there is reduction in the size of the cells with each cleavage event something else if you increase the number of cells and the shell of the zona felicita is not permissible it's not allowing expansion it means what that the cells will be squeezed within themselves that process of squeezing of the early embryonic cells is termed compaction the cells of the modular are compacted the process of compaction is very important for these cells that intimacy is important for them to communicate amongst themselves and even identify themselves as members of the same gun so to speak all right so i wanted to capture that compaction process characterize the first week of development we're on the fourth day we have a morilla the process of cleavage continues and because cleavage process is going on it means that the process of compaction is also going on so the cells are squeezed even more now to add to that a cavity appears within the modula so that cavity first appear smaller cavities that collise to form one large cavity and it accumulates fluid so it expands that stage of development when a cavity is present is generally seen from the fifth day onwards we don't call that stage the modular anymore we call it the blastocyst stage of development so it begins from the fifth day going upwards this is the blastocyst that conceptus that has a cavity inside so you realize what that the blastocyst we're talking about at this point still has its zona felicita also intact that tells you that the cells of the blastocyst are even more compacted than the cells of the mole this particular blastocyst we are talking about its cells are more compacted than the cells of this model why because definitely the number of cells in this blastocyst are more than 32. and to add an insult to that a cavity has appeared which is also competing for space and so definitely these ones are most quests so from the fifth day we have a structure called the blastocyst that blessouses that formed on the fifth day usually will be present even after the sixth day and then some things still go on during that time what are they cell division is still going on so the number of cells continue to increase the size of the cavity continued to expand and what does that mean in terms of compaction the pressure inside is even more than what we had because of those the pressure within here become more too much to bear and so the zona pelicida gives way that process where the zona policy that gives way is called the hatching process when the zona policy that gives way it degenerates at around that point not any other random point but at that point and i'll give you a name for that shortly the zonophilis that degenerates at that point when the zona pelicider degenerates at that point the blastocyst that is within the zona pelicider become extruded out of the shell of the zona felicita and that is what you see in this image so this is the shell of the zona pellucida and this is the blastocyst within the shell of the zona felicidade being extruded as you can see here after that extrusion we will have a structure which look like this such a structure will be present from the sixth day all the way to around the eighth day of development you realize that we've already gone beyond the first week and we are now threatening in the second week but all in all i want to compare these two structures the blast the structure that has been extruded is still known as the blastocyst just like the one that has not been extruded from the shell of the zona pelican however to distinguish the two we call the first one the early blastocyst and we call the second one the let blastocyst so we have an early blastocyst and a late blastocyst this is the lead blastocyst this is the early narcissist there are some differences between the alley and the lead blastocyst we can try to capture them you can compare the size first of all we can compare the presence of the zona pericida so the early one has it the left one does not have it we can also compare the size of this cavity the left one has a bigger one the early one has a smaller one and it should make sense because remember the cavity increases in size and so this one will have a bigger cavity compared to that look carefully at the cells on the periphery look at their shape the cells in the periphery seems to have a flattened outline the term given to flattened cells is squamous so these cells are relatively squamous but they are squamous predominantly how about these ones they are predominantly cuboidal last we can compare the overall size of the blastocysts this one is smaller compared to that one but this one now has expanded because of the pressure from inside so i wanted to remember those differences between the alley and the late blastocyst now we can talk about the parts of the blastocyst the individual cells of the blastocyst are still known as the blastomeres i'll use the let blastocyst to know the parts of the blastocyst the cells of the blastocyst exist in two cellular populations we have a population of cells which are on the inside this one that have outlined is known as the inner cell mass the inner cell must represent the population of cells that are designated to become the baby of course they'll form the embryo first and for that reason the inner cell mass is known as the embryoblast the cells on the periphery represent the outer cell mass this outer cell must refer to the population of cells designated to become the placenta because the placenta is a supporting structure we call the outer cell as the trophoblast so we see two population of cells in the blastocyst stage the one designated to become the baby in a cell mass the one designated to become the placenta the outer cells this space is what we call the blastocyst cavity [Music] we can also say that the blastocyst has two poles this one here the pole of the blastocyst away from the inner cell mass is known as the embryonic pole you can consider that to generally mean the absence so absent embryo basically this part that lacks the inner cell mass is known as the ab embryonic pole the embryonic pole of the blastocyst refers to the pole that leads the way during the hatching process so this is the part of the blastocyst that lead the way when the blastocyst is coming out of the shell of the zona pelicider then we have this other pole we call it the embryonic pole the embryonic pole refer to the population sorry to the pole that will lead the way during implantation so that's the pole that leads the way during implantation we will see what implantation is in the next class good so let's make a summary of the events that take place during the first week of development we have said that during the first week of development the conceptors will be propelled from the site of fertilization to the site of implantation and there's some mechanisms that will promote that movement as that takes place there is continuous cleavage this will increase the number of cells and because of the pressure inside that we call compaction hatching will eventually take place in terms of the morphological steps we talk to the zygote two-cell stage four-cell stage modular stage and the blastocyst i hope you remember the timing zygote is day one two-cell is day one today two for sales day two more or less day three to day four the blast assist in general is day five up to day eight importantly you realize that when the first week ends there are two population of cells that have been formed one designated to become the baby the inner cell mass that refers to the embryoblastic tissue and the outer cell mass the trophoblastic tissue in simple terms we can say this when the first week ends a decision has already been made in terms of the population of cells that will become the baby embryonic and the population of cells that will never become the baby the non-embryonic that distinction is made in the first week of development and the conceptus that has already undergone hatching is the blastocyst that blastocyst is ready for implantation so when the first week ends the conceptus is ready for implantation good so that is the first week of development basically thank you