Morphological evaluation of the placenta and fetal membranes during canine pregnancy from early implantation to term.

To describe the histological changes of fetal adnexa throughout the physiological pregnancy, canine samples were obtained during natural delivery and caesarean section, as well as during ovariohysterectomy performed at any stage of undesired pregnancies (N=12). The first period of pregnancy (multiple samples collected at 10, 12, 14 days) was consistent with pre- and peri-implantation events, i.e. apposition and initial invasion steps into the endometrium. The second period (multiple samples collected at 18, 38, 40, 45 days) was related to the development of extra-embryonic structures, placenta establishment and labyrinth formation. At the end of this period the maximum morphological complexity of the endotheliochorial placenta was achieved, characterized by complete erosion of the endometrial epithelium and underlying interstitium with exposure of maternal capillaries to the chorial cells. The third period of gestation (multiple samples collected at 50, 53, 57, 60, 63 days) was characterized by enhancement either of placental and extra-embryonic tissues.