 Hi there, in this video we will 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. Day two, the first mitotic division of the zygote occurs around 30 hours post conception, producing two blastomias. Blastomia means sprout segment because one blastomia is one segment of the developing human sprout. Day three, division continues, and once 16 blastomias are present, they are together known as the moriola, which means mulberry, because this bunch of cells visually resembles a mulberry. Day four, fluid enters the moriola, forming an internal cavity. The moriola 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 cyst 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 triphane, meaning to feed, because the trophoblast becomes a placenta, which feeds the developing embryo. Day six, trophoblast cells break down barriers in the uterine epithelium, allowing entry of the blastocyst. Day seven, meanwhile, the trophoblast begins to transform into two separate parts, cytotrophoblast on the inside, syncytiotrophoblast on the outside. The latter is one cell with many nuclei. Syncytiotrophoblast 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, we'll remove the endometrium now and rotate our view like so. Day eight, the inner cell mass differentiates into hyperblast and epiblast. A space develops in the epiblast, the amniotic cavity. Hypoblast meaning lower sprout, epiblast meaning upper sprout, and amniotic comes from amnion, which is Latin for membrane around a fetus. Day nine, meanwhile, cells of the hyperblast have migrated to replace the blastocyst cavity with the primitive yolk sac. Hyperblast and epiblast together are here known as the bilaminar disk. Bilaminar means two-layered. Day 10, the blastocyst is now firmly implanted in the endometrium. The syncytiotrophoblast breaks into maternal capillaries known as sinusoids, effectively co-opting their blood for the developing embryo. Day 11, on the inner surface of the cytotrophoblast, tissue known as extra embryonic mesoderm has developed in orange here. This eventually develops cavities, which form the extra embryonic ceolum. Ceolum is sort of another word for cavity. The result is extra embryonic mesoderm adhering to cytotrophoblast and amniotic sac, where it is known as somatopluric mesoderm. And so the yolk sac, where it is known as splanknopluric. Splanknopluric basically means on the side of the visceral organs, because this mesoderm adheres to the visceral side, the yolk sac eventually becoming the gastrointestinal system. And somatopluric means on the side of the body. Day 12, the yolk sac pinches off part of itself, leaving us with a remnant of the primitive yolk sac, the exocelomic cyst. And the secondary yolk sac, which we'll from now on just call the yolk sac. Exocelomic cyst means fluid filled sac outside the main cavity. The main cavity being what we will now call the yolk sac. Day 13, the space known as the extra embryonic selum has expanded and is now known as the chorionic cavity. The extra embryonic mesoderm linking the cytotrophoblast to the embryo is known as the connecting stalk, which will eventually become the umbilical cord. Chorionic comes from chorion, meaning outer membrane of the fetus. Day 14, taking a closer look at the bilaminar disc now. Cranial disc means head end of the embryo, cordal means tail end. We can see here this little dip in the middle, cells in the epiblast begin to differentiate, change type, and burrow underneath the layer of the epiblast. So this looks like a burrow down the cordal end of the epiblast, it's known as the primitive streak, and seeing that marks the beginning of gastrolation. Gastrolation means becoming a gastroler, which is a tri laminar embryonic disc, and more on that in week three. We'll bring in a slightly more detailed version of the bilaminar disc now. Day 15, at the cranial end of the embryo, there's a patch of columnar cells known as the pro-cordal 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. Day 16, by now there are thousands of cells in the epiblast layer alone. It is the mass movement of these cells that produces these changes in the shape of the epiblast. 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 endoderm, in a darker shade of yellow here, and these cells displace and replace cells of the hypoblast. We have the germ layer known as mesoderm, and these cells push in between the layers of the former bilaminar disc. Some mesoderm spreads to the edges to meet the extra embryonic 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, mesoderm, middle skin, ectoderm, outer skin. 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 notochordal process. Notochordal is from noto, 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, ectoderm 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 neuralation. We have a separate video on that if you'd like to know more about it. Day 19, the notochordal process partly fuses with the endoderm now and is known as the notochordal plate. Given that the notochordal process was hollow, this means that now there's a passage between the amniotic cavity and the oak sac from the primitive node down through the endoderm. And this passage is only temporary and it's known as the neuroteric canal. It's thought to be important in equilibrating pressure between these two spaces. Our arrow travels through the notochordal plate into the oak 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 notochordal plate has by now lengthened. It's closed its inferior opening which was the neuroteric canal and it's now become a solid rod which is the notochord. Just beneath the developing neural tube lies this structural back bone of the embryo and that's all we'll cover for the first three weeks. As I mentioned before, watch the video on Neuralation for further detail on that process. We also have one on Gastrolation. Hit subscribe if you liked this video. Thanks for watching and we will see you next time.