Study question What is the stiffness (elastic modulus) of human non-pregnant secretory phase endometrium, first trimester decidua and placenta?

Summary answer The stiffness of decidua basalis, the site of placental invasion, was an order of magnitude higher at 103 Pa compared to 102 Pa for decidua parietalis, non-pregnant endometrium and placenta. What is known already Mechanical forces have profound effects on cell behaviour, regulating both cell differentiation and migration. Despite their importance, very little is known about their effects on blastocyst implantation and trophoblast migration during placental development, because of the lack of mechanical characterisation at the human maternal-fetal interface. Study design, size, duration An observational study to measure the stiffness of ex vivo samples of human non-pregnant secretory endometrium (N=5), and first trimester decidua basalis (N=6), decidua parietalis (N=5) and placenta (N=5). The stiffness of the artificial extracellular matrix Matrigel®, commonly used to study migration of extravillous trophoblast in three-dimensions (3D) and to culture endometrial and placental organoids was also determined (N=5). Participants/materials, setting, methods Atomic force microscopy (AFM) was used to perform ex vivo direct measurements to determine the stiffness of fresh tissue samples. Decidua was stained by immunohistochemistry for HLA-G+ extravillous trophoblast to confirm whether samples were decidua basalis or decidua parietalis. Endometrium was stained with hematoxylin and eosin to confirm presence of luminal epithelium. Single-cell RNA sequencing data were analysed to determine expression of extracellular matrix transcripts by decidual and placental cells. Fibrillin 1, a protein identified by these data, was stained by immunohistochemistry in decidua basalis. Main results and the role of chance We observed decidua basalis was significantly stiffer than decidua parietalis at 1250 Pa and 171 Pa respectively (P < 0.05). The stiffness of decidua parietalis was similar to non-pregnant endometrium and placental tissue (250 Pa and 232 Pa respectively). These findings suggest it is the presence of invading extravillous trophoblast that is driving the increase in stiffness in decidua basalis. The stiffness of Matrigel® was found to be 331 Pa, significantly lower than decidua basalis (P < 0.05).
Limitations, reasons for caution Tissue stiffness was derived by ex vivo measurements on blocks of fresh tissue in the absence of blood flow. The non-pregnant endometrium samples were obtained from women undergoing treatment for infertility. These may not reflect the stiffness of endometrium from normal fertile women. Wider implications of the findings These results provide direct measurements of tissue stiffness during the window of implantation and first trimester of human pregnancy. They serve as a basis of future studies exploring the impact of mechanics on embryo implantation and development of the placenta. The findings provide important baseline data to inform matrix stiffness requirements when developing in vitro models of trophoblast stem cell development and migration that more closely resemble the decidua in vivo.
Study funding/competing interest(s): This work was supported by the Centre for Trophoblast Research, the Wellcome Trust (090108/Z/09/Z, 085992/Z/08/Z), the Medical Research Council (MR/P001092/1) an EPSRC Doctoral Training Award (1354760) and a UK Medical Research Council and Sackler Foundation Doctoral Training Grant (RG70550). Trial registration number: N/A