A three-dimensional staging system of mouse endometrial gland morphogenesis

Endometrial or uterine glands secrete substances essential for uterine receptivity to the embryo, implantation, conceptus survival, development, and growth. Adenogenesis is the process of gland formation within the stroma of the uterus that occurs after birth. In the mouse, uterine gland formation initiates at postnatal day (P) 5. Subsequently, the developing uterine glands invade into the adjacent stroma. Mouse uterine gland morphology is poorly understood because it is based on two-dimensional (2D) histological observations. To more fully describe uterine gland morphogenesis, we generated three-dimensional (3D) models of postnatal uterine glands from P0 to P21, using light sheet microscopy. At birth (P0), there were no glands. At P8, we found bud- and teardrop-shaped epithelial invaginations. By P11, the forming glands were elongated epithelial tubes. By P21, the elongated tubes had a sinuous morphology. These morphologies are homogeneously distributed along the anterior-posterior axis of the uterus. To facilitate uterine gland analyses, we propose a novel 3D staging system of uterine gland morphology during postnatal development in the mouse. We define 6 stages: Stage 0: Aglandular, Stage 1: Bud, Stage 2: Teardrop, Stage 3: Elongated, Stage 4: Sinuous, and Stage 5: Primary Branches. This staging system provides a standardized key to assess and quantify uterine gland morphology that can be used for studies of uterine gland development and pathology. In addition, our studies suggest that gland formation initiation occurs during P8 and P11. However, between P11 and P21 gland formation initiation stops and all glands elongate and become sinuous.