Proliferative versus hypertrophic growth in tissue subcompartments of human placental villi during gestation

A cross-sectional sample of human placentae was collected at 12-41 wk, and growth mechanisms within villous subcompartments (trophoblast epithelium, stroma and fetal capillary endothelium) were assessed using state-of-the-art design-based stereological methods. Physical disectors (adjacent pairs of sections) were used to count nuclei in syncytiotrophoblast and in cells of the cytotrophoblast, stroma and endothelium. The volumes of trophoblast and stroma and the surface areas and lengths of capillaries were employed to assess the overall growth of each compartment. Growth within compartments was monitored as total number of nuclei (a measure of nuclear proliferation) and compartment size per nucleus (a measure of hypertrophy). During gestation, all compartments grew dramatically. Numbers of nuclei increased exponentially and followed an uninhibited growth model. Volumetric growth of the placenta did not keep pace with the increase in total nuclear number (all types). Growth of trophoblast was exclusively hyperplastic and proceeded at an average net recruitment rate of 15 million nuclei per hour with a numerical predominance of syncytiotrophoblast nuclei of 9:1. There was no evidence of hypertrophy: trophoblast volume per nucleus averaged 1080 microns 3. Growth within stroma involved cell proliferation (17 million nuclei per hour) and the volume of stroma per nucleus declined at a rate of about 24 microns 3 per week. Capillary growth was hyperplastic (5 million endothelial squames per hour) and, possibly, hypertrophic (the mean area of a squame increased by about 18 microns 2 per week). Linear growth of vessels exceeded cellular recruitment and the density of squames along the capillaries decreased each week by about 5 per mm.(ABSTRACT TRUNCATED AT 250 WORDS)