Thy-1 differentiation protein and monocyte-derived cells during regeneration and aging of human placental villi

PROBLEM: The classification of placental villi was reviewed, and regenerati on of villous trees in mature human placentae was examined. METHOD OF STUDY: Expression of Thy-1 by placental fibroblasts and pericytes , and markers of endothelial cells and monocyte-derived cells were studied by immuno-histochemistry and image analysis. RESULTS: Villous regeneration consists of: (i) dedifferentiation of mature ramuli into young stem villi producing mesenchymal villi; (ii) differentiat ion of mesenchymal villi into immature intermediate villi; and (iii) differ entiation of immature intermediate villi into transitory intermediate villi , branching into the precursors of mature intermediate and terminal villi. These processes are associated with dedifferentiation and redifferentiation of placental monocyte-derived cells. Significant changes of Thy-1 expressi on by fibroblasts and pericytes accompany aging and degeneration, as well a s regeneration of placental villi. CONCLUSIONS: Villous aging and degeneration in normal mature human placenta is compensated by regeneration of villous trees. Lack of villous regenerat ion may cause chronic fetal distress, due to the increasing demands of the growing fetus on the remaining terminal villi.