Anatomy of the California sea lion globe

This study analyzed the morphology of the California sea lion globe to determine what features may contribute to their characteristic visual abilities.

Globes from the Comparative Ocular Pathology Laboratory of Wisconsin (COPLOW) collection were examined from gross photographs and microscopic sections stained with hematoxylin and eosin, trichrome, smooth muscle actin, and alcian blue periodic acid-Schiff (PAS). Transmission electron microscopy of the cornea and iris was also performed.

There was a round, flattened area ventromedial to the axial cornea. The pupil was tear-drop shaped. Pectinate ligaments were visible without magnification. The retina was holangiotic, containing numerous spoke-like venules and arterioles. The tapetum was green encompassing the entire fundus. The optic nerve was unmyelinated. HISTOLOGICAL RESULTS: The sclera was thinnest equatorially and thickest at the limbus and posterior pole. Bowman's layer was difficult to see by light microscopy but clear with transmission electron microscopy. The cornea had a thick epithelium, thin endothelium and Descemet's membrane, and the stroma thinned axially. The dilator muscle was absent near the pupil, but enlarged and mingled with the sphincter muscle near the iris base. A large, wide ciliary cleft with prominent trabeculae and a single continuous pectinate ligament was present. The corneoscleral trabecular meshwork was discontinuous. A round lens attached to the ciliary body via direct attachment to ciliary processes and delicate zonular ligaments. There was a circumferential muscle at the base of the ciliary processes. A thick tapetum covered the entire fundus except peripherally. The retina was characterized by sparse, large ganglion cells.