Localization of H/358 Protein in the Fetal Placenta
Partitioning of the fetal rat placenta into two morphologically distinct regions termed the labyrinth and junctional zones becomes evident early in chorioallantoic placental development. Because mice lacking functional Hp58 failed to form a chorioallantoic placenta, it was essential to investigate the distribution of Hp58 in the different placental cell types during the transition from the choriovitelline placenta, which is comprised of a single tro-phoblast cell phenotype, to formation of the chorioallantoic placenta that contains at least four differentiated trophoblast cell phenotypes. Paraffin sections from placentas at Days 10, 12, and 14 of pregnancy were reacted with Hp58 antibody (Figs. 5 and 6). At Day 10 of pregnancy, the junctional and labyrinth regions of the placenta were morphologically indistinguishable, and fetal trophoblast giant cells and erythroid stem cells were evident in the chorioallantoic primordia (Fig. 5, A and B). No other fetal cell types characteristic of the more mature chorioallantoic placenta were identified.
At this early stage, trophoblast giant cells in the chorioallantoic primordia did not react with Hp58 antibody. However, comparison of sections reacted with Hp58 antibody (Fig. 5A) versus those treated with preimmune serum (Fig. 5B) clearly showed that the small, nucleated erythroid cells within the developing fetal vasculature of the chorioallantoic primordial region were strongly immunopositive. Blood spaces were evident adjacent to the trophoblast giant cells and the maternal decidua (Fig. 5, A and B). At higher magnification of the cells in the chorioallantoic primordia (Fig. 5C), it was clear that Hp58 immunoreactivity localized to erythroid cells within the developing fetal vasculature. The immunoreactivity appeared to be both nuclear and cytoplasmic in those small fetal erythroid cells when the sections were observed under oil immersion at X100 (data not shown). Erythroid cells in sections reacted with preimmune serum did not show Hp58 immunoreactivity (Fig. 5D).
FIG. 5. Cell- and tissue-specific localization of Hp58 in the developing rat chorioallantoic placenta. A) Day 10 placenta reacted with Hp58 antibody. B) Day 10 placenta treated with preimmune serum. C) Day 10 placenta reacted with H^58 antibody showing trophoblast giant cells and fetal vasculature. D) Day 10 placenta reacted with preimmune serum showing trophoblast giant cells and fetal vasculature. E) Day 14 placenta reacted with H^58 antibody showing the fetal-maternal interface. F) Day 14 placenta reacted with preimmune serum showing the fetal-maternal interface. Arrows indicate fetal erythroid cells. Arrowheads indicate trophoblast giant cells. Bars = 25 ^m.