Revisiting the human umbilical cord epithelium. An atypical epithelial sheath with distinctive features.

The umbilical cord epithelium (UCE) is the surface tissue that covers the umbilical cord (UC). It is widely considered a single-layered epithelium composed of squamous or cuboidal cells, which are in constant contact with amniotic fluid. The objective of this study was to elucidate the distinctive structural characteristics and abundance of specific proteins in this unique epithelium, many of which have not been previously demonstrated. Samples of the UC were obtained from term pregnancies (n = 12) and processed for examination using stereo, light, electron, and 3D high-resolution confocal microscopy. Sections displayed a range of stratification, ranging from a single squamous layer to 4-5 layers of round/cuboid cells, challenging the notion of considering it as a single-layered structure. Cells are located on a well-developed basement membrane (BM), as evidenced by the expression of BM-specific proteins and PAS staining. The cells possess distinctive cytoplasmic domains that are tightly bound to each other by desmosomes and interdigitating anchoring surfaces. Desquamations and limited organelles suggest that the cells have reached the final stages of differentiation and are no longer actively synthesizing proteins, despite maintaining stratification-specific expression levels of cytoskeletal, junctional, receptor, and stem cell proteins. Although definitive keratinization was not observed, the distribution of proteins and the distinctive structural organization of the single/multi-layered cells suggest that they exhibit plasticity, likely due to adaptive mechanisms in response to chemical and/or mechanical stimuli during fetal development. These structural alterations may facilitate the active transportation of soluble ingredients between the amniotic fluid and cord blood through an intercellular route.