2-4th Week of Development

Um, good day, class. Um, let's talk about the embryology of uh second week to fourth week of development. Um, second to fourth week are very, very important in embryology. They are critically transformative. Um, think of this period as building a house in Nigeria. You know, week two is like laying the foundation. That's the bilaminar

uh disc and implantation. Week three is like putting up pillars. That's gastrulation and germ layers. Week four is is like starting the walls and roof. And that's like folding and organogenesis. Now, if you skip a step, trouble happens. Trouble arises. Just like a poorly built house collapses. So, clinically, this period explains uh

early pregnancy loss, you know, congenital malformation, and the origin of the placenta, and the extraembryonic structures. Okay, so they're very, very important. Week two, for example, uh like I said, is for bilaminar germ disc and implantation completion. The timeline is between day eight to 14. And the key events here are implantation is

completed. The trophoblast differentiate into cytotrophoblast, that's the inner proliferative layer, and syncytiotrophoblast, that's the outer invasive layer. Then there's bilaminar disc uh that forms. Uh epiblast becomes the embryoblast, while the hypoblast become the extraembryonic structures. Now, amnio- amniotic cavity, you know, forms uh within the epiblast. And uh uh this is lined by the amnioblast. So,

the yolk sac, you know, uh uh uh uh yolk sac formation then provides, you know, um uh uh uh early nutrients before the placenta then develops. Okay. Um Then also, there's extraembryonic mesoderm, which surrounds the yolk sac and amniotic cavity, uh you know, splitting it into somatic and then the splanchnic uh layers. There's also the chorionic