Differences in collagen ultrastructure of human first trimester decidua basalis and parietalis: Implications for trophoblastic invasion of the placental bed


  • Conflict of interest: None of the authors have any actual or potential conflict of interest including any financial, personal or other relationships with other people or organizations to declare. The authors confirm that the results of this manuscript have not been distorted by research funding or conflicts of interest.
  • Funding: No dedicated funding was available for these studies.
  • Author contribution: I. M. designed the study; V. S. T., I. M., K. K. and R. M. executed the study and performed the analysis; V. S. T.,I. M., B. B., K. K. and R. M. all contributed towards drafting and approving the final manuscript.



The human embryo–maternal interface in the first trimester of pregnancy is an area of extensive tissue remodeling. Because collagen is the most abundant constituent of the extracellular matrix of the placental bed, successful invasion must involve its rapid turnover. We compared the nature and distribution of collagen fibrils in decidua basalis and parietalis.


We used a direct-vision hysteroscopic technique to obtain biopsies of the decidua basalis and parietalis from 11 women undergoing pregnancy termination in the first trimester. The biopsies were subjected to light, transmission and scanning electron microscopy, and immunohistochemical studies using mouse monoclonal antibodies against cytokeratin 7 and collagen types I, III and V.


Collagen fibrils in the stroma of decidua basalis were significantly thicker when compared to those in decidua parietalis (56.48 ± 1.37 nm vs 45.64 ± 0.85 nm; P < 0.0001 [mean ± standard error]) between 9 and 12 weeks gestation, but this difference in thickness was not observed at gestations below 9 weeks. In basalis, the fibrils appeared disrupted at most places surrounding the decidual/trophoblast cells while a uniform regular arrangement was preserved throughout most of parietalis.


There are differences in the ultrastructure of collagen fibrils between basalis and parietalis, with thicker and disrupted fibrils within abundant amorphous tissue in basalis, and thinner uniform fibrils in parietalis. These differences may reflect an adaptive response by decidua or a direct consequence of the invading trophoblast cells.