Functional heterogeneity of human term cytotrophoblasts revealed by differential sensitivity to extracellular Ca2+ and nucleotides

We have prepared purified cytotrophoblasts from human term placentas and examined the sensitivity of fura-2 loaded cells to the nucleotides ATP and UTP and to changes in extracellular Ca2+ concentration ([Ca2+](o)). Purified cytotrophoblasts were obtained by collagenase digestion and separation according to density using self-generated Percoll gradients. The cytotrophoblast fraction was free of red cell and largely free of white cell contamination (as assessed by uniformly negative staining for vimentin and the failure of >90% of fura-2 loaded cells to respond to the chemotactic peptide fMet-Leu-Phe). Purified cells secreted progesterone in a linear fashion over several hours in the presence of 25-hydroxycholesterol. The cells ranged in size from approximately 75 to 50 mu m in diameter as described previously for purified cytotrophoblasts, and an analysis of cells for sensitivity to [Ca2+](o) or nucleotides suggested functional heterogeneity within the cytotrophoblast population. Small cells (7.5-10 mu m) were negative for cytokeratin-8 and, after loading with fura-2, were insensitive to extracellular nucleotides but sensitive to elevations in [Ca2+](o). Medium-sized cells (12-20 mu m) were largely cytokeratin-positive (70% of cells) and sensitive to both ATP and UTP but largely insensitive to [Ca2+](o). Large cells (25-50 mu m) were uniformly cytokeratin-positive (100% of cells) and, after fura-2 loading, sensitive to both [Ca2+](o) and extracellular ATP or UTP. We examined the likely origin of small, medium and large cytotrophoblasts using an immunomagnetic cell sorting procedure that separates villous cytotrophoblasts (which do not express major histocompatibility class I antigens) from extravillous cytotrophoblasts. This procedure resulted in the selective sedimentation of almost all medium and large cells, leading to the conclusion that the small cells were villous cytotrophoblasts whereas medium and large cells were predominantly extravillous in origin. The data suggest that small, medium and large cytotrophoblasts have distinct roles in the function of the term placenta.