Schematic of the proposed model of primary ciliogenesis in polarized epithelial cells. The transitions of the midbody remnant (release or movement to a central position) and the beginning of the ciliogenesis process are controlled in a cell area–dependent manner by two cell area thresholds: a first threshold of ∼400 and a second of ∼200 µm2. For cell areas above the second threshold, the remnant is only partially conserved, whereas it is fully conserved for cell areas below it. With regard to the remnant location, it locates peripherally in the apical surface at cell areas above a first threshold. When the area of the cell is between the first and a second threshold, it moves toward the center of the apical membrane to meet the centrosome, and this transition generates a pool of cells ready to start ciliogenesis. When the area of the cells falls below the second threshold, cells continue dividing and generating new midbody remnants that move to the cell center and enable ciliogenesis. Successive cycles of cell division (only one cycle is represented) increase the number of cells with a midbody remnant and, subsequently, the number of ciliated cells. In this way, the percentage of nonciliated cells becomes progressively smaller. Cell area reduction is produced by occupancy of the room of the mother cell by the daughter cells or compression of a cell by cells that divide in its proximity. Black lines represent cell divisions; blue and green arrows represent midbody remnant transitions (loss or peripheral to central movement) and PC formation, respectively.
