[Music] to understand how the body cavities develop we're going to start at the traminer embryo stage the trilaminar embryo is basically three stacked sheets of undifferentiated cells these consist of the not cord which induces signaling changes in all three layers the ectoderm the mism and the endoderm now I have a whole video on development of the embryo from one cell up to the triamer stage if these terms are a little bit unusual or unfamiliar to you I'd recommend you go and review it I'll put the link below and basically these tissues will create the entire body now up above the ectoderm is a space called the amniotic cavity and it will eventually surround the entire embryo and the formation of the body cavities is actually intimately tied to how that cavity surrounds the embryo surrounding the primary Yol Sac is the endoderm and the primary Yol Sac will dwindle to pretty much nothing as development proceeds but the endoderm will form the lining first of the gut tube and a little remnant of the yolc will be present just inside that lining the mism itself needs to be subdivided into the paraxial mism which is going to differentiate into somites the intermediate mesoderm which will become the urogenital system and the lateral plate mism and this is going to be one of the major players in this process as we move a little further along in development the ectoderm has developed a neural gr this is the start of the central nervous system nearby we have spaces developing in between the lateral plate mism and they're actually going to separate the lateral plate mism into two parts the parietal layer and the visceral layer you may also hear these referred to as the sematic layer and the splank niic layer parietal is going to form the body wall and visceral is going to form the surrounding of the organs and that's a pretty useful generalization we'll follow a little bit further as we go as development proceeds the split becomes very apparent between the two layers of lateral plate mesoderm and we wind up with an actual indentation there the parietal layer of lateral plate maderm is riding along with the ectoderm and it's going to start moving inferiorly and starting to wrap around the remainder of the embryo meanwhile the primary Yol sack gets pinched very tightly leaving the visceral layer of lateral plate mesoderm enveloping what will become the gut tube the gut tube has become so pinched off that even though it's lined by endoderm and even though it's still connected to the body by the visceral layer of lateral plate maderm the connection that it has to the Yol sack becomes incredibly tenuous we only have a very thin what's referred to as a vitaline duct connecting it and eventually the yolac will resend completely and that's because we have the parietal layer of lateral plate maderm moving around on either side lined by ectoderm and these two edges are going to meet each other and completely seal off the body creating a cavity called the intraionic or the intraionic cavity as that happens it's pulling the amniotic cavity around the embryo and it's entirely and the amniotic fluid is the water that we refer to as when someone's water breaks when they're giving birth this cavity and the fluid within it will almost completely surround the embryo throughout further development now the ectoderm remains in contact with the surface of the embryo it's going to form the epidermis and it also forms the spinal cord and brain it gets surrounded by some of the Semite mism specifically the osteotome forming the actual skeleton and nearby are the myoto and dermatome forming some of the muscles of the body and the dermal layer of the body in particular the intermediate maderm does a lot of interesting things making the kidneys and gonads we're not going to follow it any further in this talk but it is still moving along and the dorsal aorta is now present very close to the center of all this development and it will be supplying blood to all these developing tissues now we've completely sealed off the embrionic cavity the anterior body wall has been completely formed from thorax down to pelvis by the parietal layer of lateral plate mesoderm and its surrounding ectoderm the in embrionic celum is a completely separate space from the outside and the gut tube lined by endoderm is surrounded and suspended by its own mesentary and you can see a little vessel coming through the mesentary from the aorta to supply it and those support tissues and the mesentary come from the visceral layer of lateral plate mesoderm amniotic cavity more or less surrounds the embryo completely there's the epaderm again and now we're going to take a little step back and come back to this picture after we take a quick look at a video our a little quick aside to understand how the intraionic celum forms and will eventually make the peral and paracardial Cavities I've used my vast resources in my research lab AKA pool noodle to demonstrate that the embryo is basically a disc and that seon that space forms on the lateral side and the cranial side of the developing embryo but as the heart develops and the Brain enlarges the cranial part actually folds anteriorly to make a little Bend like this the heart grows into it as the head and brain roll a little more interiorly so this is basically the space that's present at the time and notice it's connected from bottom to top and the partitioning of these spaces is what we're going to be discussing next so we've seen that the intraionic seom forms a little bit of a horseshoe shaped cavity around not only the lateral side of the body but also around the head here what we're viewing is the ectoderm but we're looking at it from A sagittal cut so a sideways view of a midsection cut of the developing embryo endoderm is just below it and there's a place just ahead of where the brain is forming right here called the stomodeum and it's a place where the ectoderm and endoderm remain completely adherent to each other now just cranial to that interestingly enough is our developing heart our heart actually develops in front of our brain early on and the intraionic celom passes through that space as well so it's just at the kind of Midway point of that horseshoe at the bend now outside of that is a layer of mism and in particular a spot we call the septum transversum and it's going to be a big important player in just a moment because as the embryo enlarges and folds that heart developing area gets tucked underneath ventral to the body and that's why the heart winds up in our chest now the developing brain coming from ectoderm is still present getting bigger the gut tube is developing from endoderm but we have mism present between them at this point however the stomodeum remains a thin membrane which will eventually rupture making the mouth that happens well after all this folding has occur meanwhile the developing heart and the intraionic celum near it which is going to become the paracardial cavity keeps tucking Up and Under more ventrally into the thoracic region and that big wedge of mism called the septum transversum comes along for the ride and quick preview of what's coming up it's going to form the diaphragm or at least a significant part of it as we keep folding the heart's been tucked in ventrally here we have the developing brain right there stomodeum forming an early mouth connecting to the gut tube from the outside and that's why once it ruptures we'll have communication from the outside into our for gut the vitaline duct leading to the Yol Sac is present a little further along but now I want you to note that septum transvers it's hanging out between the abdominal region and the thoracic region and that's why it's going to contribute to the diaphragm please note however we've got the developing heart pushing into the pericardial cavity little pink layer here is going to form pink layer lining the heart is going to form the visceral pericardium I'm not going to continue drawing it in any further illustrations but do note that it's there and the paracardial cavity that it's pushing into will form the pericardial cavity of the adult and at this point that horse Sho still exists it's still connected to the perianal cavity but that's about to change in a process I'm going to describe right now if we take a cut right through that septum transversum and look at it in an axial view we'd see something like this there's that septum transvers and we still have the intraionic seum but in this region they have a special name for it called the pericardio parital canals are connecting the pericardial and peronal Cavities as development proceeds little folds from the lateral wall start to close those canals those little folds are called plur perianal folds and they get bigger and bigger until they actually close off this space entirely and fuse to the septum transversum and that takes us to our next picture they're going to push in and now we've got something a little more complicated looking this septum transversum is going to become the central tendon of the diaphragm it's still very much in the middle the plural folds contribute a little bit laterally and posteriorly by fusing completely to the septum transvers and then the majority of the muscle in our diaphragm actually comes from the body wall it actually grows in from the outside now failure of this process particularly of the plop perenial folds to fuse to the seum transversum can result in a hernia of gut into the thorax during development the abdominal contents are under a great deal of pressure during development and if there's a hole because these plop peronal folds didn't close properly then the gut will push up usually into the left plural cavity compressing the lungs and maybe even pushing the heart further to the right and that's incompatible with life outside the womb and has to be caught early and corrected in utero so that's development of the diaphragm and how it affected this and how we got a separate cavity in the thorax from the abdomen to see a little bit more about how that proceeds we're going to do a similar cut at the level of the heart and here's what we see there the heart's right there it sits within a pericardial cavity which at this point is still connected to the peral cavity by means of those paric cardio perianal canals and now I've drawn the forut but I've also drawn the developing lungs the lungs come from a respiratory diverticulum that blbs off the for gut and little tracho sophal ridges separate it from the for gut and it's going to form little lung buds on the right and left that are going to form the lungs nearby is a fold of tissue containing the frenic nerve which inates the diaphragm and some of the Cardinal veins which will form the great veins leading up to the heart [Music] as development proceeds those lung buds get bigger and bigger and from front to back we have the aorta the for gut a completely separate trachea and as I mentioned before these expanding lungs push into that space they fill the space that's there and they compress the space around the heart the plural space enlarges and as it does so it doesn't invade the pericardial space it pushes that fold of tissue containing the frenic nerve ahead of it and that's going to essentially seal off the paracardial cavity we're going to go another step further as the lungs continue to develop it's like they carve out some of that tissue and wind up completely surrounding the lateral aspect of the heart and as that proceeds even further we get to the mature confirmation where the lungs have wrapped more or less around the heart completely we have a a totally separate pericard space we have a right and a left plural space here and here we've got the frenic nerves inside lining the wall of the pericardium and that is how we get the spaces of the body I hope this has been helpful and happy studying