Extracellular fibrillar matrix architecture of human placental villi at term.

The spatial organisation of the extracellular fibrillar matrix of normal human placental villi at term can be directly visualised by scanning electron microscopy after 2N-NaOH maceration technique. By these methods, the extracellular fibrillar matrix of placental villi appears as a continuous network of isolated collagen fibrils and/or small fibrillar bundles interwoven each other. This sort of "collagenous fibrillar skeleton" forms the axis of chorionic villi and connects them with the basal plates running through the whole villous system of the placenta. Significant variations in the spatial arrangement as well as in the quantity of the extracellular matrix is observed at different levels of the villous ramification. Within the stem villi, the fibrillar extracellular matrix are abundant and, whereas the fibrils near the villous surface run parallel to the longitudinal axis of the villous (outer fibrils), those located in the inner core of the villous are arranged circularly around the wall of the fetal vessels (inner fibrils). In mature intermediate and terminal villi, viceversa, the extracellular fibrillar matrix is significandy reduced and the fibrils are mainly organised in a thin circular layer around the capillaries and sinusoids. The present study demonstrated the existence of a diverse spatial architecture of the extracellular matrix that results to be peculiar to the various levels of the ramification of the villous tree. Therefore, these morphological data strongly suggest a "compartmentalisation" of the villous tree as suggested by previous immunohistochemical study. Such a highly organised "collagenous fibrillar skeleton" stresses the important mechanical role of the extracellular matrix in sustaining the chorionic fetal vessels and the trophoblastic layer. Furthermore, the fine reticular-meshed network observed within the terminal villi suggests that at this level an additional role ensuring a favourable milieu for active feto-maternal exchanges may exist.