Editing Embryology (section)

== Second week ==

* Blastocyst slowly becomes completely embedded, and the surface defect in the mucosa has healed
* Trophoblast
** Trophoblast differentiates into cytotrophoblast (inner, actively proliferating layer) and syncytiotrophoblast (outer layer which erodes maternal tissues)
** Lacunae develop in the syncytiotrophoblast which maternal blood can enter - the first uteroplacental circulation
* Embryoblast
** Differentiates into the bilaminar disc (epiblast and hypoblast)
** A small cavity appears within the epiblast, which becomes the amniotic cavity
** Epiblast cells lining the amniotic cavity, and later give rise to the germ layers
** Hypoblast cells - small layer surrounding the primitive yolk sac (exocoelomic cavity)
** The yolk sac provides nutrition to the embryo until the blood supply to chorionic villi is established