Shifting Concepts of the Fetal–Maternal Interface: A Historical Perspective

Structure of human placental villus according to Carpenter [5]: “Extremity of a placental villus: a, external membrane of the villus, continuous with the lining membrane of the vascular system of the mother; b, external cells of the villus, belonging to the placental decidua; c, c, germinal centres of the external cells; d, the space between the maternal and foetal portions of the villus; e, the internal membrane of the villus, continuous with the external membrane of the chorion; f, the internal cells of the villus, belonging to the chorion; g, the loop of umbilical vessels” (original legend). In reality layer b represents the syncytiotrophoblast which is overlying the cytotrophoblast layer f. The space d probably is a processing artefact.

Structure of an anchoring villus according to William Turner [7]: “ds, ds, represent the decidua serotina of the placenta; t, t trabeculae of serotina passing to the foetal villi; ca curling artery; up utero-placental vein; x a prolongation of maternal tissue on the exterior of the villus outside the cellular layer e', which may represent either the endothelium of the maternal blood vessel, or delicate connective tissue belonging to the serotina, or both. The layer e' represents maternal cells derived from the serotina. The layer of foetal epithelium cannot be seen on the villi of the fully formed human placenta” (original legend). Turner's strands of maternal decidual cells e' moving to the villi are in reality the cytotrophoblastic cell columns of anchoring villi.

Diagram summarizing Duval's interpretation of placental development in mice. (1) Progressive outgrowth of allantoic vessels (All) into the placenta results in formation and gradual expansion of the labyrinth (Lab – yellow arrows) shifting the labyrinthine–trophospongial border. By this process increasingly more nutrients are extracted for the developing fetus. (2) Outgrowth of the trophospongium (Ts) into the decidua results in incorporation of decidua-derived glycogen cell islands (Gl) into the placenta. By this process the placenta “vegetates” into the decidua to capture more maternal nutrients.

Friedländer's [16] illustrations of “uterine sinuses”. “Fig 6. Uterine sinus of the placental site, 8 month's pregnancy. a: musculature; b: thickened homogeneous membrane containing dispersed cells; c: multinuclear granulated cells filling the sinus. Fig 7. Similar structure with beginning organization of thrombus. a: musculature; b: connective tissue surrounding the sinus; c: thickened membrane; d: blood corpuscles and multinuclear cells embedded in a fibrous network; e: young connective tissue, an organized thrombus. Fig 8. Multinuclear cells isolated from such a sinus; in between a few blood corpuscles. Fig 9. Uterine sinus of the placental site 3 months' post partum. a: musculature; b: folded membrane; c: organized thrombus” (translated from the original legend). This figure clearly shows different stages of spiral artery remodelling by invading trophoblast. The “thickened membrane” of his Fig. 7c is the fibrinoid layer with embedded trophoblast, which is partially covered by a thickened intima (e). Endovascular cells (his Fig. 8) are no longer considered as multinuclear.