Running title: FETAL AND MATERNAL PLACENTA OF NEW ZEALAND RABBITS Academy Section: BIOLOGICAL SCIENCES

The onset of gestation is characterized by growth, morphological and functional changes of the placenta. We aim to evaluate the placental compartments in New Zealand rabbits by means of stereological methods. The fetal and maternal portion of placenta (12, 14, 18 and 20 gestational days) was randomly sampled for the stereological analysis. Histological sections were scanned to estimate fetal (labyrinth and junctional) and maternal (decidua) compartment volumes. The total volume of the placenta for the ages of 12, 14, 18 and 20 days was, respectively, 320 mm, 340 mm, 940 mm and 1300 mm. The volume of the labyrinth was 56 mm, 119 mm, 231 mm and 481 mm, respectively. The volume of junctional zone was 75 mm, 76 cm, 238 mm and 314 mm, respectively. The volume of decidua was 174 mm, 143 mm, 469 mm and 504 mm, respectively. We concluded that the rabbit ́s placenta compartments varied according to the gestational period, increasing continuously over the 20 gestational days. However, on the onset of the development of the placenta the decidua presented faster growth, whereas after the 20 days of development, the labyrinth developed more quickly. This study represents an aid to the understanding of placentation in humans.