## 4 Discussion  
QSOX1 is primarily expressed in the luminal and glandular epithelium in the non-pregnant uterus but is weakly expressed in the muscle and only background stained in the stroma. Although the fertilization signal significantly increases QSOX1 expression in the luminal and glandular epithelium, QSOX1 levels in the stroma and muscle do not fluctuate. Implantation signal significantly up-regulates increases protein levels in decidua cells, which mainly derive from stroma cells. However, QSOX1 levels in the decidua are conspicuously downregulated with growing gestation days, especially in the anti-mesometrial decidua. In addition, uterine longitudinal muscle tends to increase protein levels in the later stages of pregnancy. The dramatic increases and decreases in QSOX1 levels in the pregnant uterus depict that the expression of QSOX1 in the pregnant uterus is delicately regulated. The expression of QSOX1 in the primary TGCs around the embryo implantation site is relatively weak, while the cells surrounding the 9.5 dpc embryo are even feeble to almost undetectable; however, secondary TGCs of the ectoplacental cone are precursors of the  
Figure 7: Cellular expression and localization of QSOX1 in tissues of the embryo at 17.5 dpc. The cross-section of the 17.5 dpc embryo is shown on the top left. The detailed QSOX1 cellular distribution is enlarged from six subregions (a to f). (a) the nose and vibrissae follicles; (b) the tongue and tooth; (c) the muscle and sterrebrae; (d) the thymus, trachea, and esophagus; (e) the spinal cord and ganglion; (f) uterine muscle, yolk sac, embryo membrane, and skin. Red colors show QSOX1 staining, and blue shows the counterstain of hematoxylin in the nucleus. Scale bar = 1 mm. Abbreviations: cans, cartilage primordium of the nasal septum; car, cartilage; foli, follicles of vibrissae; gan, ganglion; mus, muscle; nc, nasal cavity; ee, olfactory epithelium; oes, esophagus; oss, ossification with sterrebrae; spi, spinal cord; sub, submandibular gland; thy, thymus; to, tongue; too, tooth; tra, trachea; vo, vomeronasal organ; ys, yolk sac. spongiotrophoblast and p-TGCs, two types of main TGC in the junction zone. Indeed, QSOX1 was prominently expressed in the placental junction zone. The differentiation of these two TGCs is inherently different, and the regulation of QSOX1 in the two TGCs is also intriguing. QSOX1 is highly expressed in the brainstem, including the spinal cord, midbrain, and medulla oblongata. This finding was also confirmed in the rat brain [19; 20]. QSOX1 was detected in the central neuron system of the rat brains of embryos and adults [20]. [PERSON] et al. hypothesized that QSOX1 could play a role in guiding migrating cells, their settling, and neuronal maturation [20]; however, the results are contradictory with the investigation of QSOX1 in the mouse embryo at late embryogenesis and postnatal day 1 [15], which only slightly stained in the mouse brain by the immunohistochemical staining. The study also reported that fetal and postnatal epithelia and epidermis were not or only weakly stained. In the present study, QSOX1 was detected in the epithelia of numerous tissues such as the uterus, lens, olfactory, yolk sac, amnion, and chorion. The epidermis also showed substantial expression of QSOX1. Previous studies also demonstrated that QSOX1 is expressed in the epidermis of the rat [8; 13] and can be cloned from the mouse epidermis [21]. The possible problem is that the antibodies used in that study were made from recombinant proteins produced by _Escherichia coli_ (_E. coli_). _E. coli_-derived protein does not equip with the post-translational modification of glycosylation nor the secretory pathway of the eukaryotic cell; thus, the conformation of the recombinant protein is different from the native one. Its antibody may have specificity to QSOX1; however, the sensitivity may be insufficient. When QSOX1 levels in a tissue are low, it may be difficult for the antibody to detect the expression of QSOX1.