Palisade Endings in Cat Extraocular Muscles Develop Postnatally Following Different Time Courses

The eyes are the most mobile organs of the body and knowledge of the eye position is important for vision. It has been supposed that eye position signals arise from specialized sense organs (proprioceptors) in extraocular muscles (EOMs). Surprisingly, classical proprioceptors (muscle spindles and Golgi tendon organs) are absent in the EOMs of most mammals and are substituted by particular nerve specialization termed palisade endings. Palisade endings are regularly present in frontal‐eyed but not in lateral‐eyed species and are formed by axons that establish nerve terminals around single muscle fiber tips. Although palisade endings exhibit some structural features of sensory Golgi tendon organs, they are cholinergic and their axons originate from motor nuclei in the brain stem. Because of these chimeric characteristics, the function of palisade is still unclear. Little is known about the development of palisade endings. Here we analyzed their postnatal maturation in a frontal‐eyed species (cat). Cats of different ages (P0, P2, P8, P22, P45, P95 and adult) were used and whole mount preparation of the rectus muscles were triple‐immunolabelled and analyzed in the CLMS. Triple labelling included antibodies against neurofilament (to label axons), synaptopysin (to label nerve terminals), growth associated protein43 (GAP43, to label developing axons), choline acetyltransferase (ChAT, to label cholinergic neurons). Phalloidin was used to label muscle fibers and α‐bungarotoxin to label acetyl choline receptors. Findings showed that palisade endings developed postnatally. In each rectus muscle they passed through the same developmental steps but in a heterochronic sequence and to a different final density per muscle. Specifically, around birth only in the medial and inferior rectus, precursors of palisade endings were found. At P8, simple, immature palisade endings were first found in the medial rectus. Two weeks later (P22), simple palisade endings were seen in the inferior rectus but were still at precursor level in the lateral and superior rectus. At P45, palisade endings were found in all rectus muscles and at P95 they were identical to those in adult animals. The number of palisade endings was higher in the medial rectus compared to the other rectus muscles. GAP43 expression decreased in palisade endings with age and was nearly absent in adult animals. Cats open their eyes 7–10 days after birth and later develop a complex three‐ dimensional visuomotor climbing and jumping behavior depending on accurate binocular vision and fine tuning of the ocular movements. Our findings indicate that palisade ending development correlates with important landmarks in visuomotor behavior and support our previous notion that palisade endings play an important role for convergence eye movements in frontal‐eyed species. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .