Proliferation increases as PGCs move along their migratory route. (A) The frequency of EdU incorporation in WT PGCs increases by age during the period of migration. n = 708–1,966 cells from 18 embryos; ***, P < 0.0001 by χ² and Fisher’s exact test, χ² = 103.03, correlation coefficient = 0.998. (B) Distribution of PGCs by progressive location along the migratory route from the hindgut to three locations within the mesentery (mes.) to the gonad in combined ages E9.25 to E11.5. (C) The frequency of EdU incorporation in WT PGCs increases by location during migration. Anatomical cartoons show migratory PGCs as black dots. n = 197–1,817 cells from 18 embryos; †, P = 0.06 by Fisher’s exact test; ***, P < 0.001 by χ² and Fisher’s exact test; χ² = 116.19, correlation coefficient = 0.991. NS, not significant.(D) The frequency of EdU incorporation in WT PGCs cultured ex vivo is unaffected by age. n = 537–1,305 cells from four to eight experimental replicates; *, P = 0.04; †, P = 0.05 by Student’s t test. (E) Survival of WT PGCs decreases by age when cultured ex vivo. n = 1189–2986 cells from 6 to 13 experimental replicates; ***, P < 0.001 by Student’s t test; error bars in A, C, D, and E indicate standard error of the mean.