Bilaminar embryonic disc

The one-celled zygote, a eukaryotic cell formed by a fertilization event between two gametes at the start of embryonic development, undergoes cleavage by mitosis as it travels through the fallopian tube to the uterus. As the zygote undergoes cell division to form two, then four, then eight and then 16 cells (typically by day four after fertilization), it becomes a ball of cells called a morula. During these cellular divisions, the zygote remains the same size, but the number of cells increase. The morula enters the uterus after three or four days—during which a cavity, called the blastocoel, is formed to produce the blastocyst.[4] Once the blastocyst is formed, it undergoes implantation into the endometrium.[4] During implantation the blastocyst, which contains the inner cell mass, undergoes cellular differentiation into the two layers of the bilaminar embryonic disc. One of which is the epiblast, also known as the primitive ectoderm. The epiblast is the outer layer of the bilaminar embryonic disc and consists of columnar cells. The hypoblast, also known as the primitive endoderm, is the inner layer, closest to the endometrium, which consists of cuboidal cells. The epiblast will develop into the 'embryo proper', and the hypoblast into the outer layer of fetal membranes (extraembryonic membranes). The blastocyst serves as a source of nutrients for the growing cells by diffusion from the surrounding fluid.[5]

Image showing the bilaminar embryonic disc with its surrounding structures by day eight of embryonic development

Beginning on day eight, the amniotic sac is the first new cavity to form during the second week of development.[4] Fluid collects between the epiblast and the hypoblast, which splits the epiblast into two portions. The layer at the embryonic pole grows around the amniotic sac, creating a barrier from the cytotrophoblast. This becomes known as the amnion, which is one of the four fetal membranes and the cells it comprises are referred to as amnioblasts.[6] Although the amniotic sac is initially smaller than the blastocyst it becomes larger by week eight until the entire embryo is encompassed by the amnion.[4]

The process of the formation of the gestational sac (chorionic cavity or extraembryonic coelom) and the yolk sac (umbilical vesicle) is still debated. The main theory states that formation of the membranes of the yolk sac begins with an increase in production of hypoblast cells, followed by different patterns of migration. On day eight, the first portion of hypoblast cells begin their migration and make what is known as the primary yolk sac, or Heuser's membrane (exocoelomic membrane). By day 12, the primary yolk sac has been disestablished by a new batch of migrating hypoblast cells that now contribute to the definitive yolk sac.[4]

While the primary yolk sac is forming, extraembryonic mesoderm migrate into the blastocyst cavity and fill it with loosely packed cells. When the extraembryonic mesoderm is separated into two portions, a new gap arises called the gestational sac. This new cavity is responsible for detaching the embryo and, its amnion and yolk sac, from the far wall of the blastocyst, which is now named the chorion. When the extraembryonic mesoderm splits into two layers, the amnion, yolk sac and chorion also become double-layered. The amnion and chorion are composed of extraembryonic ectoderm and mesoderm, whereas the yolk sac is composed of extraembryonic endoderm and mesoderm. By day 13, the connecting stalk, a dense portion of extraembryonic mesoderm, restrains the embryonic disc in the gestational sac.[4]

In the third week, gastrulation begins with the formation of the primitive streak.[4] Gastrulation occurs when pluripotent stem cells differentiate into the three germ cell layers: ectoderm, mesoderm and endoderm.[7] During gastrulation, cells of the epiblast migrate towards the primitive streak, enter it, and then move apart from it through a process called ingression.[4]

• Diagram at manchester.ac.uk