Fisher S K, Steinberg R H
J Comp Neurol. 1982 Apr 1;206(2):131-45. doi: 10.1002/cne.902060204.
The apical surface of the retinal pigment epithelial cells (RPE) in the cat extend long sheetlike membranes that wrap concentrically above and around cone outer segments forming the cone sheath. The origin and organization of these sheetlike projections were studied in serial sections by electron microscopy. The apical surface of the RPE cells was found to consist of a thin zone of anastomosing ridges, or microplicae, from which longer projections extend. The lamellar projections forming the cone sheath originate from the microplicae as small cytoplasmic tabs that rapidly expand into broader sheets. Growth of individual sheets to their final size and shape continues by lateral and longitudinal expansion, fusion, and subdivision of the membrane. The small area of connection to the cell body allows the lamellae to overlap and interdigitate in forming the complex organization of the sheath. Microfilaments but not microtubules extend into the apical processes. RPE cilia (9 + 0 microtubules) with associated basal bodies, striated rootlets, and microtubules mark the location of retinal cones. These structures may be part of a microtubule organizing center that participates in morphogenesis of the cone sheath. They also may be involved in anchoring the apical projections forming the sheath, or in the movement of apical projections during the phagocytosis of outer segment discs shed from cone tips.
猫视网膜色素上皮细胞（RPE）的顶端表面延伸出长的片状膜，这些膜在视锥细胞外段上方和周围同心包裹，形成视锥细胞鞘。通过电子显微镜在连续切片中研究了这些片状突起的起源和组织。发现RPE细胞的顶端表面由一个薄的吻合嵴或微褶区域组成，更长的突起从这里延伸出来。形成视锥细胞鞘的片状突起起源于微褶，最初是小的细胞质薄片，然后迅速扩展成更宽的片层。单个片层通过膜的横向和纵向扩展、融合和细分，持续生长到其最终大小和形状。与细胞体连接的小区域使得片层在形成鞘的复杂结构时能够重叠和交错。微丝而非微管延伸到顶端突起中。带有相关基体、横纹小根和微管的RPE纤毛（9 + 0微管）标志着视网膜视锥细胞的位置。这些结构可能是参与视锥细胞鞘形态发生的微管组织中心的一部分。它们也可能参与锚定形成鞘的顶端突起，或者在吞噬从视锥细胞尖端脱落的外段盘期间参与顶端突起的移动。
