The conjunctival crypt of the equine third eyelid

The purpose of this study is to characterise the gross anatomy of a relatively large, cryptic structure within the bulbar conjunctival surface of the equine third eyelid that has commonly been observed by the authors during equine ophthalmic examinations. In this study, cadaveric eyes and adnexa were obtained from 13 Quarter Horses without history of ocular disease. The gross anatomy of the bulbar surface of the third eyelid was characterised by digital macrophotography and with an image capture system using an operating microscope. A well-defined cryptic structure was readily identified in the majority of eyes (17/26). The location of the crypt varied but was most commonly observed within the central aspect of the bulbar surface of the third eyelid, often in close proximity to prominent blood vessels oriented perpendicular to the leading edge of the third eyelid. A patch of lymphoid follicles was often observed surrounding the crypt. The rectangular shape and upward facing orientation of the crypt was akin to a shirt's breast pocket. Width and depth were variable but ranged between 1 and 8 mm in either dimension. Some crypts were complex, with multiple channels observed within the primary opening. In conclusion, this study confirms the presence of a well-organised mucosal crypt within the bulbar surface of the equine third eyelid capable of harbouring foreign material and ocular irritants. Additional anatomical characterisation of the macro-crypt by light microscopy, immunohistochemistry and electron microscopy is underway. Evaluation of this structure by equine practitioners and veterinary ophthalmologists should be performed during the routine ophthalmic examination to better understand its role in ocular disease.