Three-dimensional visualization of the cytotrophoblast layer of human term chorionic villi.

In the human placenta, trophoblast cells give rise to two unique cell layers that envelop the surface of chorionic villi: the outer syncytiotrophoblast (STB) layer and the inner cytotrophoblast (CTB) cell layer. The structural changes in CTB cells during gestation are still not fully understood. This study examined the ultrastructural integrity of the CTB layer within chorionic villi of the human term placenta using Fast Red immunohistochemistry in conjunction with proteinase K/Triton X-100 tissue-clearing and serial block-face scanning electron microscopy (SBF-SEM). Visualization of the CTB layer in whole-mount peripheral villous trees was facilitated by Fast Red immunohistochemistry of SPINT1, a marker specific to CTB cells, under both bright-field and fluorescence imaging modes in light microscopy. CTB cells displayed a thin, flattened morphology and extended multiple cellular projections, resulting in a spider-like transformation that envelops grape-like terminal villi. In contrast, in more proximal villi (e.g., stem villi), CTB cells showed a thick, cuboidal, or polygonal appearance, covering the villous subsurface. SBF-SEM imaging demonstrated the structure of a thin, mesh-like CTB layer, where the basal domain of the villous surface STB infiltrated through the small gaps of the CTB layer and contacted fetal capillaries via the basal lamina in terminal villi. Our data suggest that terminal villi undergo structural changes to facilitate fetomaternal exchange.