RETINAL PIGMENT EPITHELIUM The retinal pigment epithelium is a monolayer that lies between photoreceptor outer segments and Bruch’s membrane (
Figure 2-21). This epithelium has many functions, including matrix production for photoreceptors, phagocytosis or outer segments, barrier protection, and active transport. These cells are large; are polygonal in shape; and contain abundant cytoplasm, round nuclei, and single nucleoli. The cytoplasm contains large distinctive ovoid and elliptical pigment granules (
Figure 2-22). The retinal pigment epithelium has a remarkable potential to proliferate and undergo metaplastic transformation. CHOROID The choroid underlies the retinal pigment epithelium and is continuous anteriorly at the ora serrata with the ciliary body. It is extremely vascular and contains a layer of capillaries, the choriocapillaris, directly under Bruch’s membrane (
Figure 2-21). Numerous melanocytes are interspersed with collagen in the choroids. The choroid is not sampled in vitrectomy. However, because many uveal tumors arise from the choroids, it is quite possible to sample choroidal melanocytes in fine needle aspiration. These melanocytes have a stellate shape and contain pigment granules. The cells contain small oval nucleoli (
Figure 2-23). They are usually accompanied by the fibrovascular stroma of the choroid. These cells should not be confused with melanoma cells.
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