Survey of Homeobox Genes Expressed in Hemochorial Placentae of Mice (Mus musculus) and in Epitheliochorial/Syndesmochorial Placentae of Shiba Goats (Capra hircus var. Shiba)

Abstract

The homeobox genes expressed in highly invasive murine hemochorial placentae and in non-invasive caprine epitheliochorial/syndesmochorial placentae were surveyed by cloning and sequencing RT-PCR products generated by using sets of degenerate primers. All HOX9 paralogues, i. e., HOXA-9 - HOXD-9, were expressed both in the murine and in the caprine placenta. Among the non-clustered HOX genes, the placental expression of EMX2 and ATBF1 was newly demonstrated in the present study. In the murine placenta, Hoxa-10 was the dominant homeobox gene expressed. In the intercotyledon region of the caprine placenta, HOXA-10 was expressed at a low level, while HOXB-7 transcript levels were high. By contrast, no Hoxb-7 expression was found in the murine placenta. The high HOXB-7 mRNA levels in the caprine placenta might represent the expression in the allantois which enormously expands in the caprine epitheliochorial/syndesmochorial placenta. Despite the difference in the pattern of HOXA-10 and HOXB-7 expression, the homeobox genes expressed in two morphologically and functionally different types of placentae were surprisingly similar with each other. In the mouse plcenta, the expression levels of Hoxa-9, Emx2, Msx2 and ATBF1 did not show any consistent changes during the pregnancy period examined. Hoxb-9 expression levels exhibited a decline between Day 13 and Day 15 of pregnancy, while the expression levels of Hoxc-9 and Hoxd-9 gradually increased from Day 13 to Day 19. The Hoxc-9 expression level reached to the maximum level on Day 19. The functional significance of these homeobox genes remains to be clarified.