Murine uterine gland branching is necessary for gland function in implantation

Uterine glands are branched, tubular structures whose secretions are essential for pregnancy success. It is known that pre-implantation glandular expression of leukemia inhibitory factor (LIF) is crucial for embryo implantation, however contribution of uterine gland structure to gland secretions such as LIF is not known. Here we use mice deficient in estrogen receptor 1 (ESR1) signaling to uncover the role of ESR1 signaling in gland branching and the role of a branched structure in LIF secretion and embryo implantation. We observed that deletion of ESR1 in neonatal uterine epithelium, stroma and muscle using the progesterone receptor Pgr^Cre causes a block in uterine gland development at the gland bud stage. Embryonic epithelial deletion of ESR1 using a mullerian duct Cre line - Pax2^Cre, displays gland bud elongation but a failure in gland branching. Surprisingly, adult uterine epithelial deletion of ESR1 using the lactoferrin-Cre (Ltf^Cre) displays normally branched uterine glands. Intriguingly, unbranched glands from Pax2^Cre Esr1^flox/flox uteri fail to express glandular pre-implantation Lif, preventing implantation chamber formation and embryo alignment along the uterine mesometrial-antimesometrial axis. In contrast, branched glands from Ltf^Cre Esr1^flox/flox uteri display reduced expression of glandular Lif resulting in delayed implantation chamber formation and embryo-uterine axes alignment but deliver a normal number of pups. Finally, pre-pubertal unbranched glands in control mice express Lif in the luminal epithelium but fail to express Lif in the glandular epithelium even in the presence of estrogen. These data strongly suggest that branched glands are necessary for pre-implantation glandular Lif expression for implantation success. Our study is the first to identify a relationship between the branched structure and secretory function of uterine glands and provides a framework for understanding how uterine gland structure-function contributes to pregnancy success.