in this video we'll talk about nervous system embryology let's get started the important thing to note here is that you have something called a blastula and over time that blastula will sort of fold into itself shown here in red in doing so the blastula will give rise to a blasto pore for the purposes of nervous system embryology we don't really care about the blastopore because that goes on to form a cavity for the GI system but what we do care about is that after the blastula folds into itself we get the formation of three germ layers those three germ layers are the endoderm the mesoderm and the ectoderm and in talking about nervous system embryology we are particularly focused on the ectoderm the ectoderm is what goes on to become the entire nervous system so here's where we are right now we've got an ectoderm shown in purple mesoderm shown in Orange and I'm not even showing you the endoderm because it's kind of Irrelevant for the purposes of this embryology discussion so between where the mesoderm sits you get the development of something called the notochord and the notochord will secrete growth factors and those growth factors will seep out and act on the ectoderm those growth factors are going to tell the ectoderm to start to differentiate into a neural plate now as that neural plate begins to form in the center it pushes out the ectoderm laterally the combination of the ectoderm moving laterally and the growing neural plate forming in the medial section this is all referred to as the neuro ectoderm because the ectoderm is giving rise to all of the nervous system now what's going to start to happen here is that the neural plate will sort of start to fold over and as it grows and continues to fold it's going to push the ectoderm up and out and it's going to look like this so that growing if we're on this side that growing neural plate starts to fold and then the ectoderm forms these folds that will curve out at the top now between the ectoderm which is shown in purple and is continually moving out laterally and folding over and the neural plate which is shown in teal which is growing in a sort of circular fashion that area in between them shown here with the brown circles that is going to ultimately give rise to neural crest so again in teal we have neural plate in brown we have the tissue that will ultimately become neural crest and in purple we have the ectoderm that's folding out laterally and this is referred to as the neural folds now as these neural folds start to invaginate they sort of pinch medially and as this happens the neural plate area of the neural folds will move toward the midline and so when all of these motions are happening in 3D in real time you're slowly going to get the formation of a circular object and what we end up with is a neural tube which was derived from the neural plate the neural crest which sits right above it which was the area in between the ectoderm of the neural fold and the neural plate of the neural fold and then we've got the remnants of the ectoderm up on the top and so this entire process that I've just described is what's known as norulation and on your exams nor relation is really the meat of all of your questions so if they're going to ask you a black and white question about embryology in terms of how these things are folding and moving it's probably going to be about neuralation so here's where we are and let's pause for a moment because what becomes really high yield is not only knowing how neuralation Works in terms of the neural ectoderm giving rise to the neural crest and the neural tube but also pathology that can occur if this doesn't happen the way that it's supposed to so embryology if you want to really be a reductionist here and think about this in a really simple and stupid kind of way embryology is normal but when embryology doesn't occur normally you get pathology and congenital abnormalities so let's talk about spina bifida spina bifida in general refers to varying degrees of congenital abnormalities that result from the failure of the neural tube to close quarterly in other words if neuralation as I've just described it doesn't occur the way that it's supposed to one of the manifestations of that is spina bifida so let's go and talk about the three different types of spina bifida we'll start with spina bifida occulta spina bifida occulta there is no protrusion of spinal contents okay so look at the image below there's three different types of spina bifida there's spina bifida occulta spina bifida with meningocele and spina bifida with Milo meningocele and as you move from left to right across these three different types it gets more severe so in spina bifida occulta no protrusion of the spinal contents will be coming out through the vertebrae and in this case Alpha feed a protein when it's measured will be normal you would only see elevated levels of alpha feta protein if there is an open hole in the low back through the vertebrae because that's where the AFP will leak out and that's where it's measured now in these patients with spina bifida occulta you could see a tuft of hair or a sacral dimple right in that area of the low back where in more severe versions of spina bifida you would see protrusion but again in occulta it's called occulta because it's a cult you don't see it it's kind of there hidden with no protrusion so this is the most benign version of spina bifida now in spina bifida with meningocele we do see protrusion of the meninges and because we're seeing protrusion and because there's that opening in the low back levels of alpha feta protein will be increased if measured and in these patients the spinal cord is undamaged and that's important to know because even though you have protrusion of something it's just the meninges it's not the spinal cord itself so look in the image the spinal cord is still in the canal where it's supposed to be it's just the meninges that are protruding through lastly we have spina bifida with Milo meningocele and this is the most severe type of spina bifida in myeloma meningocele you see protrusion of both the meninges and the spinal cord and because of this because there's a hole in the low back fluid can leak out so if measured you'll see increased levels of alpha feta protein what's very important to know for exams is that spina bifida with myelomaingocele is highly associated with Chiari type 2 malformations now these are the three different types of spina bifida and I want to show you this summary chart so that you can keep all of this straight again occulta is a cult it's not protruding through it's just there in the low back so there's no protrusion and AFP is normal in these patients you could see a tuft of hair or sacral dimpling in spina bifida with meningocele it's meninge a seal it's just meninges so protrusion of the meninges through the low back there's a hole there so AFP will be increased it's just meningocele so it's the spinal cord is fine it's just meninges in spina bifida with Milo meningocele it's Milo plus meningoceles so it's spinal cord myelo plus meningo plus meninges so it's protrusion of meninges and spinal cord there's a hole in the low back so AFP is increased and this is highly associated with Chiari 2 malformations so these are the three different types of spina bifida again this pathology occurs if nor Elation does not happen normally the other pathology I want to talk about is anencephaly and this is a really sad and unfortunate congenital malformation where you have partial absence of the brain calvaria or skull because the neural tube didn't close at the cephalic end so spina bifida is the is at the other end not this phallic end but anencephaly is the cephalicin right so it's up at the top where the head would be so in anencephaly you can you really see incompatible with life right so this isn't compatible with survival if if a developing baby has anencephaly then unfortunately it's it's either going to be stillborn or it will probably pass within a few hours of birth and in anencephaly you're going to see increased levels of AFP and polyhydramnios again there's a hole up where the head should be and that allows the leakage of fluid so we'll see increase AFP and polyhydramnios now again this is what we were talking about so we were talking about neuralation and this is where we ended and what's important to note now is that at that neural tube at the top of the neural tube or that black star is shown this is what's going to lead to primary vesicle formation so let's let's talk about that now so primary vesicle formation is the formation of the developing nervous system and the first thing that's going to form are three primary vesicles so we have the prosencephalon which gives rise to the forebrain the mesencephalon which gives rise to the midbrain and the ram encephalon which gives rise to the hindbrain again all of this is derived from the top part of the neural tube now each of these three areas shown in different colors for your convenience gives rise to more and more structures so the prosencephalon will give rise to both the telencephalon and the diencephalon the mesencephalon will give rise to the mesencephalon so that's pretty easy to memorize the ramencephalon will give rise to both the the met encephalon and the myel encephalon now I'm going to pause for a moment I have another video on my channel that goes through this and gives you mnemonics to remember all of this so I'm just describing the embryology here but if you want the mnemonic for how all of this is easily memorized then see that other video so now we have the telencephalon diencephalon mesencephalon met encephalon and myel encephalon and these are going to give rise to neural derivatives okay so just just to keep things stupid and simple we talked about these are your primary vesicles they give rise to secondary vesicles here are your secondary secondary vesicles that will give rise to neural derivatives okay so the telencephalon will give rise to the cerebral hemispheres and the Globus pallidus and the diencephalon will give rise to the thalamus and the hypothalamus the mesencephalon the met encephalon and the myelencephalon give rise to midbrain ponds cerebellum and medulla and if you're a little confused right now in terms of how you're supposed to learn all of this and keep all of this straight aside from using my mnemonic which I have in my other video on this topic really if you know neuroanatomy this should make sense because the telencephalon is at the top the myelencephalon is at the very bottom and then you got varying degrees of these vesicles in between and so from top down if you know where the cerebral hemispheres are what the Globus pallidus is Thalamus hypothalamus as I go down this list etc etc I'm moving from top to bottom so if you know that the telencephalon is at the top and the myelencephalon is at the bottom and then you're asked which one gives rise to the medulla well in your head you could be like I know that the medulla is at the bottom or the base of the brain so which of the secondary vesicles is is at the bottom or is at the base and so Mile and cephalon gives rise to medulla conversely if you're asked what gives rise to the cerebral hemispheres in your head you could be like well I know that the cerebral hemispheres are sort of like the this at the outside or at the top of the brain and so it's probably the top most secondary vesicle so it's probably tell encephalon and so it's easier to guess if you have a rough idea of the general neuroanatomy from top to bottom and that's a theme that will carry over into the next slide as well so these neural derivatives each give rise to cavities or where CFS or CSF flows that's a tongue twister so the telencephalon goes to cerebral hemispheres and Globus pallidus that gives rise to the lateral ventricle the diencephalon gives rise to the thalamus and hypothalamus that gives rise to the third ventricle mesencephalon gives rise to midbrain that creates the cerebral Aqueduct met encephalon Pond cerebellum that gives rise to the upper part of the fourth ventricle and the myel encephalon gives rise to the medulla that gives rise to the lower part of the fourth ventricle and the central Canal so just like we saw with the neural derivatives if you have a rough understanding of top to bottom neural Anatomy you can make a very educated guess about with which ventricle or which cavity is formed from which structure and so here is an image of where how CSF flows from top to bottom so lateral ventricle to third ventricle to cerebral Aqueduct to Fourth ventricle to Central canal and top to bottom you see on this slide it's identical to this slide so tell encephalon at top then diencephalon mesencephalon met encephalon myel encephalon top to bottom and so keeping this color-coded top to bottom we go lateral to third cerebral fourth Central again the takeaway here is that you can either memorize it you can learn a mnemonic from my other video or you can just if you know neuroanatomy work top to bottom this should all make sense so those are the derivatives so here's what we talked about the top part of the neural tube gave rise to primitive vesicles or primary vesicles those gave rise to secondary vesicles those gave rise to neural derivatives and those gave rise to cavities so that's what we just talked about that was all from the top part of the neural tube the bottom part of the neural tube that gives rise to the spinal cord not really a lot to talk about there just know that the bottom part of the neural tube becomes the spinal cord top part of the neural tube basically becomes everything else so I flew through that but that is nervous system embryology again I'd probably re-watch this video once or twice because it is a complex topic but at the end of the day if they're going to ask you questions about this they'll either ask you about how neuralation Works they'll ask you about pathology you know spina bifida that sort of thing they'll ask you about the derivatives from the top of the neural tube or they'll just be really really nice to you and they'll say where does the spinal cord come from and that's the bottom of the neural tube good luck