PAs localization. Expression of uPA (A-K) and tPA (L-V) in preimplantation developing rat embryo stages grown in vivo and in vitro. (A, L)-Zygote. (B, G, M, R)-2-cells. (C, H, N, S)-4-cells. (D, I, O, T)-8-cells. (E, J, P, U)-Morula. (F, K, Q, V)-Blastocyst. (A-F)-uPA in vivo. (G-K)-uPA in vitro. (L-Q)-tPA in vivo. (R-V)-tPA in vitro. (X)-2-cell uPA negative control. (XX)- 2-cell tPA negative control.