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. 2021 Jan 13;11:1110. doi: 10.1038/s41598-020-79987-4

Figure 6.

Figure 6

Low RA levels and RT-derived factors may mediate the non-cell autonomous regionalization of the SV epithelia. A model schematic illustrating a potential contribution of RT-derived FGFs and local low-level RA to the regionalization of the SV epithelia. Through the region-specifically enriched HSPG in the SV region, FGFs derived from the RT (e.g., FGF9) get immobilized and perceived in the SV region, and then possibly mediate the constitutive AKT phosphorylation in the SV Sertoli cells. Such RT-derived FGF ligands can also contribute to the maintenance of stem/progenitor cell population in the SV region by promoting their self-renewal. On the other hand, SV-specific expression of CYP26A1 degrades RA within the SV region to maintain the local RA at low levels, repressing the Aundiff–Adiff transition of stem/progenitor cells. The absence of ALDH1A1A1-3 expressing meiotic germ cells within the SV region may also contribute to the physical maintenance of the valve-like structures, as well as the low-RA levels within the SV region.