In this second embryology lecture we looked at how we develop from a zygote (fertilised ovum) to a simple embryo with the three germ layers that will form all parts of the adult.
Cells of the zygote divide until a mass of smaller cells forms a morula. The cells inside will form the embryo and are called the embryoblast, and the cells on the outside will form part of the placenta and are called the trophoblast. Next, a fluid-filled space forms inside the morula. The morula has become a blastocyst, and the embryoblast has been pushed to one end. The blastocyst will continue its journey along the uterine tube to the uterus, and begin to implant in its wall. This all occurs in the first week post-fertilisation.
Implantation is the major event of the second week. The blastocyst will begin to penetrate the uterine epithelium and stroma at the end holding the embryoblast. The cells of the blastocyst trigger the process of decidualisation, which gives the formation of the placenta from both maternal and blastocyst derived cells. The embryoblast cells differentiate to form the epiblast and hypoblast layers. The trophoblast cells differentiate and take part in the formation of the placenta. By the end of the second week the blastocyst is fully implanted within the uterine wall, and the syncytiotrophoblast (the developing placenta) is producing human chorionic gonadotrophin (hCG).
Gastrulation occurs in the third week and describes (in the case of human embryos) the process by which the epiblast becomes the three germ layers: ectoderm, mesoderm and endoderm. Cells of the epiblast move to the midline of the sheet of epiblast and dive down through the primitive streak to form the new layers beneath.
