## 3 Results
### Expression of QSOX1 in the Pregnant Uterus and Placenta at 9.5 to 17.5 dpc  
The expression of QSOX1 presented a sharp contrast in the 9.5 dpc uterus. QSOX1 was highly expressed in mesometrial decidua but regraded severely in anti-mesometrial decidua (Figure 2a). QSOX1's levels decreased at the mesometrial decidua near the ectoplaental cone (Figure 2a,b). The intriguing feature of 9.5 dpc is that QSOX1 was intensively expressed in the trophoblast giant cells (TGCs) of the ectoplacental cone (Figure 2c). Some uterine glands still retained the considerable QSOX1 expression, but the QSOX1 staining in anti-mesometrial decidua was already feeble (Figure 2d,e). The TGCs around the embryo showed weak QSOX1 staining (Figure 2d,e). The protein was expressed relatively weak in the uterine muscle, while its levels in the circular muscle were close to the background staining (Figure 2d,e). Figure 2: Expression of QSOX1 in the pregnant uterus and placenta at 9.5 and 11.5 dpc. (**a**) Representative image of immunohistochemical staining of QSOX1 in the pregnant uterus at 9.5 dpc. (**b–e**) Enlarged views of the marked area in (**a**). (**f**) Representative image of immunohistochemical staining of QSOX1 at 11.5 dpc. (**g–j**) Enlarged views of the marked area in (**f**). The blue arrow indicates the parietal trophoblast giant cells. Red colors show QSOX1 staining, and blue shows the counterstain of hematoxylin in the nucleus. Scale bar = 500 \(\upmu\)m. Abbreviations: amd, anti-mesometrial decidua; ch, chorion; cm, circular muscle; cp, chorionic plate; dec, decidua; epc, ectoplacental cone; glyT, glycogen trophoblast; jz, junction zone; lab, labyrinth; lm, longitudinal muscle; md, mesometrial decidua; mg, mesometrial gland; pTGC, parietal trophoblast giant cell; spT, spongiotrophoblast; tgc, trophoblast giant cell. At 11.5 dpc, QSOX1 had a further down-regulation trend in the uterine mesometrial decidua, but the staining was still visible (Figure 2g,h). The main QSOX1 in the mouse placenta was still the short form (Figure S3). The parietal trophoblast giant cells (p-TGCs), spongiotrophblast, and glycogen trophoblast cells were all highly expressed in the junctional zone of the placenta (Figure 2g,i). At the same time, QSOX1 levels were relatively low in the mesometrial gland. The trophoblasts in blood vessels near the mesometrial decidua also showed QSOX1 expression (Figure 2h). In addition, the staining signal also was intense in fetal and maternal blood vessels in the labyrinth, and even fetal erythrocytes exhibited QSOX1 staining signals (Figure 2i). The chorionic plate also expressed strong QSOX1 (Figure 2i). Compared with 9.5 dpc, QSOX1 in the uterine muscle layer seemed to have a more appreciable expression at 11.5 dpc (Figure 2j). At 13.5 dpc, QSOX1 was weakly stained in some mesometrial decidua but strongly expressed in the mesometrial gland (Figure 3a-c). The p-TGC, the placental border, always showed considerable staining for QSOX1 (Figure 3b-d). Spongiotrophblasts in the junction zone and their extension in the labyrinth showed strong QSOX1 staining signals (Figure 3b,d). Moreover, QSOX1 was highly expressed in the chorion; the anti-mesometrial decidua had almost regressed. The staining in the uterine muscle was still remarkable (Figure 3e). QSOX1 was intensively stained in the uterus and placenta on day 17.5 (Figure 3f). Uterine mesometrial glands (Figure 3g) and the spongiotrophblasts (Figure 3g-i) exhibited a robust QSOX1 expression. At the same time, glycogen trophoblasts that invaded the de  
Figure 3: Expression of QSOX1 in the pregnant uterus and placenta at 13.5 and 17.5 dpc. (**a**) Representative image of immunohistochemical staining of QSOX1 at 13.5 dpc. (**b–e**) Enlarged views of the marked area in (**a**). (**f**) Representative image of immunohistochemical staining of QSOX1 at 17.5 dpc. (**g–j**) Enlarged views of the marked area in (**f**). The blue arrow indicates the parietal trophoblast giant cells. Red colors show QSOX1 staining, and blue shows the counterstain of hematoxylin in the nucleus.