At Day 12 of pregnancy two separate populations of trophoblast giant cells were identified; however, Hp58 expression was not detected in trophoblast cells at either of these stages. As maturation of the chorioallantoic placenta progressed to Day 14 of pregnancy, Hp58 was detected in the trophoblast giant cells, and it was particularly evident in the trophoblast giant cells located at the maternal interface (Fig. 5E). The fetal erythroid cells strongly expressed Hp58 at Day 14 of pregnancy (Fig. 5E), while in sections reacted with preimmune serum the fetal erythroid and tro-phoblast giant cells were negative (Fig. 5F).
As pregnancy progressed from Day 10 to Day 12, demarcation of the junctional and labyrinth regions in the fetal placenta were unquestionably distinguishable (Fig. 6, A and B). The fetal vasculature now extended throughout the labyrinth region, and it was clearly developed and more extensive than that at Day 10. The Hp58-positive fetal ery-throid cells were evident throughout the labyrinth zone within the fetal vasculature (Fig. 6A), while erythroid cells in sections treated with preimmune serum showed no im-munoreactivity (Fig. 6B). The junctional zone itself was poorly vascularized compared with the labyrinth region except at the interface between the maternal decidua and the trophoblast giant cells (Fig. 6, C and D). Nucleated fetal erythroid cells were evident within the developing vascular spaces between the trophoblast giant cells at the junctional zone juxtaposed to the maternal decidua (Fig. 6, C and D), and those cells were immunopositive for Hp58 (Fig. 6C).
FIG. 6. Spatial expression of H^58 in the junctional and labyrinth regions of the rat placenta. A) Day 12 placenta reacted with H^58 antibody. B) Day 12 placenta reacted with preimmune serum. C) Day 12 placenta reacted with H^58 antibody. D) Day 12 placenta reacted with preimmune serum. J, Junctional zone; L, labyrinth zone; M, maternal decidua. Bar = 25 ^m.