A re-appraisal of the morphophenotype and basal lamina coverage of cytotrophoblasts in human term placenta.

A recent study of human placental villi [Mori et al., The cytotrophoblast layer of human chorionic villi becomes thinner but maintains its structural integrity during gestation, Biol Reprod 76 (2007) 164-172] concluded that cytotrophoblast (CT) cells occupy 80% of the basal lamina (BL) surface at term and that syncytiotrophoblast (ST) does not make direct contact with the BL. Based on SPINT-1 localisation using immunofluorescence on cryosections, these conclusions run counter to previous light and electron microscopic data suggesting that term CT cells cover no more than about 24% of the BL surface. To resolve these discrepancies, we have undertaken a stereological study of term placenta using transmission electron microscopy (TEM) and a novel immunofluorescence approach. Test line lattices were randomly superimposed on TEM images of villous trophoblast from 13 normal term placentae. Intersections with the test lines were counted to assess the fractional surface of BL occupied by CT cells. After trypsin-mediated removal of syncytium, cells in whole-mounted term and first trimester villi were stained with cytokeratin 7 to identify CT and then visualised by confocal microscopy. CT formed an almost continuous layer in the first trimester. In contrast, term CT cells and their processes were found to cover only 44% (SD 14%) of the BL surface with intervening regions occupied by ST. TEM and confocal images were consistent with the concept of a network of 'octopoid' CT cells with fine processes extending from a central cell body. Our estimates of CT coverage are lower than the recent immunofluorescence estimate but greater than earlier TEM estimates. The former may have been biased by overprojection (section thickness) effects whilst the latter may be underestimates due to failure to include the fine CT cell processes. We conclude that CT cells transform from a cuboidal phenotype early in gestation to flattened cells with multiple interconnecting processes. The CT layer thins but maintains a functional network within which cells intercommunicate without compromising substance transfer via the syncytium.