Stereology is a rigorous method, based on mathematical principles and rules for counting, which enables quantitative results in a three-dimensional structure to be obtained using two dimensional samples .
Here, we used paraffin embedded placentae for histology and immunohistochemistry. Immunohistochemistry all-owed a better identification of structural components of the labyrinth and consequently lead to a precise analysis of the volumes fractions of N. lasiurus and M. musculus domesticus placentae.
In contrast to Muridae for which qualitative and quantitative data are available in the literature, only qualitative aspects about the placenta and placentation in a few number of cricetids species were investigated [39–43]. In addition, especially Necromys lasiurus have been investigated by our group and detailed qualitative studies about the characteristics of the chorioallantoic and yolk sac placentation were performed using light microscopy, immunohistochemistry, and scanning and transmission electron microscopy [44, 49].
In detail, a significant increase in the absolute volume of the placenta and the main regions (labyrinth, junctional zone, and decidua) from early to mid-gestation was observed in N. lasiurus, followed by subsequent decrease near term. Thus, maximal placental extension was reached during mid-gestation. This is similar to what was observed in the mouse [9, 48, 50], supporting the interpretation that general patterns of placental structure and development are largely conserved within the rodent suborder Muroidea. In contrast, the placenta grows continuously in the human, at least under normal conditions, although external factors such as multiple pregnancies and disease could influence this development [5, 7, 8, 51, 52]. The differences in growth pattern between the human on the one side and the mouse and its relatives on the other side may have an important influence during gestation. Thus, differential and growth aspects should not be uncritically investigated in mouse model. For instance, Carter  provided a balanced and critical discussion on the suitability of the mouse as model species in comparison for human placentation.
It is important to note, however, that the absolute volume of the mouse placenta at the end of gestation ([9, 28, 50, 53], our results) was 3-fold higher than that of N. lasiurus, which had a greater body mass. Placental efficiency near term was also more than 10 in the mouse [9, 53, 54], but only 0.9 for N. lasiurus.
The labyrinth was the most prominent placental structure in N. lasiurus throughout gestation. In the mouse, at the end of gestation, the absolute volume of the labyrinth was similar to the junctional zone (, own results) and its relative size in comparison to the junctional zone was not very different than in N. lasiurus. Only in the labyrinth, the fetal capillaries and maternal blood channels are in contact; thus this area has a key role for fetomaternal exchange processes and fetal growth. We can only speculate that the low placental efficiency in sigmodont rodents limits the relative decrease of the labyrinth during pregnancy. The junctional zone contains giant cells, glycogen cells and spongiotrophoblasts (syncytial and cellular trophoblasts) [9, 26, 44], which may have important endocrine functions in early gestation, but which may not be essential to maintain the exponentially growing fetuses towards term.
Within the labyrinth, the relative proportion of fetal vessels and sinusoidal giant cells increased, whereas that of labyrinthine trophoblast decreased and maternal blood spaces did not exhibit significant changes. These changes may contribute to meet the demands of the developing fetus for adequate nutrition and probably to compensate the low placental efficiency. In that regard, the relative increase in fetal vessels seems to be most important. Moreover, the relative reduction of the labyrinthine trophoblast layer in advanced pregnancy corresponds to a thinning of the three-layered interhemal barrier that optimizes the diffusion distance between the maternal and fetal blood systems . In the mouse near term, the labyrinthine trophoblast becomes the relative most abundant tissue, whereas particularly giant cells are reduced (, own results). Optimization for exchange seems to be less significant compared to N. lasiurus, which may reflect the effective placentation in the mouse. Finally, in early mouse gestation trophoblastic giant cells maintain important functions related to invasion processes, including the modulation of hormones and growth factor activities [34, 55]; their reduction towards term correlate with the establishment of the placenta and the end of invasion. Further studies are necessary to reveal additional functions that may take place especially in the labyrinth by the sinusoidal giant cells in sigmodontine rodents and that may explain the relative explosion in their relative volume during advanced pregnancy, even though trophoblast in general was reduced.