The phrase "apical surface" refers to the top or uppermost part of a cell or tissue layer that is in direct contact with the environment or a neighboring structure. It is the surface that is exposed or faces outward. Full definition
In the Intestine Chip, the epithelium forms villi - like projections lined by polarized epithelial cells that undergo multi-lineage differentiation similar to that of intestinal organoids, however, these cells expose their apical surfaces to an open lumen and interface with endothelium. (emulatebio.com)
We take advantage of the accumulation of centrosomes and mitotic figures at the apical surface of these polarized tissues to improve the quality of staining and imaging. (science.institut-curie.org)
Cell division always occurs at the apical surface and when nuclear migration is inhibited, cells fail to enter mitosis, leading to dramatic developmental defects. (science.institut-curie.org)