Morphology and growth curve of primary and immortal CEF cells. A) Representative photomicrographs of primary passage 6 CEF cells, senescent passage 18 CEF cells, passage 43 SC-2 cells, passage 95 SC-2 cells and immortal passage 280 DF-1 cells . B) Growth curve of SC-2 cells. SC-2 cells were plated at a density of 3 × 105 cells/10 cm dish. At 80% confluency cells were trypsinized, counted and the number of population doublings per day calculated.