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E. Knop, N. Knop; Lymphoid Follicles in Conjunctiva–Associated Lymphoid Tissue (Calt) of the Rabbit Reveal Ultrastructural Characteristics of M–Cells . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2688.
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Purpose: The mucosal immune system of the conjunctiva (CALT) is of increasing importance for the understanding of defence, immune regulation and ocular surface disease, and lymphoid follicles serve for antigen uptake. The ultrastructure of CALT follicles in the rabbit is largely unclear, particularly the presence of antigen–transporting M–cells. Methods: Conjunctival tissues from young adult NZW rabbits were investigated as stained and cleared whole–mounts (6), in paraffin histology (6), scanning (SEM, 4) and transmission electron microscopy (TEM, 4). Results: Prominent secondary lymphoid follicles were more frequent in the lower lid and aggregated nasally into a dense group. Goblet cells terminated at the follicle flanks and the apex was covered with a follicle–associated epithelium (FAE) that contained intraepithelial lymphoid cells (mainly lymphocytes and occasional plasma cells). Darker epithelial cells formed pillars or fence like structures between groups of lymphoid cells. TEM showed that the lymphoid cells in the superficial epithelial layer were located inside the cytoplasmic pockets of larger cells with brighter cytoplasm, reminding of M–cells. These were located between and on top of the darker pillar cells that contained distinct intermediate filament bundles, both were connected by desmosomes. The M–cell cytoplasm was filled with a meshwork of filaments. It formed a thin layer containing numerous small vesicles towards the epithelial surface and a thin wall paper like lining with occasional openings at the basolateral side as also seen in SEM.. Conclusions: Follicles in rabbit CALT show characteristics of M–cells and indicate that CALT may be able to detect ocular surface antigens.
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