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(AB) S U M M A R Y The placenta has a dynamic and continuous capacity for self-renewal. The molecular mechanisms responsible for controlling trophoblast proliferation are still unclear. It is generally accepted that the simultaneous activity of proteins involved in cell proliferation, apoptosis, and extracellular matrix degradation plays an important role in correct placental development. We investigated in depth the expression of the serine protease HtrA1 during pregnancy in human placenta by in situ hybridization and immunohistochemistry, we demonstrated that HtrA1 displayed a low level of expression in the first trimester of gestation and a strong increase of HtrA1 expression in the third trimester. Finally, by electron microscopy, we demonstrated that HtrA1 was localized either in the cytoplasm of placental cells, especially close to microvilli that characterized the plasma membrane of syncytiotrophoblast cells, or in the extracytoplasmic space of the stroma of placental villi, particularly in the spaces between collagen fibers and on collagen fibers themselves. The expression pattern of HtrA1 in human placentas strongly suggests a role for this protein in placental development and function. Moreover, on the basis of its subcellular distribution it can be postulated that HtrA1 acts on different targets, such as intracellular growth factors or extracellular matrix proteins, to favor the correct formation/function of the placenta.

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