DIP Episode 322 - Introduction to General Embryology

Okay, welcome. My name is Divine. This is episode 322 of the Divine Intervention podcasts and the title of this podcast is going to be an introduction to general embryology. Introduction to general embryology. And before I start this podcast again I just want to make an announcement for those of you taking the two C Ks, step three. Sometimes soon I draw for 20 hour review course. It's going to be taking place on Friday and Saturday this week and then an MBME testing strategy course which is going to be taking place tomorrow. So if that's something you're interested in just shoot me an email through the website and I'll give you some more details on that. Again I've had many people taking these courses and I've done that I've done pretty well. Okay, so let's just jump right into it and really like my goal today is to go through a embryology from the very beginning to basically when the child is essentially you know placed in the you know basically the child baby comes out. Again just many of these things again people just memorize things but again a lot of this thing is just being able to I feel like embryology the name of the giving embryology is just being able to know how things logically happen and what triggers one thing after the other. So again just think of this again as a nice survey nice just introduction to generic embryology. I'll emphasize quite a number of details that you definitely do need to know for for your exam. So let's just get ready into it right.

So the major events of early life right in a child right for the most part you'll form your gametes first right and then after you form your gametes you're gonna have fertilization between the egg and the sperm and they are gonna form a zygote and in that zygote is going to implant in the uterus and then that zygote is just gonna keep cleaving cleaving cleaving cleaving cleaving cleaving. All of this stuff happens in week one. By week two you then have an embryo that has two layers formed. By week three you have an embryo with three layers formed. By week four you start having all these body folding processes beginning to happen. Let's just kind of like a nice overview of what what actually happens right. So this just hit on those processes one by one and talk about the high old things you need to know for your USML exams right. So the thing is in a developing embryo the gonads of that embryo like the testes the ovaries they actually begin to form right around the fifth week of development. So how does this process happen? Well if you really think about it there are some structures right. So if you if you kind of look at the fetus and you look at the posterior abdominal wall right. So like looking the back of the fetus around the abdominal area there are some structures in that region that are called the urogenator ridge right. They are called the urogenator ridge right.

They actually derived from misoder and really if you look at this term urogenator ridge it should basically give you some clues that this leaves structures right they give rise to genital related entities and kidneys remember kidneys urine euro right kind of put those put one two together there right. So kidney related structures genital related structures they actually come from these are urogenator ridges that's why many times when people have like reproductive organ and atomic problems they also will usually have problems with your kidneys or with your collecting systems. So the thing is this urgen these are urgenitor ridges they're actually not going to be unique in the beginning and becoming testes or ovaries they actually have the ability to become one or the other right. So the thing that makes that determination right as to oh you know what you're going to become an ovary you know what you're going to become a testes they actually some cells that come from the yoke sac those cells actually derived from endoderm right. So there are some endodermal direct cells that come from the yoke sac. Now then tell these urgenitor ridges you're going to you're going to become an ovary you're going to become a testes right. So the thing is these cells that are you know dry from endoderm that come from the yoke sac they could either have an xx chromosome right. Obviously that's female or they can have an xy chromosome that's male.

So the thing is they basically come from again the yoke sac to the urgenitor ridge the highway that kind of gets them there is your medical cord right. And one thing I guess maybe I should say is the urgenitor ridge if you really think about it the default program is to become female genital structures but again these endodermal lead derived cells have a big say big seen that process right. So again they will come from the yoke sac through the umbilical cord to the urgenitor ridge right. So if those endodermal lead derived cells are like xy well the presence of that y chromosome is what will then determine that oh you know what go ahead and form male genital structures the thing is the presence of that y chromosome is what tells you go your urgenitor ridge become male versus if there is an absence of the y chromosome that's what the term means oh urgenitor ridge become female. So it's very important to understand that you know again that y chromosome is what makes you form male genital structures but the thing is in addition to that y chromosome being there you do need to produce certain things so that you can shut down the urgenitor ridge from becoming like that pre-programmed female thing right. So that's how many of these things work and I'll talk about these specific factors right. So the thing is if you look at the y chromosome why is the y chromosome important.

So the thing is if you look at the y chromosome it actually has this tiny little genetic segment called the SRI region many of us have heard about it right. SRI, SRI literally just means six determining region on the y chromosome right. So essentially this SRI region it actually quotes for a protein that's called test is determining factor. So the thing is this test is determining factor as those cells that come from the endoderm from the yoke sac right as they come towards the urgenitor ridge right. The release of this test is determining factor by those cells is what converts that on differentiated urgenitor ridge to a differentiated test is right and remembering the test is they are two very important cell types right. There's the lady cell which is very important and then there's the certulli cell and exact what exactly do these lady cells do. These lady cells excuse me they make testosterone right and then they also make a more potent derivative of testosterone that's known as DHT. Remember the thing that makes you convert testosterone to DHT is five-offer adoptees. Remember DHT is responsible for the growth of the prostate that's why we use five-offer adoptees inhibitors like finasterite and deutastorite to treat BPH right. Benin prostatic hypereplegian right. So again lady cells they make testosterone right and again they can make it's very potent causing DHT dihydrotestosterone.

So basically by releasing this testosterone right again is going to convert other elements of the urgenitor ridge again to male genital structures but remember so that's objective number one. Objective number two comes from the certulli cells the certulli cells they actually make more lyrian inhibiting factor. More lyrian inhibiting factor sometimes it's called anti-molirion hormone on MBME exams. What it does is it basically prevents the urgenitor ridge from becoming those pre-programmed female structures right. So again to become a guy you need two things to happen. One you need to become male and then two you need to prevent female development right. Again that comes from molirion inhibiting factor also called anti-molirion hormone that comes from the certulli cells right. And one thing I will just say is those cells that I talked about again that are endordomally derived that come from the yoke sac and then demigrate through the umbilical cord to the urgenitor ridge. They actually become gametes in the end those are the things that ultimately become become your gametes right. So again those cells just to summarize the old calm from the yoke sac they are derived from endoderm right they are like primordial germ cells. They are derived from endoderm they will come through the yoke sac go through the umbilical cord go to the posterior abdominal wall in the fetus right which continues your urgenitor ridge right.

And then that the presence of test is the trombinine factor right. And molirion inhibiting factor you go down that test is pathway you from the male genital system right. But if you don't have those factors you go down the ovary pathway and from the female genital system right. From the paramedic onephyric duct right the molirion duct remember the the duct for the guys is the is the wolfian duct right is the wolfian duct. Okay so now uh so you know let's say fertilization happens right. Although again remember for fertilization to happen the sperm must meet with the egg right. So I think maybe one thing that maybe hopefully is to kind of talk about the process of gameteogenesis in females right. I think that's the one that's probably especially important. The males can always be discussed later right. But remember those germ cells right those primordial germ cells they are deployed right they're called ugonia right. O ugonia like W o g o n i a right. So ugonia right. So they are deployed right. So they are 2 N they have 46 chromosomes in them right. So the thing that happens is you'll have the process of DNA replication that will happen at first and then you'll form primary o-sites right. You'll form primary o-sites. Remember those primary o-sites contain 46 chromosomes but now you're at the 4 N stage because you're basically duplicated each each chromosome right. Now the thing that happens is that you have an arrest at this primary primary o-site phase.

You actually have an arrest in my uses 1 right in prophes of my uses 1 until the lady hits puberty right. And then when puberty happens right. You then have you then have my uses 2 beginning to happen right. So that primary o-sites will divide and form secondary o-sites right. Remember those secondary o-sites I'm gonna have 23 chromosomes they're gonna be 2 N right. 23 chromosomes they're gonna be 2 N right. And the thing is in following that secondary o-site because you think that o- divine from the primary o-site which was 46 for N will form 2 secondary o-sites. No that's not true. You actually form 1 secondary o-site and then the second thing that is supposed to become the secondary o-site becomes a polar body right. So that 1 secondary o-site that survives right. Then starts dividing again but again it's gonna be arrested one more time. It's gonna be arrested in my uses 2 right. It's gonna be arrested in my uses 2 in metaphise of my uses 2. So remember the first arrest happens in prophase of my uses 1. The second arrest happens in metaphise of my uses 2 right. And when that arrest then happens you then need to wait for the sperm to meet the egg right. When the sperm meets the egg then that secondary o-site will then complete my uses 2 and then you'll form the matro o- that is 23 chromosomes and is 1 N it's haploid right. It's haploid and then you'll actually form a second polar body as well. So you don't form too much over from 1 secondary o-site.

No you form 1 matro over right. And the second polar body after fertilization right. So the sperm needs to meet the egg for the egg to complete my uses 2 to basically proceed from metaphise of my uses 2 right. So let's say fertilization then happens right. So usually where those fertilization happen. Usually fertilization is gonna happen like in the ampula of the fallopian tube. Remember another name for the fallopian tube when in beam exams is the o-v-duct right. So you have fertilization between the sperm and the egg happens in the ampula of the fallopian tube right. Then you're gonna form a zygote and that zygote is obviously deployed right. The thing that happens with the zygote is that it begins to divide very very very rapidly right. The thing is in regular mitosis that we're used to learning right. Regular mitosis as you're having replication happen then you'll have an increase like a doubling of the size of the cytoplasm. That actually does not happen at all with my doses of the zygote. The zygote actually has this kind of cell division called cleavage division. Basically you're increasing the number of cells but you actually not increase the amount of cytoplasm that's very high to know. You're increasing the number of cells but you're actually not increasing the amount of cytoplasm right. You're not increasing the amount of cytoplasm right. So the thing is the zygote it keeps dividing dividing dividing dividing until it becomes a solid ball of cells.

That solid ball of cells is the 16 cell phase. That's what's known as the morilla okay. Is the 16 cell phase is known as the morilla. Then the thing that will happen is you keep having again more division more division more division. The thing is as you keep dividing you then that morilla will then form like a whole of like it's almost like a hollow right like a cylinder right. And inside that cylinder there's like fluid in it right. So because there's fluid in it is almost like a cystic structure is called a blastocyst right. The thing is that blastocyst is actually like the first level of specialization in the embryo. That's very high to know that blastocyst is actually the first level of specialization in the embryo right. So the thing is the two cell types that make up that blastocyst right. You have an inner cell mass because basically that blastocyst has like two populations of cells. First population of cells they are called the inner cell inner cell mass right. So you're gonna have the inner cell mass that is known as the embryo blast right. The thing is whenever you see the term blast in something it means that it has the ability to give rise to other stuff right. And since this is the embryo blast that means oh it has the potential to give rise to the embryo which it does right. And then you have the outer cell mass right which is known as the truffle blast right. Truffle means nutrition right.

I mean you've probably learned of trophism from maybe like biology in college or whatever right. But the truffle means nutrition right. So that truffle blast is well ultimately from the placenta right. So again you start at the 16 cell phase the moreyla then it keeps dividing and then you form a whole ball of cells right. That whole ball of cells has fluid in the center and then that whole ball of cells has two cell populations and inner cell mass called the embryo blast. We becomes the embryo and then an outer cell mass which becomes the truffle blast. Again remember the word truffle means nutrition is gonna from the placenta. Now right around these six right those truffle blasts excels. The thing they actually do is they invaded the endometrial lining of the ladies' uterus right. They will invade the endometrial lining and again when they invade those truffle blasts will then form two more cell lines right. So the first cell line before me something called the cytotrophoblast right. Look at the name cytotrophoblast. cytotomy cells right. These cells actually have the ability to just keep undergoing continuous mitosis continuous mitosis continuous mitosis but they also form the syncytrophoblast right. The thing is the syncytrophoblast cells remember the word sensation. Sensation means like a network of cells right. Sensation syncytrophoblast cells they actually the cells that have direct contact with the mom's uterus. This is very high autonome right.

This is extremely high autonome. Between the cytotrophoblast and the syncytrophoblast the one that has direct contact with mom's cells with mom's uterus with the endometrial lining of the mom's uterus at the syncytrophoblast right. So the thing is so you may be like okay the thing was the point of the cytotrophoblast. The cytotrophoblast their job is to actually replenish the syncytrophoblast then syncytrophoblast by the way spelled as C no syn cytiotrf no synsysio so syn yn cytiotrf p-h-o-b-l-a-s-t syncytrophoblast right. So the cytotrophoblast is the thing that replenishes your syncytrophoblast but then the job of the syncytrophoblast is actually to produce beta-hcg right. It's actually to produce beta-hcg right. Remember the job of beta-hcg is to maintain the corpus lurium right because remember the corpus lurium on its own is almost like one friend helping out the other. Syncytrophoblast will produce beta-hcg which maintains the corpus lurium and then the corpus lurium then meets progesterone look at the name progesterone it is pro gestational the progesterone from the syncytrophoblast is what then maintains the endometrial lining right. And the thing is that beta-hcg that comes from the syncytrophoblast that's actually what you detect in the urine pregnancy test right. So if you notice if you do urine pregnancy test like at B2 of conception you're not gonna see any effect right.

You need to wait around the 6 days 7 when the syncytrophoblast starts producing beta-hcg for you to detect pregnancy through the urine pregnancy test. Again this stuff is very very high-yotes to know right. Very very high-yotes to know. So again remember Zygot right is deployed for the six chromosomes. It keeps dividing dividing dividing right you have a group of 16 cells that's the morial right. And then the five you're from the blastocyst the blastocyst has an inner cell mass which is the embryo blast and an outer cell mass which is the trophoblast. The six that's right around where implantation happens right. So again just very important to make sure you understand these things if you do the thing about embryology is embryology just involves a lot of processes right a lot of just daunting processes sometimes but if you have it organized and you repeat it often enough you begin to get it into your memory easily that's why you see I'm repeating things and summarizing things over and over again over and over again right. So now let's keep marching on right so we've dealt with week one right. Now week two right we will remember I said earlier at the beginning that only week two you form a two layered embryo that's a nice way to remember that and week two you form a two layered embryo right so what in the world happens in week two.

The thing is in week two remember those two cell populations I talked about I've already talked about the trophoblast and how it forms the cytotrophoblast and the syncytotrophoblast the thing is in week two you have the embryo blast right remember the embryo blast was that inner cell mass right so that embryo blast actually differentiates into two cell types right so remember the trophoblast differentiated into two cell types cytotrophoblast and syncytotrophoblast the embryo blast on its own differentiates into two cell types right the two cell types one is the epi blast the epi blast is like right on the top right remember whenever something is on top that's the dorsal orientation anatomically right and then the second cell population it forms is the hypoblast hypoblast is below it's ventral right so really to be honest with you the best way to visualize this because this is an audio podcast but I still want you to understand is just think of these things as having some kind of vertical orientation the thing that is on top right an upside down orientation the thing that is on top is the epi blast right is at the top and when something is on top it's dorsal the hypoblast is at the bottom it's ventral right so the thing is it's actually very high you to know that the epi blast is what ultimately gives rise to all the cells that will eventually constitute the embryo right the epi blast is the thing that ultimately gives rise to all the cells that will constitute the embryo right now the thing is for the most part that epi blast and hypoblast again they are just stuck on top of each other right they just form these two distinct cell layers the thing is at this point we don't know what is the head of the fetus we don't know what is the tail of the fetus right we don't know any portion that oh you know this side is the head this side is the tail we don't know right so how

can we fix that problem how can we establish some kind of polarity like oh you know what this will be the fetal head oh you know what this will be the fetal tail the thing that happens is that though that epi blast and hypoblast that are stuck on top of each other there is actually a point where they fuse the fuse right so like just think of again like two arrays of cells and then there is one point in that are really really really fuse together that place place where the fuse together is called the pre-cordle pleat right the pre-cordle pleat the thing is this point of fusion right is what will ultimately determine where your future head or your future mouth is going to be right really like this is like the thing that gives rise to like oh you know we're going to define what is the head of this fetus we're going to define what is the tail of this fetus right so the thing is the body is almost like you know what we have epi blast on top we have hypoblast at the bottom let's just pick a spot bam and from this spot we will then define every other thing of the body in relation to this one spot right so that one spot is called the pre-cordle pleat right it's called the pre-cordle pleat again that's very high you to know and that's very high yield to understand again it's a point of fusion between the epi blast cells which are dorsal which are on top and the hypoblast cells which are ventra which are below remember the epi blast and hypoblast they are derived from the embryo blast which was that inner cell mass I talked about in relation to the in relation to the to the blast to system right so that's essentially what happens in week two right so what in the world happens in week three well in week three remember I said in week two you formed a two layered embryo when in week three you're going to form a three layered embryo right you're going to form a three layered embryo

and again remember what I said earlier I said that oh between the epi blast and the hypoblast is the epi blast that is going to give rise to all of the future embryo right so the thing is again let's continue from let's continue our journey from that pre-cordle pleat right so remember the pre-cordle pleat is where the future mouth and the future head is going to be right so once you know that oh this is the place where the future mouth this is where the future head is going to be you know that that has to be the cranial part of set fetus right and then any other thing that is like distal to that you can then begin to make denotations as to oh this is this this is this this is this for the fetus right so what happens to that cranial part this pre-cordle pleat part well the thing that happens is that the cranial axis of the fetus right it keeps enlarging enlarging enlarging enlarging in large in fact it grows a lot bigger than the cottolaxis right it grows a lot bigger than the cottolaxis so the thing is if you really look at a fetus right if you let's say you become a new parent or whatever right and you're looking at the baby at like eight weeks nine weeks ten weeks usually you see like a part of the baby's body that is very big that is actually the head again a lot of that starts around week three the baby's head grows a lot faster than the rest of the body but obviously that's not going to be the case for the whole of pregnancy right that's obviously not going to be the case for the for the whole of pregnancy so the thing is so again that pre-cordle pleat tells us where the head is right so that's the cranial axis of the fetus the caudal axis of the fetus where we're going to form the tail in a region around that caudal axis of the epi-blast one thing that happens is you begin to have some cells dying right so let's say again we have the epi-blast on top we have th

e hypoblast at the bottom some cells around the epi-blast they just die right the thing is when cells die the body then reabsorbs them and that creates a hole literally that creates a hole right that creates a hole this is very high you to know right so again in the epi-blast somewhere around the caudal axis right around the tail region of the epi-blast some cells begin to die some cells die as they die you create a hole right an invagination so the thing that then happens is that the place where those cells die right gives you a structure right a structure begins to form within that epi-blast called the primitive streak right it is actually very important to think of this primitive streak as just like a highway that is being created I'll say that again this place where you have the selective death of cells of the epi-blast it creates a hole that hole essentially forms a highway right think of that fancy highway as being the primitive streak right because basically that highway you create that hole you've created allows some cells from the epi-blast above to gain access to the region of the hypoblast they use that hole it's almost like they fall from the top and fall towards the bottom right so you create a fancy highway of sorts for those epi-blast cells which are bursally oriented on top to gain access to the region around the hypoblast below which is ventrum right so the thing is cells from the hype from the epi-blast right again they migrate through this primitive streak hole right they migrate downwards they migrate downwards right and the thing is as they migrate downwards they'll begin to push the hypoblast out of the way remember I said that is these epi-blast cells that ultimately form the embryo in the future right see these e-structures from the embryo right embryo blast epi-blast right you see they both stab with ease that's a nice way to keep that straigh

t right so those epi-blast cells they migrate through this highway they migrate downwards so they migrate inventory right and then they push the hypoblast cells out of the way right so those cells those epi-blast cells that come in through that hole and displace the hypoblast they didn't take over that layer that was previously occupied by the hypoblast those epi-blast cells that take over that layer will ultimately form endorder right they'll ultimately form endorder I'll see that again they'll ultimately form endorder that's the first layer right now remember I said that oh remember I said earlier on that again you had the epi-blast on top again I'm repeating myself so many times just to make sure that you fix this facts in your mind right and you understand it remember I said that oh initially you had the in week two you had the epi-blast you had the on top right door saw and then you had the hypoblast on the bottom ventral and we said that they fused in one spot and that place where they fused is the pre-cordle pleapt well the thing is they fused in one spot that means they did not fuse in other spots so there's literally some space between the epi-blast and the hypoblast so the thing is that potential space that existed between the epi-blast and the hypoblast again some of those cells that are coming in through that fancy highway through that primitive streak they come and feeling that in between space the cells that feeling that in between space actually the cells that become misoder right those are actually the cells that become misoder right so again we've already talked about the cells that feeling the space that was occupied by the hypoblast that's endorder and in the space between epi-blast and hypoblast the epi-blast cells that fill up that space that have traveled through that highway right that highway of the primitive streak become misoder right and the

n the cells right that fill up that you know that middle space that become misoder they actually arrange themselves in a defined orientation they arrange themselves like in a lateral orientation and in a medial orientation so some cells are closer to the middle they are closer to that primitive streak and then some other cells are lateral beyond the sides right beyond the sides the thing is those misoderum cells that are the most medial right again remember this is the space that was between the epi-blast and hypoblast so those misoderum cells that are the most medial they actually form a structure right that structure is known as the nodal cord right you'll see that the nodal cord is extremely important right and actually it's very high you to know for the USML exams that that nodal cord is what becomes the nucleus purposes of the interpretive rodisc and the adult right now those cells of the epi-blast that did not migrate because remember I said that oh some cells of the epi-blast they come down through that hole that highway of the primitive streak they displace the hypoblast those become endorder some staying that space between the endorder and the misoder I mean between the epi-blast and hypoblast they become misoderm and we said that the ones in the middle of that misoderm layer from the nodal cord right because some of them are arranged medially they're very close to the primitive streak some are arranged more lateral right now the thing is the epi-blast cells that remain at the very top that just never migrated in the first place they actually acquire a new name we call them ectoderm right we call them ectoderm and the thing is just like misoderm some of those cells in the ectoderm layer we actually define with respect to their location along like a medial lateral axis right around the medial lateral axis so the thing is those cells from the epi-blast that did

not migrate that are most medial the ones that are close to the primitive streak they actually become neuro ectoderm I'll say that again they become neuro ectoderm again this should make sense because that nodal cord remember the nodal cord I said is like almost like the most medial part of your misoderm layer that nodal cord it actually releases factors during the embryonic period that causes differentiation of the ectodermal cells just above it right so just in the middle just above it right in that epi-blast layer right and those cells become neuro ectoderm right remember the neuro ectoderm is what will ultimately give rise to all the cells of the CNS and the cells of the retina right and then the thing is those cells that are ectoderm that are most lateral right if you're going lateral away from the primitive those are cells that actually become the neuro crest cells right they become the neuro crest cells and remember those lateral line cells right they're on the periphery of the yet because remember I said neuro ectoderm forms the CNS the central that that's literally that's literally like a nice way to remember that oh central nervous system neuro ectoderm it's in the center it's in the middle right and then if you go for the lateral you're having cells on the periphery right they're literally like on the periphery of the central nervous system right that's why it's called the peripheral nervous system right so you see many things in life they just make sense if you understand it in fact I'm smiling as I'm explaining this now like you just make so much more sense as you begin to actually like wow oh so this term central nervous system didn't come out of thin air this term peripheral nervous system didn't come out of thin air right so again those cells that lie lateral right they give us to the peripheral nervous system and give us to shwan cells right and the

thing is the most lateral the most lateral of all these ectoderm cells they are many times we describe them best as a surface ectoderm right as surface ectoderm so the thing is this overall process where we have found the endoderm right that's layer one the mesoderm that's layer two and the ectoderm that's layer three that process is referred to as gas relation gas relation as a process happens in week three of development right and then the thing that ultimately happens is again that flat embryo that's arranged in a vertical axis as I've described earlier right it then folds laterally to form a tube within a tube within a tube within a tube right so this flat embryo is folds laterally right and then form a tube within a tube within a tube obviously the inner most tube is going to be the endoderm the outer most tube is going to be the ectoderm the tube in the middle is going to be the mesoderm right so again remember the ectoderm and endoderm they are actually the things that all form epithelial tissue right the mesoderm is what ultimately forms connective tissue right it literally connects the ectoderm to the endoderm right and again the thing is unfortunately the embryonic layers you kind of need to know their derivatives for the USML exams but again I will encourage you to not just straight up try to memorize them the thing is as you learn as you're studying for your USML step one exam as you learn the different like embryology of the different body systems try to pick up those derivatives in that format that's like the best way to remember those things right well let me give you some useful generalizations that I feel will help you out with a lot of the reasoning a lot of the reasoning the thing is first things first ectoderm and endoderm from epithelium that's a given mesoderm forms connective tissue right so things like think of the connective tissues in your bo

dy right bone muscle heartilage blood vessels dermis the heart right those things are all the right from mesoderm right they are all the right from from mesoderm right that's very high you to know right and then the endoderm remember it forms the epithelial lining of the major tracts of your body right like your respiratory tract the lining of your GI tract even a portion of your your genital tract comes from endoderm right and they remember that ectoderm which is on the outside right it's going to form the epithelial lining of the skin right so like the epidermis remember your epidermis is what comes from ectoderm your dermis is not derived from endoderm from ectoderm your dermis is derived from mesoderm right and then again that ectoderm also gives rise to a lot of neuro structures right so you know gives rise to neuro structures right so again let's just summarize right so we start off at the egg and sperm come together from the zygote zygote becomes moreyla moreyla becomes a blastocyst which has an inner cell mask that becomes the epi blast and an outer cell mask that becomes a truffle blast the truffle blast gives rise to the side of truffle blast and the sincere truffle blast the inner cell mass right or the embryo blast gives rise to the high pool blast which becomes not useful and the epi blast which undergoes the process of gas relation in week three to form ectoderm, mesoderm and endoderm so I think I'm going to go ahead and stop here but that's pretty much a nice overview of general embryology so that you can just know like because again the body just did not happen randomly right you know it kind of came from came from somewhere so I'm going to go ahead and pause here again as I do at the end of every podcast I do offer review courses for the USMLA exams if you're interested in any of them just shoot me an email through the website and I'll give you some m

ore information but then I do have a You Tube channel Divine Intervention USMLA Podcasts and videos that's where I post the videos that I make subscribe to that and if you hit that bell right you'll get a notification whenever I make a video and but if you want the slides do associate with those videos go to the website divineinterventionpodcasts.com and the thing is if you subscribe to the website whenever I make a new podcast you'll actually get an email notification and then I do have these podcasts on Apple podcasts on Google podcasts and on Spotify again if you subscribe to those you get those you get my new podcast in your in your podcast feed at least the most recent 150 it's a it's a role of try to circumvent there's really no way around it right so if you want all my podcasts from episode one then you would need to go to the website everything from episode one is on the website you don't need any logins you can access them you can download them right but again one thing that be helpful is if you subscribe to the website that support definitely helps me but then in addition to that if you if I make a new podcast you you get an email notification and then again many of you are used to my life lessons I do have a new website that I've formed Divine Intervention Life Lessons in fact those I make podcasts again many of you listen to this podcast know that I'm a Christian right so I put like some just very short podcasts pretty much all of them are going to be less than 10 minutes for the most part I've made three episodes right now you know one episode one was on was on focus episode two was on diligence episode three which I just made this morning what is on stewardship so if you subscribe to those you know I have it on Apple podcasts right and again the website is Divine Intervention Life Lessons.com if you have any questions just feel free to reach out to me and

I'll be more than happy to point you pointing the right direction so thank you for listening to this podcast I hope you you've gained a lot from this and you have a very thorough solid understanding of embryology so I'll see in the next podcast thank you good bless you

Ectoderm, Mesoderm, and Endoderm.

The Trophoblast (specifically the cytotrophoblast and syncytiotrophoblast).

To maintain the corpus luteum.

The primitive streak.

Inner Cell Mass (Embryoblast $\rightarrow$ embryo) and Outer Cell Mass (Trophoblast $\rightarrow$ placenta).

Cleavage increases cell number but does not increase the amount of cytoplasm.

Endoderm.

Testis-Determining Factor (TDF).

Neuroectoderm.

Cytotrophoblast.

The ampulla of the fallopian tube (oviduct).

Folding/Coiling (or Gastrulation leading to folding).

Endoderm.

Testis-Determining Factor (TDF).

Neuroectoderm.

Cytotrophoblast.

The ampulla of the fallopian tube (oviduct).

Folding/Coiling (or Gastrulation leading to folding).