Choroidal all- trans -retinoic acid (atRA) may play a key role in the control of postnatal eye growth in a variety of vertebrates through modulation of scleral extracellular matrix synthesis and may therefore play a crucial role in the development of myopia. In the chick eye, choroidal atRA synthesis is exclusively regulated by its synthesizing enzyme, retinaldehyde dehydrogenase 2 (RALDH2). In chicks and humans, RALDH2 has been detected in a population of hitherto uncharacterized choroidal cells.Therefore, the aim of this study was to identify the RALDH2+ cell type(s) in the choroid and determine how these cells modulate atRA concentrations during periods of visually guided eye growth. Chicks wore translucent goggles on one eye for 10 days and choroids were analyzed for RALDH activity and RALDH2 protein expression at days 0, 1, 4, 7, 15 following removal of the goggle (“recovery”); choroids from contralateral eyes served as controls. The presence of RALDH2+ cells was assessed in chick choroid wholemounts using multiphoton microscopy. RALDH2 protein expression was measured by western blot and RALDH2 activity was assessed via HPLC quantification of atRA. Cell proliferation was assessed by BrdU-labelling in combination with RALDH2-immunohistochemistry. For characterization of RALDH2+ cells, immunohistochemistry for various tissue specific markers was applied in chicken (Ia antigen, CD5, Col1-propeptide, desmin, IgY, L-Cam, Cadherin1, MHC-II; Tcr-γδ, vimentin) and human donor tissue (α-smooth-muscle-actin, CD’s 31/34/68/146, desmin, IBA1, LYVE-1, PGP9.5, vimentin) followed by confocal microscopy. In the chick and human choroid, RALDH2+ cells with variable morphology were present in the stroma and adjacent to choroidal blood vessels. In chick wholemounts, RALDH2+ cells were concentrated toward the choriocapillaris, and their number increased nearly linearly between 1 and 7 days of recovery and plateaued between 7 and 15 days compared to corresponding controls. A significant increase in choroidal RALDH2 protein concentration and atRA synthetic activity was observed by four days of recovery (↑107% and ↑120%) by western blot and HPLC, respectively. A 3-fold increase in RALDH2+/BrDU+ cells was observed following 4 days of recovery compared to controls (12.43 ± 0.73% of all RALDH2+ cells in recovering eyes as compared with 4.46 ± 0.63% in control eyes, p < 0.001). In chick choroids, the vast majority of RALDH2+ cells co-expressed Col1-propetide, but did not co-label with any other antibodies tested. In human choroid, some, but not all RALDH2+ cells colocalized with vimentin, but were negative for all other antibodies tested. RALDH2+ cells represent a novel cell type in the chick and human choroid. Our findings that some human RALDH2+ cells were positive for vimentin and all chick RALDH2+ cells were positive for Col1, suggest that RALDH2+ cells most closely resemble perivascular and stromal fibroblasts. The increased number of RALDH2+/BRDU+ cells following 4 days of recovery suggests that choroidal atRA concentrations are partially controlled by proliferation of RALDH2+ cells. The identification of this choroidal cell type will provide a broader understanding of the cellular events responsible for the regulation of postnatal ocular growth, and may provide new avenues for specifically targeted strategies for the treatment of myopia.

脉络膜全反式视黄酸 (atRA) 可能通过调节巩膜细胞外基质合成在控制多种脊椎动物出生后眼睛生长中起关键作用，因此可能在近视的发展中起关键作用。在鸡眼中，脉络膜 atRA 的合成仅受其合成酶视黄醛脱氢酶 2 (RALDH2) 的调节。在小鸡和人类中，已在迄今为止未表征的脉络膜细胞群中检测到 RALDH2。因此，本研究的目的是确定脉络膜中的 RALDH2+ 细胞类型，并确定这些细胞如何在视觉期间调节 atRA 浓度引导眼睛生长。小鸡在一只眼睛上佩戴半透明护目镜 10 天，并在第 0、1、4、7 天分析脉络膜的 RALDH 活性和 RALDH2 蛋白表达。15 摘下护目镜后（“恢复”）；来自对侧眼的脉络膜作为对照。使用多光子显微镜评估鸡脉络膜整体中 RALDH2+ 细胞的存在。通过蛋白质印迹测量 RALDH2 蛋白表达，并通过 atRA 的 HPLC 定量评估 RALDH2 活性。通过 BrdU 标记结合 RALDH2 免疫组织化学评估细胞增殖。为了表征 RALDH2+ 细胞，对鸡（Ia 抗原、CD5、Col1-前肽、desmin、IgY、L-Cam、Cadherin1、MHC-II；Tcr-γδ、波形蛋白）和人类供体组织应用了各种组织特异性标志物的免疫组织化学（α-平滑肌肌动蛋白、CD 31/34/68/146、desmin、IBA1、LYVE-1、PGP9.5、波形蛋白），然后是共聚焦显微镜。在鸡和人的脉络膜中，具有可变形态的 RALDH2+ 细胞存在于基质中并与脉络膜血管相邻。在小鸡整群中，RALDH2+ 细胞向脉络膜毛细血管集中，与相应的对照相比，它们的数量在恢复的 1 到 7 天之间几乎呈线性增加，并在 7 到 15 天之间趋于稳定。通过蛋白质印迹和 HPLC 分别观察到四天的恢复（↑107% 和 ↑120%）后脉络膜 RALDH2 蛋白浓度和 atRA 合成活性显着增加。与对照相比，在恢复 4 天后观察到 RALDH2+/BrDU+ 细胞增加 3 倍（恢复眼中所有 RALDH2+ 细胞的 12.43 ± 0.73% 与对照眼中的 4.46 ± 0.63%，p < 0.001）。在鸡脉络膜中，绝大多数 RALDH2+ 细胞共表达 Col1-propetide，但没有与任何其他测试的抗体共同标记。在人脉络膜中，一些但不是所有的 RALDH2+ 细胞与波形蛋白共定位，但对所有其他测试的抗体均为阴性。RALDH2+ 细胞代表了鸡和人脉络膜中的一种新型细胞类型。我们的发现，一些人 RALDH2+ 细胞波形蛋白呈阳性，所有鸡的 RALDH2+ 细胞都呈 Col1 阳性，这表明 RALDH2+ 细胞与血管周围和基质成纤维细胞最相似。恢复 4 天后 RALDH2+/BRDU+ 细胞数量的增加表明脉络膜 atRA 浓度部分受 RALDH2+ 细胞增殖的控制。这种脉络膜细胞类型的鉴定将提供对负责出生后眼部生长调节的细胞事件的更广泛的理解，