Head & Neck Anatomy | Embryology & Pharyngeal Arches | INBDE

hey everyone ryan here welcome to this brand new series on head and neck anatomy now we're going to cover a lot of great content here and this material appears on the integrated national board dental exam it also appears on the part 1 board exam and it also comes up on a lot of other exams both medical and dental now that being said there are a lot of great resources out there to learn anatomy and i'll be posting and referencing those resources throughout this series if you ever want to check them out for some more high

quality information but my job here is to present this information so that's easy to understand and gives you the high yield facts that you need to know for your dental board exams so with that let's get started with some embryology so we're going to start from the very beginning that being the germinal stage otherwise known as the egg stage now this is from fertilization to the second week after fertilization this is also when a miscarriage is most likely to happen now as we go through the steps of this process i'm going to have in the

top right the week that we're currently in and that weak number is based off the starting time point being fertilization and fertilization occurs when the sperm and egg unite in the fallopian tube to create a diploid cell called a zygote before this though ovulation had to happen which refers to the release of an egg from the ovary now over the next few days that zygote cleaves into a ball of two cells and then four cells and then eight cells and so on to form what's called a morula that morula will eventually develop an inner cavity

or space called a blastocell at which point it becomes a blastula it can also be called a blastocyst in mammals that blastocyst is going to eventually implant itself into the uterine wall implantation occurs when the blastula contacts the endometrium or that uterine wall now we're still in week one here but the blastula is going to get a little bit more complex so we have an embryoblast which refers to the inner cell mass that's in green that will become much of the embryo proper so basically it's going to be differentiating and dividing into cells that become

much of what we consider the embryo there's also the trophoblast that's this outer purple layer and that outer cell mass will form much of the placenta so let's zoom in on this blastocyst and get a little closer look so now we are two weeks in and at two weeks we now have two layers it's an easy way to think of it week two we have two layers and that inner cell mass becomes a bilaminar disk that consists of those two layers an epiblast and a hypoblast so if we look at this image we have two

cell layers right at the center comprising this disk the upper layer is the epiblast this lower layer is the hypoblast now you also notice two cavities that have formed here this white cavity up here is called the amniotic cavity and that will house the developing baby the yolk sac cavities this yellow one down here and that will provide nutrients and gas exchange before the placenta eventually takes over and that placenta remember is derived from the trophoblast from the last slide so the epiblast is going to do most of the work here it's going to become

the three germ layers of the eventual embryo the hypoblast not so much it's going to disappear and not really contribute to a whole lot amniotic cavity and yolk sac cavity we talked about those and there's also what's called a primitive node and primitive streak on the dorsal surface on the caudal side of the embryo which basically identifies the midline it separates right from left we'll get a closer look at what that actually looks like in the next slide also notice at this point in the process implantation is now complete the entire egg is completely within

the endometrium now also by the end of the second week we have this space that's kind of surrounding the developing egg that's called the extra embryonic selum or a chorionic cavity and as that cavity expands we eventually have this connecting stalk which eventually becomes the umbilical cord that is suspending this developing egg so for week three let's zoom in on this part of the diagram the most exciting part where most of the action is happening so now that we're three weeks in we have three layers so remember week two we had the epiblast and hypoblast

in week three we now have three layers the ectoderm mesoderm and endoderm but first now we have to talk about a new period that we're in we're no longer in the egg stage now we're in the embryonic period which goes from the third week to the eighth week after fertilization so now we can call this the embryo now during this embryonic period this is when the organ systems are being established and also when major malformations can be caused by genetic or environmental insults things like alcohol or retinoids and so to start this off we have

this process called gastrulation this refers to a transition from a single sheet of cells to three distinct germ layers again we have the ectoderm in blue here and that's on the dorsal side of this embryo it's going to contribute to things like the epidermis the skin the nervous system the teeth and the facial skeleton so really really important stuff here and the surface ectoderm specifically is going to form the enamel it's kind of more this part of the ectoderm and more this part we're going to we're going to talk a little bit more about neuralation

in week four but the neural crest is going to form the rest of the two structures i talked more about tooth development odontogenesis in the first video of my pediatrics series so go check that one out if you haven't already also notice this part right here that those arrows are pointing to that is the primitive streak and this little tiny dot at the end is the primitive node now these things are located again on the caudal side of the embryo that's the tail side of the embryo on the dorsal surface and so that is just

a really important kind of landmark to determine right from left and it's mediating a lot of this development process so to cover the rest mesoderm contributes to body skeleton so everything but the facial skeleton all the muscles all the connective tissue and then that leaves endoderm to do the lining of the hollow organ systems like your gastrointestinal tract for instance this is a really good image from bioninja and talks about the different cells that differentiate from these different these three different germ layers a nice little image there so this is a this is a really

difficult thing to visualize but these images do a very nice job of showing what goes on next from the transition from week three into week four so here we have the ectoderm mesoderm and the endoderm that we just looked at and on the right we have a few days later it says day 25 it's a little bit more further along in development it's also a different view so the left we have a cross-sectional slice and we're kind of peeking in from the the caudal side dorsals up here ventrals down here now on the right we

also have dorsal appear ventral here but we're looking at a side view so cranial is up here caudal is down this way and so really on the left we're taking like a cross section through this image now how this works is a lot of things are going on right around here and that's where that epiblast layer started remember that epiblast contributes to all three germ layers so that layer is going crazy and it's producing ectoderm cells that get migrated here it's producing endoderm cells that are being migrated here and it's producing mesoderm cells that stay

down here in the middle but they also spread out and create these nice little protective membranes and coverings um and there's just a lot going on a lot of differentiating going on now the right image like i said it's a little bit further along in development so kind of what happens structurally to get to this point this is a very different looking thing and what happens is this ectoderm in all three planes of space kind of grows these appendages almost and it grows out and eventually is going to pinch in on this endoderm kind of

like a claw game like you'd see at a boardwalk or an arcade getting a prize and so it's going to push on either side of that endoderm and it's going to pinch a little piece and it's going to involute into the actual embryo so this is the pinching off point that little piece that's getting pinched up here becomes the gut the forka the midga and the hindgut over here the rest that gets left behind becomes the yolk sac so that's basically what's going on here is there's this ectoderm that's pinching the endoderm and it's able

to um get that endoderm internalized and you can start to see how that might contribute to the lining of the hollow organs within that body so a lot of complex things happening in this week three to four transition period hopefully this visualization and explanation helps you out the next we're going to zoom in on what's going on right at this part here so now we're in week four and this is the neurulation process nervulation is happening in week four whereas gastrulation happened in week three so neurulation refers to the process where this flat neural plate

rolls into a tube kind of like a yoga mat that's being rolled up and so i want to call your attention back to this picture where i drew that black line again we're looking at the caudal side of the embryo with that primitive node and primitive streak here for neuralation it's happening on the same structure but it's happening on the cranial side it's happening on the cranial side on the dorsal surface but on the cranial side of that embryo so this is that same structure we were just looking at just on the cranial side so

we start with a neural plate that's in purple here and a notochord which isn't shown but it's a little tube derived from mesoderm that's sitting below everything going on here the notochord is the signaling device is signaling this plate to start folding and start this process and that notochord is going to eventually become the vertebral column so again think of this plate kind of like a yoga mat and you're bringing the ends together allowing that middle portion to sag down now those edges in green once they come meeting together are called the neural fold the

neural folds that's in the sagging area in purple is called the neural groove so we have the neural plate then we have the neural fold and the neural groove in this drawing now that right and left fold eventually converge into a point pinching off into a tube of ectoderm tissue and that tube is called the neural tube the top of it that's left behind these blue ends that eventually meet together contributes to the epidermis now note that that green portion is very special and it stays together and it's called the neural crest the neural crest

that's what used to be the neural fold that part that actually pinched together and stayed on top of the neural tube so that neural crest consists of cells that are going to migrate all over the place and contribute to some really important structures that are far away from the dorsal surface so again this top part is the dorsal surface it's the epidermis and those neural crest cells are going to be able to be sent out all over the place to contribute to all sorts of structures a lot of the tooth again is formed from neural

crest cells so as all of this is happening the aortic arch vessels and corresponding pharyngeal arches and somites form in a cranial to caudal sequence so these are some really important things what we're going to spend most of the rest of the video talking about the pharyngeal arches are segments by which the head and the neck develop they start with we start with six pharyngeal arches and we go down to five the somites are segments by which the body develops so the rest of the body we start with about uh anywhere between 42 and 44

somites we eventually end with 37 individual somites so 5 pharyngeal arches 37 somites and they develop in a cranial to caudal sequence from head to tail all right so following neural tube closure this buccopharyngeal membrane or oral membrane perforates so it finally opens allowing a communication with the outside world between the the outside world and the foregut this primitive oral cavity is called the stimodium or stomatodium it's a really really important word and comes up on board exams all the time some other good terms i wanted to point out for you the optic plecode that's

the primitive i the oticode somewhere up here is the primitive ear it pops up a little bit later in development and the blastopore is where the cloacal membrane eventually perforates to communicate with the hindgut to create the primitive anal cavity all right so let's focus on the pharyngeal arches they're also called the branchial arches branchial for gills and they're a series of these they kind of look like gills but they're not they're a series of externally visible anterior tissue bands lying under the early brain that give rise to head and neck structures the sixth arch

you can't see here you can only see one two three four maybe five but five doesn't stay around for a whole lot for a whole long time and eventually we're just left with arch one two three four and six that contribute to really important structures all of which we're gonna talk about very soon so each of these arches contains very specific structures and if we're looking at we're basically looking at a cross-section of the embryo and we see that each of these physical rib like structures rib like arches has an internal area this internal area

in yellow or orange that is called in a pouch and that's endoderm tissue we have a mesenchymal core that's on the inside of each of these arches you can see it blown up on the right side that's made of mesoderm and neural crest cells and that's going to contribute to arteries nerves muscles and cartilage we also have an outside layer this is an external ectodermal cleft so this is coming from ectoderm tissue so we have endoderm on the inside of each of these arches ectoderm on the outside of each of these arches and each of

those arches has all of these components artery nerve muscle and cartilage so let's unpack this a bit more in the next slide and talk about specifics for each arch all right so i cannot stress how high yield this table is going to be for you there are so many possible test questions that can be asked on this material and they certainly come up all the time so a lot of this is honestly going to be just kind of rote memorization using flash cards and things like that i'm going to try to talk about some things

that might help you remember some of these things a little bit better so how this is laid out we have the five important pharyngeal arches one two three four and six and then we have the nerve the bones the cartilage the muscles and the ligaments associated with each of those arches so i want to go through the cranial nerves really quick we're going to have a separate video talking about each of the cranial nerves and what they actually do so don't worry too much about the function part of it but certainly remember these numbers and

these cranial nerves because it's just so important it's gonna inform a lot of the later things we talk about so pharyngeal arch one is innervated by cranial nerve five pharyngeal arch two cranial nerve seven three goes with 9 and 4 and 6 are both innervated by cranial nerve 10. so 5 is trigeminal 7 is facial 9 is the glossopharyngeal and 10 is the vagus nerve again we'll talk about the specifics a little bit later but if you can remember 5 7 9 10 5 7 9 10 this is so so important all right so let's

go now across the rows the first pharyngeal arch is also called the mandibular arch this it's in this red or orange let's call it orange because i see red down here a little bit more so the orange arch here is the first one and it's called the mandibular for a good reason some of the things that are going to come from this arch are the meccals cartilage meccals cartilage maxilla mandible malleus do you see where i'm going with this all of these m's are coming from the mandibular arch also the the incus is another inner

ear bone also the zygoma and the temporal bone of the skull also being contributed from this first arch over for muscles we have the all of the muscles of mastication i call them the moms muscles of mastication are the masseter the temporalis and the medial pterygoid and lateral pterygoid we're gonna have a separate video uh just talking about those muscles but those are all innervated by cranial nerve five the third division of cranial nerve five to be exact and they're all coming from pharyngeal arch one also the myelohyoid is coming from this arch so just

look at all the m's i think that's a really nice quick way to remember some of these things for this first row also the anterior digastric and these two tensor muscles the spheno mandibular ligament is the ligament coming from this one so again a lot of uh a lot of commonalities among that row for the second one hyoid it's also called the hyoid arch this one is coming up with the reichert's cartilage the stapes that's the third inner ear bone that we got left out from the first arch the styloid process oid rhymes with hyoid

and the upper half of the hyoid body and the lesser cornu or lesser horns of that hyoid bone the muscles of facial expression the mfe come from this one they're all innervated by the facial nerve and then we have the posterior digastric so the anterior diagastric muscles were for the first row the posterior for the second we also have the stylohyoid muscle and the stapedius muscle that goes along with the stapes and we have this stylohyoid ligament so a lot of you know oids and hyoids that appear in that row as well so hopefully some

of those things can kind of stick out to you and help you at least fill in some of this table because honestly this is like one of those really high yield things that i would almost just reproduce on your scrap paper as soon as you start the exam because so many things could uh refer back to this the hyoid arch is in blue on this right image and then three is in yellow four is in red and finally six is in green down here so uh i won't go through all the rest of this but

you certainly can look at this and memorize it one thing that's a little nice is that with the third arch there's only one muscle to remember no ligaments and the skeleton is a little pretty straightforward if you get the the second row done and you know its upper half of the hyoid and the lesser horns you just have to do the opposite lower half of the hyoid and the greater horns to fill in that arch so there's some things that can help you out with reproducing this table again cannot stress enough how high yield this

is now some people ask is there a fifth arch well yes and no in humans the fifth pharyngeal arch exists only transiently during this embryogenesis process eventually it goes away and doesn't contribute to a whole lot kind of like the hypoblast that we saw before all right so once we understand the previous table this table makes a whole lot more sense so this one looks at the ectodermal clefs and the endodermal pouches that we talked about before and also i included the neurogenic placodes these are focal thickenings of the ectoderm layer that give rise to

to neurons essentially so let's look at the placodes first because i think they're actually the most straightforward so remember i talked about the cranial nerves for each arch five seven nine ten well guess what for arch one we have the trigeminal ganglion which goes with cranial nerve five the geniculate ganglion is part of cranial nerve seven and then we have the inferior sensory ganglion of the ninth cranial nerve inferior sensory ganglion of the 10th cranial nerve so if you just remember 57910 you already have everything you need with that column all right now let's go

over to the clefs so i just want to clarify the cleft of arch one is is really between arch one and two so each of these clefts and pouches are kind of between the arches so so cleft one is between these two cleft two is between those two etc so they're really listed below the arch of the row that they're in so the cleft of arch one which is really between arch one and two is the external auditory meatus that's this opening right over here that's the opening of the outer ear and as you'll soon

see the outer ear develops from three segments of pharyngeal arch one and three segments of pharyngeal arch two so and the fact that the opening is between these two arches makes perfect sense of course there's going to be one on the right side and one on the left side of the developing embryo clefts two three and four all contribute to the cervical sinus which is a temporary space that eventually goes away however if it doesn't go away there can be remnants of the sinus located laterally along the anterior border of the sternocleidomastoid muscle of the

neck and these cysts that don't go away it's called a branchial cyst or branchial sinus so if something goes wrong there that's what that becomes lastly the pouches so there's a bit more going on in this column but i promise that it'll all make sense so remember cleft one was for the outer ear opening now guess what pouch one is it's the inner ear opening so right over here we have the eustachian or auditory tube and the tympanic cavity in there so it's all part of the inner ear and that's gonna obviously connect with the

outer ear opening so together they form the entire ear canal now for pouch number two we move further down along the back of the throat essentially and this is where we get to the palatine tonsils the palatine tonsils now for pouch three and four it's where things get a little bit weird basically everything in this area migrates and descends from its starting position and some things descend further down than others so some of these things aren't going to make a whole lot of sense where they start it'll make more sense where they finish so pouch

three makes the thymus which is an immune organ where the t cells mature and so here's the thymus over here and that's going to go way down that's going to migrate way down to eventually sit between the two lungs now also from this arch or from this pouch i should say is the inferior parathyroids the inferior parathyroid glands one on either side that are embedded within the thyroid gland and these make a parathyroid hormone to regulate calcium and phosphorus levels in the body and those also descend those also descend but not quite as far as

the thymus does now pouch four contributes to the superior parathyroid glands again one on either side of the thyroid and that's the confusing part because they don't migrate as far down as the inferior ones did so the inferior start out higher and then end up lower the superior ones start out lower but they don't have to migrate down as far so they stay on top of where those inferior glands end up the same thing with the ultimo branchial body that's contributes to the parafollicular cells or c cells of the actual thyroid gland those stay a

little bit closer to where they developed into where the thyroid gland eventually was going to end up now if one of these pharyngeal pouches doesn't close properly you can end up with a tunnel of tissue or a branchial fistula that extends from a pharyngeal pouch to the surface of the neck so a branchial fistula so a cyst is if a cleft messes up a fistula as if a pouch messes up all right so a lot of information there hopefully all of that made sense i know some of the stuff going on with the third and

fourth pouch is a little bit confusing but as long as you figure out you know where they start versus where they end up it makes a little bit more sense all right so how about the pituitary gland well remember back to our week four setup over here where we had the um the roof of the oral cavity we have the roof of the oral cavity the oral cavity is the stimodium and the floor of the forebrain are essentially right next to each other and each of these things contributes to the pituitary gland formation so a

nice image from teach me anatomy we have the floor of the forebrain and the roof of the stomata diem or stomodium those are synonymous right next to each other both of these things are going to basically fold up and the rafke's pouch is an imagination at the roof of the developing mouth close to that buccopharyngeal membrane that is eventually going to give rise to the anterior pituitary and then up here the roof of the forebrain we have the diencephalon that's the caudal part of the forebrain and that evaginates downward to merge with the anterior pituitary

and becomes the posterior pituitary so the anterior part originates from oral ectoderm the posterior part originates from neural ectoderm so pretty cool how that how that happens so the pituitary gland developed from two entirely different tissues and the tongue is another cool example because it's one organ that develops from four different pharyngeal arches and that explains why its innervation pattern is essentially all over the place we'll have a separate video dedicated again to all the cranial nerves but for now let's just cover how the tongue feels and moves so the first two columns again should

look really similar again we just follow the pattern 5 7 9 10 for those four arches we also have a fifth source that i want to mention for the tongue being not an arch but actually occipital myotomes that's part of the somites that develop into muscles so let's move over to a diagram i made over on the right and we'll start with general sensation general sensation so that's things like touch pain and temperature in the anterior two-thirds that's this bottom part down here the anterior two-thirds of the tongue is innervated by v3 that's the third

division or the mandibular branch of the trigeminal nerve the posterior third oops that's up here is innervated by cranial nerve 9 and then the root or the base of the tongue as well as the epiglottis which is way back there is innervated by cranial nerve 10 and all that stuff is is reiterated on the chart this is just a fun little visual tool that i like to reproduce uh on test day now for taste we move over to this column the anterior two-thirds is innervated by cranial nerve seven the facial nerve so that's different an

entirely different nerve is is responsible for an entirely different sensation in the same region of the tongue it just blows my mind how cool that is and then if we go to the posterior third as well as the base of the tongue those two things are the same as they were for sensation so cranial nerve nine is responsible for the posterior third cranial nerve 10 for the base the root whatever you want to call it those are synonymous as well as the epiglottis now how about for a motor for actually moving the t the tongue

around well the vagus nerve once again plays a role here in innervating what's called the paladoglossus muscle that is a tongue muscle that's innervated by cranial nerve 10. so 10 is the only cranial nerve here that does all three of these things for the tongue however that's the only tongue muscle that a pharyngeal arch contributes to and the rest of the tongue muscles are from those occipital somites that are innervated by cranial nerve 12 the hypoglossal nerve again we're going to talk more about cranial nerves we'll also talk more about these tongue muscles in an

entirely separate video we're focusing more on the embryology part of it here again notice that the pituitary the tongue and also a lot of the other things we're going to talk about in this video all of this is happening starting week four week four is a really big week where a lot of organ development is going on how about the thyroid gland the thyroid gland so we talked about this a little bit already we talked about how the um parafollicular cells originate and how they descend what about the rest of the thyroid gland well it

has a very interesting path of development so it actually develops from tissues at the base of the tongue which actually makes perfect sense the thyroid cartilage comes from arch four and the vast majority of the thyroid comes from the cleft under arch iv so it's only natural that the thyroid would originate from tissue of the tongue that's innervated by the vagus nerve which is also part of arch four so all of that kind of connects together so from this view this is the same image i had in the last slide but i just re-masked it

for some anatomy labeling so we have the median sulcus goes across the tongue we have these big circumvallate papillae that kind of form a v shape at the end of our tongue and then we have this little dot here called the foramen cecum and that's a vestigial depression and it starts out as the development site for the primitive thyroid gland so the thyroid gland starts right there at the back of the tongue and it descends through the neck so it descends and it goes quite a distance carrying with it the thyroglossal duct the thyroglossal duct

and that makes sense because thyro thyroid and glossal for tongue all that um goes together and that stays connected to the foramen cecum on the surface of the tongue the thyroids pyramidal gland that's right at the center right here it kind of sticks up a little bit usually extends up along the path of this duct at the midline and that's a result of the travel that the thyroid gland had to take an undescended thyroid gland might sit at the base of the tongue as a lingual thyroid so that's pathology related to a failure of this

development process how about the ear this happens a little bit later about week six and it develops from six individual auricular hilox from the first and second pharyngeal arches and that's three helix on either side of that ear opening so three from first star is three from the second arch and that little opening is going to happen right between those so it all again it all kind of comes together which is really really the cool thing about all of this so um one here becomes the the tragus we have three up here becomes the helix

four is the anti-helix six is the lobe and you don't have to remember all of those things but you can see how kind of the one two and three form a good portion of that four five and six form this portion and so you have the the external auditory meatus that gets left in between those two groups and that's how it all kind of works together now important thing to know for a possible case question on a dental board exam retinoic acid also known as accutane is a medication that's used for severe acne and it

can cause first and second arch defects that manifest if if the mother is taking it during this stage of development it can cause defects such as microtia which is a small ear and it's frequently accompanied by micro mafia which is a small mandible again in the context of embryology it makes sense because they all share the same arch portions of the ear and the entire mandible are coming from arch number one all right so we've covered this content in our orthodontics series on cleft lip and palate i'll cover it again from an embryology standpoint so

a cleft is the result of a failure of fusion of tissue during early development and we're still in the embryo stage here so let's start with cleft lip lip formation occurs during weeks four five and six and the lip is derived from medial nasal prominence the medial nasal prominence that's in red in this diagram and the maxillary prominence which is in green the lateral nasal prominence in blue forms the allah or the sides of the nose now cleft lip occurs when the maxillary prominence fails to fuse with the medial nasal prominence anteriorly so if these

two primitive tissues fail to fuse properly that's how you get a cleft lip now due to the location of these prominences and where they're fusing that's going to typically result in either a left side or a right side a unilateral cleft but not always sometimes it can occur bilaterally sometimes even at the midline now let's go to cleft palate the palette is a little bit later primary palette forms at around six weeks in utero the secondary palette forms at around eight weeks in utero so lip is four to six palette is six to eight the

primary palette is this red part up front uh it also comes from the medial nasal prominence now we're looking at a an axial view looking up at this baby's uh palette and so the primary palette can also be called the premaxilla the intermaxilla or the incisive bone and it carries lateral incisor to lateral incisor which explains why someone who has cleft palate often has missing or malformed lateral incisors because those teeth develop right next to where this junction is the secondary pallet is everything else it's the green part originally these two palatal shelves one and

two are developing vertically they're actually located lateral to the tongue in a vertical orientation so think of them like doors that are swung wide open and as the oral cavity grows taller the tongue relatively moves downward and it allows those shelves to close to a horizontal position and fuse at the midline like you see here now the palatal shelves begin to fuse starting starting at the incisive foramen which is right here and they will zip together caudally or posteriorly so a lot of things happen in this kind of cranial to caudal or anterior to posterior

development direction now an incomplete cleft palate occurs when the palatal shells fail to fuse with each other a complete cleft palate so that would be you know if the failure diffusion ended up like that now you could also have a complete cleft palate where in addition to that the primary palate fails to fuse with the palatal shelves so there's a clean break all the way through there between the left and right sides in a complete palatal cleft now once you hit the end of week eight the baby is considered a fetus and we could go

on and on with embryogenesis but the rest of it is just not as high yields for dental board exams so we're going to stop right here now i do want to bring in some clinical stuff because i know everyone's always asking me about some case questions and can we see more patient cases well here we have a case of a child with degeorge syndrome it's a rare syndrome that's caused by genetic or environmental influence on neural crest cells during development so again things like alcohol or retinoids could play a role it's known as third and

fourth pouch syndrome and now we have all the information we need to explain this it affects the third and fourth pharyngeal pouches if we go back to our chart we see the thymus and the parathyroid glands are going to get hit the hardest and that's exactly what we see in its manifestation a great mnemonic to remember for this one is catch 22 catch 22. so the c stands for cardiac abnormalities a stands for abnormal faces it's things like a cleft palate short philtrum small mandible hypertelorism the eyes are far apart the t stands for thymic

aplasia again the thymus is getting hit hard because this is affecting among other things the third and fourth pouches cleft palate and hypocalcemia hypocalcemia results from a bad a parathyroid gland which means that there's no parathyroid hormone being produced if there's no parathyroid hormone you can't increase your blood calcium levels so the blood calcium stays low which is hypocalcemia and then it involves a 22q11 chromosomal deletion and so catch 22 is a really nice like clean mnemonic to remember for degeorge syndrome and just to wrap things up i just wanted to stress to you guys

that it's so important for embryology and really everything anatomy to draw it out to draw it out make nice clean drawings and use lots of colors and play around with different views and i think that's the best way to learn this stuff so i just did this quick sketch a few minutes before i recorded just to show you what you can kind of come up with and this is just going through the things we talked about starting from ovulation through fertilization the zygote morula blastula with that cavity the embryoblast which will become the epiblast and

hypoblast that epiblast will kind of go crazy become our three germ layers that we have here then we have the pinching off of that gut to form uh this more familiar cross section with the neural tube notochord and the primitive gut so stuff like this it's just so helpful to draw it out look at those diagrams that i included throughout the video and kind of make your own interpretations and drawings as you go along and study this stuff all right well that's it for this video guys thank you so much for watching please like this

video if you enjoyed it and subscribe to this channel for more on dentistry if you're interested in supporting this channel and what i do please check out my patreon page thank you to all of my patrons here for their support you can unlock extras like access to my video slides to take notes on and practice questions for the board exams so go check that out the link is in the description thanks again for watching everyone and i'll see you in the next video