Foxa2-dependent uterine glandular cell differentiation is essential for successful implantation

Uterine receptivity (the window of implantation) is essential for successful implantation. The concept of uterine receptivity was first discovered 50 years ago by Psychoyos. In mice, uterine receptivity begins with the secretion of LIF from uterine glands stimulated by estrogen on the morning of day 4 pregnancies. However, some implantation failure is due to the lack of responses to the estrogen secretion. The current study reveals that uterine glands undergo a differentiation process with increases in more branches during the preimplantation period. The single cell RNA profiling of glandular cells identifies that LIF is expressed exclusively in a Prss29+ subgroup of glands in response to estrogen secretion on day 4 of pregnancy. Previous studies have shown that Foxa2 deficiency in uterine glands results in implantation failure due to lack of LIF production. Interestingly, Foxa2-deficient glands fail to develop branches and the functional Prss29+ subgroup of gland cells, indicating FOXA2 is obligatory for normal glandular differentiation prior to implantation. We found that this group develops by the evening of day 3 of pregnancy prior to estrogen secretion on day 4 mornings. We noticed that Lif mRNA signals are detected in the uterus comprising the Prss29+ subgroup. Our findings indicate that uterine glands undergo a FOXA2-dependent maturation process to acquire the competence, which we have termed the “transitional phase,” necessary to respond to estrogen. This newly identified glandular "transitional phase" represents a landmark concept in the study of uterine receptivity and implantation biology.