The early development of embryos is not well understood. One necessary stage is the formation of the blastocyst, a collection of cells present by about 4 days after fertilization of the egg, before implantation in the uterus. From that tiny collection of cells, all the different cells in the body must be created. Accordingly, there need to be mechanisms in the blastocyst to ensure normal gene expression and cell differentiation; those regulatory mechanisms are still largely unknown.
* This article was originally published here
