Morphological and functional aspects of the canine placental barrier: evidence supporting a deciduo-chorial model

The dog has a deciduate placenta, meaning that specific maternal cells (decidual cells) form part of the placenta. This placenta shows restricted invasive behavior of fetal cells (trophoblast) into maternal uterine structures. Decidual cells are involved in the hormonal processes leading up to birth. They are the only placental cells expressing the nuclear progesterone receptor (PGR), which allows them to respond to progesterone needed for the maintenance of pregnancy. This study focuses on the number, shape, and arrangement of decidual cells. We also found a significant reduction in PGR-expressing cells shortly before birth, when progesterone levels decrease. Interestingly, the drop in progesterone signaling is associated with a reduction in the number of decidual cells, indicating maturation of the placenta in preparation for delivery. Using electron microscopy, decidual cells were found to be irregularly shaped with long, branching extensions. For the first time, 3D reconstructions highlighted the complex network of decidual cells, which were shown to form part of the contact zone between mother and fetus - supporting the existence of a new type of interface in the canine placental labyrinth.