Morphological evaluation of the orbit and peri-ocular glands of the African grasscutter (Thryonomys swinderianus)

The morphological features of the orbit and peri-ocular glands of the herbivorous African grasscutter were evaluated in this study. Heads of the rats were processed using the cold-water maceration technique, while the peri-ocular glands were obtained and processed for light microscopy using the conventional paraffin technique. The orbit was laterally located and associated with an enlarged maxillary infraorbital foramen. This orbital configuration imparted the grasscutter with a panoramic field of vision and a hystricomorphous improved masticatory ability. The Zeis and meibomian glands were located in the eyelid and showed holocrine histological features with myoepithelial cells surrounding their secretory acini. The gland of Moll was a simple coiled tubular gland that, together with the gland of Zeis, were associated with the palpebral cilia. The large harderian gland situated posterior to the eyeball was a lobulated compound exocrine gland whose secretory cells contained basally-displaced nuclei and foamy cytoplasm. The protruding glandular mass located adjacent to the rostral half of the upper eyelid, which we named the suprapalpebral gland had similar histological features as the harderian gland. These peri-ocular glands produced mainly mucoid and lipid secretions required for maintenance of structural and functional integrity of the cornea and eyelid. The qualitative and quantitative data from this study will be useful in the understanding of the biology of the African grasscutter and in the identification of orbital and peri-ocular glandular pathology in this species.