Placental morphogenesis and the star volumes of villous trees and intervillous pores.

In 27 human placentae collected at 13-39 weeks, the growth and morphogenesis of villi and of the maternal intervillous space were assessed using a design-based stereological approach for estimating star volume. The purpose was to quantify changes occurring during these processes and to derive parameters pertinent to physiological performance (notably transport capability and haemodynamics). Placentae were sampled by cutting paraffin sections at random locations and orientations. Estimates of the global volumes of peripheral (terminal and intermediate) villi and intervillous space were derived from placental volumes via point counting. Total villous surface was estimated by intersection counting and length by transection counting. Treating villi like the branches of tree-like networks and the intervillous space as a system of confluent pores, estimates of the star volumes of these compartments were also made. These volumes were calculated by measuring point-sampled intercept lengths. The total volumes of villi and intervillous space increased steadily throughout gestation. After the second trimester, the increase in volume of villi was accompanied by a decrease in villous star volume. This can be explained by the continued elaboration and maturation of terminal villi, the combined length of which increased whilst mean diameter declined. The star volume of the intervillous space also declined and this, too, is consistent with villous growth and proliferation.