Comparative anatomy of mammalian conjunctival lymphoid tissue: A putative mucosal immune site

Organized mucosa-associated lymphoid tissue (O-MALT) is defined by mucosal lymphoid follicles with unique overlying lymphoepithelia, and classically a ppears in tissues with a simple columnar epithelium, Within follicle-associ ated epithelium, goblet cells are characteristically absent, replaced by ul trastructurally distinct antigen-absorptive cells, termed M cells (or micro fold cells) for the appearance of their apical cell membranes. To determine if mammalian conjunctiva, with its stratified squamous epithelium, can be considered as a site of O-MALT, we compared the Light and electron microsco pic anatomy of conjunctiva from fourteen species of non-human adult mammals , and the conjunctiva of human adults harvested at autopsy. Lymphoid follic les in the conjunctiva were demonstrated in all mammals studied except for mice and rats. In those mammals with conjunctival lymphoid follicles, the f ollicle-associated conjunctival epithelium was notable for an absence of go blet cells, Transmission electron microscopy demonstrated an intimate assoc iation of lymphocytes with surface epithelial cells, but epithelial cell mo rphology was uniform overlying the follicle, and other ultrastructural feat ures of M cells were absent. Therefore, conjunctival lymphoid follicle-asso ciated stratified squamous epithelium demonstrates some but not all feature s of O-MALT lymphoepithelia. Further studies are necessary to determine wha t role conjunctival lymphoid tissue may play in mucosal immunity. (C) 1998 Elsevier Science Ltd. All rights reserved.