At P0 and P2, typical, mature palisade endings were absent in all rectus muscles. Nevertheless, there were differences between muscles in the degree of axon growth toward the tendon. Specifically, all axons in the superior and lateral rectus stopped at variable distances away from the muscle-tendon junction and no axons were observed penetrating the tendon (
Figs. 2A, A', 2B, B'). In both muscles, axons stopped more far away from the tendon at P2 than at P0 and a possible explanation is the differential growth between muscle and axons resulting in a shift of axons away from the tendon. In contrast to the superior and lateral rectus, some axons in the inferior rectus and many axons in the medial rectus extended straightly from the muscle into the tendon (
Figs. 2A'', A''', 2B'', B'''). High resolution images showed that axonal extensions exhibited few, if any, branches in the tendon of the inferior rectus but, to the contrary, many branches in the medial rectus tendon (
Figs. 3A, A', 3B, B'). In both muscles, axonal branches established vesicle-filled terminal varicosities as demonstrated by synaptophysin-immunoreactivity, most of them at the level of the tendon and only few close to the muscle fiber tip (
Figs. 3A, A', 3B, B'). Such terminal varicosities were features consistent with palisade endings of adult animals. However, in contrast to adult animals, where axons turned back in the tendon to form palisade endings, axonal expansions in perinatal kittens did not exhibit such a feature. Therefore, nerve structures observed in the tendon of newborn kittens (in medial and inferior rectus) were considered as palisade endings at an early stage of development and such structures were further on termed precursors of palisade endings. Due to extensive axonal branching at the tendon level and higher number of terminal varicosities, palisade ending precursors were more complex in the medial than in the inferior rectus.