Unusual structure of rat conjunctival epithelium. Light and electron microscopy

The conjunctiva of the adult Sprague-Dawley rat was studied by light microscopy of 3 micron glycol methacrylate sections of whole eyes with lids and by electron microscopy of conjunctiva from the lower fornix. Rat conjunctiva is unique among species studied. All the superficial epithelial cells are squamous cells rather than polyhedral or columnar cells. Furthermore, the goblet cells are aggregated into clusters rather than distributed randomly throughout the epithelium. These clusters are not found at the lid margin or limbus, but are present in the palpebral and bulbar conjunctivae and achieve maximal size and number near the fornix. The stratified squamous epithelium is typical, composed of a layer of basal cells, an intermediate zone of wing cells, and an upper zone of several layers of squamous cells. Dividing cells are seen only in the basal layer. Occasional mononuclear leukocytes are found in the basal and intermediate layers. The goblet cell clusters are largely composed of columnar cells. Goblet cells predominate, but there are also occasional tuft cells, characterized by thick microvilli at their apices. Basal cells form only an incomplete layer beneath the columnar cells, which in places span the entire epithelium. The conjunctiva of the adult rat has few cells with potential for immunological activity. It does not contain appreciable numbers of plasma cells, and lymphoid follicles are absent.