Ontogenetic and phylogenetic evaluation of the presence of fibrin-type fibrinoid in the villous haemochorial placenta

The hypothesis that fibrin type fibrinoid deposition on villi is unique to the term human placenta was tested. Bright-field microscopy was used to examine sections of first and second trimester human placental villi and tissues from three animal species that have villous haemochorial placentae similar to the human: the armadillo, the baboon and the rhesus. Sections stained with haematoxylin and eosin showed fibrin type fibrinoid deposits were hypocellular, eosinophilic masses attached to the surface of villi examined from both the human and the animal specimens. The deposits were located at discontinuities in the syncytiotrophoblast layer, and the fibrinoid provided a matrix for trophoblast re-epithelialization of the villous surface. It is concluded that fibrin-type fibrinoid is not unique to the term human placenta. The presence of the syncytiotrophoblast discontinuities associated with the fibrinoid deposition must be considered in models of maternal-fetal exchange in the villous haemochorial placenta.