Recent applications of the new stereology have thrown fresh light on how the human placenta grows and develops its form.

The availability of design-based stereological methods has made it possible to readdress certain key and contentious issues in placental growth and morphogenesis. Three particular questions are: (i) does the population of cytotrophoblast cells decline during gestation?, (ii) is placental growth biphasic or monophasic? and (iii) what are the consequences for intervillous porosity of the elaboration of terminal villi? These questions cannot be answered definitively without recourse to the new stereology. Applying the disector to estimate nuclear number and star volume to assess pore size, recent studies have helped to resolve these issues. Their findings are reviewed. Nuclei were counted in the trophoblastic epithelium, stroma and vascular endothelium of placental villi. It was found that growth is monophasic and proliferative. All types of nuclei increased in number throughout gestation and this included cytotrophoblast. Trophoblast grows by the continuous recruitment of new proliferative units of uniform mean volume. The so-called 'loss' of cytotrophoblast cells is a misinterpretation of what is seen on microscopical sections and is attributable to disproportionate growth in villous surface area. Cells simply become more widely dispersed. Elaboration of finer terminal branches on villous trees leads to a decline in the star volumes of villi and intervillous pores. Some of the functional implications of these findings are discussed.