Abstract
Somatic nuclear transfer allows scientists to produce identical copies of individuals (clones) and offers the potential for research in various fields. However, the production of cloned bovines is inefficient, partly because of high rates of embryonic mortality associated with abnormal placental development, although the exact mechanisms are still unclear. In the present study, we have applied design-based stereological methods to quantify placental morphology in cloned and non-cloned Nelore bovines (280-297 days of gestation). We have found no differences for any of the following parameters: number and volume of placentomes, villous volume and surface area, volume and number of star volume units and placental efficiency. Therefore, placental deficiency is unlikely to be attributable to its villous component. Nevertheless, new studies should focus on other placental constituents, e.g. capillaries, trophoblast cells and feto-maternal exchange surface area.