Implantation of the blastocyst into the uterus is obviously a critical step in mammalian development, yet we understand very little about the three-dimensional environment into which the embryo implants. It is known that, in mouse at least, blastocysts attach in uterine crypts, but how these form and whether such structures are also found in human is unclear. Here (p. 4749), Diana Laird and colleagues seek to provide new insights into uterine architecture before, during and after implantation. The authors develop sophisticated imaging and computational tools to characterise the 3D structure of the mouse uterine luminal and glandular epithelium, showing that the pattern of folding alters dramatically prior to implantation, giving rise to folds that overlap with structures described as crypts. Moreover, uterine glands reorient towards the site of implantation and show structural changes. This technology is able to detect architectural defects in mutant animals (such as aberrant luminal folding in Wnt5a mutants) and can also be applied to human uterus samples – as well as, potentially, other organs. This work provides an unprecedented view of the environment into which the embryo implants, and opens up avenues for further analysis of the mechanisms underlying uterine restructuring during early pregnancy.
