Morpho-functional uterine and placental dynamics during establishment and maintenance of canine pregnancy, from early trophoblast invasion to parturition

The placenta is an important, endocrinologically active organ that forms the interface between the maternal and fetal compartments. Evolutionarily, the canine placenta lies between the less invasive and more invasive placental types, positioning the dog as a physiologically relevant system for integrating molecular, endocrine, and structural aspects of placental biology. This narrative review updates key mechanisms underlying canine pregnancy, emphasizing progesterone receptor-mediated signaling, maternally-derived decidual cells, and their interactions with trophoblast within the recently proposed deciduo‑chorial placental interface, framed within a comparative and evolutionary context. It also provides, for the first time, new morphological insights into the initiation of invasive placentation in the dog. By linking these mechanisms to clinically relevant disorders of placentation, it emphasizes the translational relevance of canine placentation for studies in other species, including humans.