Embryology Animated - the First Three Weeks

in this video we'll take a simplified tour through the first three weeks of embryology each day finishes with some etymology because understanding of the language behind embryology is crucial to comprehending the topic well day one the egg is invaded by a sperm together forming the zygote it contains male and female pronuclei its cytoplasm is surrounded by a tough glycoprotein shell zygote means two things joined in a close relationship because sperm and egg have joined to become one cell they - the first mitotic division of the zygote occurs around 30 hours post conception producing two blastomeres

blasphemy amines sprouts segments because one Blass Tamiya is one segment of the developing human sprout they three division continues and once sixteen blastomeres are present they are together known as the morula which means mulberry because this bunch of cells visually resembles a mulberry they for fluid enters the Marla forming an internal cavity the Marella is now known as blastocyst from now on we'll stop drawing individual cells and we'll just show the overall shapes of structures blastocyst again sprout and sis just means small fluid-filled sac day five the blastocyst has shed its outer shell cells have

separated into two distinct areas the trophoblast which develops into the placenta and will feed the embryo and the inner cell mass which develops into the embryo proper trophoblast comes from trevean meaning to feed because the trophoblast becomes a placenta which feeds the developing embryo day6 trophoblast cells break down barriers in the uterine epithelium allowing entry of the blastocyst they seven meanwhile the trophoblast begins to transform into two separate parts Saito trophoblasts on the inside since CDO trophoblasts on the outside the ladder is one cell with many nuclei since CDO trophoblast is appropriate as a name

because it is not many separate cells it is one together cell to simplify things and match the imagery in most textbooks will remove the endometrium now and we rotate our view like so they the inner cell mass differentiates into hyper blasts and epiblast a space develops in the epiblast the amniotic cavity hypo blast meaning lower sprout epi blast meaning upper sprout and amniotic comes from amnion which is Latin for membrane around a fetus day nine meanwhile cells of the hyper blast have migrated to replace the blastocyst cavity with the primitive yolk sac hyper blast and

epi blast together are here known as the bilaminar disk bilaminar means to lad they tend the blastocyst is now firmly implanted in the endometrium the since idiot rifle blast breaks into maternal capillaries known as sinusoids effectively co-opting their blood for the developing embryo day 11 on the inner surface of the site o trophoblast tissue known as extra embryonic mesoderm has developed in orange here this eventually develops cavities which form the extra embryonic coelom coelom is sort of another word for cavity the result is extraembryonic mesoderm adhering to cider trophoblast and amniotic sac where it is

known as somato cleric mesa dough into the yolk sac where it is known as splanchnic ler ik splanchnic lurex basically means on the side of the visceral organs because this museum adheres to the viscera side the yolk sac eventually becoming the gastrointestinal system and somato cleric means on the side of the body they twelve the yolk sac pinches off part of itself leaving us with a remnant of the primitive yolk sac the EXO ceramic cyst and the secondary oak sac which pull from now on just called the yolk sac EXO Salaam ik cysts means fluid-filled

sac outside the main cavity the main cavity being what we will now call the yolk sac they thirteen the space known as the extraembryonic coelom has expanded and is now known as the chorionic cavity the extraembryonic mesoderm linking the cider trophoblast to the embryo is known as the connecting stalk which will eventually become the umbilical cord chorionic comes from carrion meaning outer membrane of the fetus day 14 taking a closer look at the bilaminar disk now cranial dis means head end of the embryo caudal means tail end we can see here this little this little

dip in the middle cells in the EPI blast begin to differentiate change type and burrow underneath the layer of the epiglottis so this looks like a burrow down the caudal end of the Apple bust it's known as the primitive streak and seeing that marks the beginning of gastrulation gastrulation means becoming a gastrula which is a tri laminar embryonic disk and more on that in week 3 we'll bring in a slightly more detailed version of the bilaminar disk now day 15 at the cranial end of the embryo there's a a patch of columnar cells known as

the pro caudal plate at the cranial end of the primitive streak cells are burrowing thickly underneath the epiblast forming as they do so this circular depression known as the primitive node a sixteen by now thousands of cells in the Epley alone it is the mass movement of these cells that produces these changes in the shape of the app you blast the cells that burrow through the primitive streak and the primitive node differentiate into cell populations known as germ cell layers these are progenitor cells which go on to differentiate into the bulk of our bodily constituents

the germ layers are in the derm in a darker shade of yellow here and these cells displace and replace cells of the hypo blast we have the germ layer known as mesoderm these cells push in between the layers of the former bilaminar disk some Meza don't spreads to the edges to meet the extraembryonic mesoderm so these are now continuous epiblast cells differentiate into ectoderm the third and most superior germ cell layer the word germ literally means bud seed or embryo endoderm means internal skin measured a middle skin ectoderm outer skin and the translation skin sort

of makes sense given they are very thin layers to begin with they look like individual skins day 17 by this stage cells burrowing down the primitive node have started to form a hollow rod called the notochord 'l process not a chordal is from natto which means back and cord meaning cord because the back cord aka the notochord later serves as integral structural support for the early embryo day 18 between the primitive node and the cranial end of the embryo begins to thicken and is known as the neural plate seeing this is the beginning of another

process called neural ation we have a separate video on that if you'd like to know more about it day 19 the notochord will process parley fuses with the endoderm now and it's known as the notochord or plate given that the notochord process was hollow this means that now there's a passage between the amniotic cavity and the yolk sac from the primitive node down through the endoderm in this passage it's only temporary and it's known as the neuro enteric canal it's thought to be important in equilibrating pressure between these two spaces our arrow travels through the

notochord all plate into the yolk sac day 20 the neural plate has begun buckling in the midline we call this dip the neural groove and the mounds either side the neural folds day 21 these folds progress toward each other and begin to fuse in the center around day 21 where they have fused they form a hollow cylinder underneath which is the neural tube the very beginning of the central nervous system the notochord plate has by now lengthened its closed its inferior opening which was the neuro enteric canal and it's now become a solid rod which

is the notochord just beneath the developing neural tube lies this structural backbone of the embryo and that's all we'll cover for the first three weeks as I mentioned before watch the video on your relation for further detail on that process we also have one on gastrulation it's subscribe if you like this video thanks for watching and we will see you next time [Music]