The extracellular matrix of normal human placental villi at term: a scanning electron microscopy study

The three-dimensional organisation of the extracellular fibrillar matrix in normal human placental villi at term is investigated by scanning electron microscopic after 2N-NaOH-maceration technique. The latter method consists of a chemical digestion of cellular components of the villi that allows the visualisation of the extracellular fibrillar network in a real three-dimensional fashion by scanning electron microscope. By this approach, the extracellular fibrillar matrix, mainly composed by collagen fibrils, forms a continuous skeleton within the whole villous system of the placenta. This sort of collagenous fibrillar skeleton forms the axis of chorionic villi and connects them with the basal plates. Significant differences in quantity and arrangement of the extracellular matrix are observed at the various levels of the villous ramification. In fact, in the stem villi, the fibrillar extracellular matrix is considerable and the fibres are arranged longitudinally at the villous surface (outer fibrils) and circularly in the inner core of the villous around the wall of the fetal vessels (inner fibrils). On the contrary, in mature intermediate and terminal vili, the extracellular fibrillar matrix is significantly reduced and the fibrils are mainly arranged in a thin circular layer around the capillaries and sinusoids. The present study confirms morphologically the existence of a diverse organization of the extracellular matrix specific for the various levels of the villous ramification as suggested by previous immunohistochemical studies. Moreover, the presence of a highly organized collagenous fibrillar skeleton as observed in our study, stress the important mechanical role of the extracellular matrix in sustaining the chorionic fetal vessels and the trophoblastic layer. Furthermore, the fine reticular-meshed network of this skeleton observed in the terminal villi suggests an additional role of the extracellular matrix to ensure a favorable milieu for active feto-maternal exchanges at this level of the villous tree.