Human pre-implantation embryos developing in vitro. Following fertilization, the embryo divides while remaining within the translucent zona pellucida (zp, arrowed). Shown are the 8 cell stage (d), the early morula stage of approximately 16 cells (e), the early blastocyst stage (f) - note the fluid filled blastocoelic cavity and the small cluster of cells at the top which will develop into the fetus. (g) shows the blastocyst hatching through the zona at the top left, and (h) shows the hatched blastocyst with the empty zona lying beneath it. The hatched blastocyst is now ready to attach to the luminal epithelium, which lines the uterus. It is this attachment process that is disrupted when SGK1 expression in the epithelium is abnormally high. (photographs courtesy of Professor Peter Braude and Professor Martin Johnson, published in Essential Reproduction).