Figure 1.

Timing of events and histology of during the first half of mouse pregnancy. A. Timeline of Days 0.5 to 11.5 of pregnancy. Implantation begins in the dark phase between Days 3.5 and 4.5. B. With the onset of implantation of the blastocyst the first macroscopically observable sign it has begun is the increased vascular permeability or “bluing reaction” (arrows) at implantation segments of the uterus where the implantation has begun along the uterine horns. C. Conceptus (egg cylinder embryo) and uterine implantation site on Day 5.5. With the onset of decidualization, a primary decidual zone (PDZ) is fully formed in the antimesometrial endometrium by Day 5.5 as shown by Tjp1 protein immunohistochemical staining (blue). D. Conceptus and uterine implantation site on Day 7.5. As decidualization continues in the antimesometrial region and mesometrial region, an antimesometrial (AD) mesometrial (MD) decidua are formed by Day 7.5. Conceptus acronyms: amniotic cavity (AC), conceptus (C), ectoplacental cone (EPC), exocelomic cavity (EC), ICM, inner cell mass (ICM), parietal endoderm (PE), trophectoderm (TE), trophoblast giant cells (primary, PTG; secondary, STG), Reichert’s membrane (RM). For more detail on conceptus histology during decidualization please see books by Kaufman or Theiler (Kaufman, '02; Theiler, '89).