Heterochronic neuromuscular junction development in an Australian marsupial (Macropus fuliginosus)

Abstract

The neuromuscular junction (NMJ) provides a connection between the somatic nervous and muscular systems. The present work documents, for the first time, the mature form of the NMJ in an Australian marsupial, the western gray kangaroo Macropus fuliginosus. Marsupials give birth to highly altricial young and the majority of motor system development thus occurs postnatally. We demonstrate that NMJ formation in the forelimbs of kangaroos precedes that in hindlimbs by several weeks, consistent with evidence of precocial forelimb development from other indices of motor system maturation in marsupials. Forelimb and hindlimb NMJs reach a similar level of development by the end of the first postnatal month, an outcome that results from accelerated hindlimb development in the early postnatal period together with a more unexpected stagnation or even regression of forelimb NMJ maturity over the same period. This study suggests that the pattern of NMJ maturation may be adjusted in forelimb as opposed to hindlimb. Elucidation of the underlying cellular control mechanisms may inform current understanding of NMJ formation, and as such the broader processes involved in synaptic formation.

Adaptations of the Marsupial Newborn: Birth as an Extreme Environment

“…In the newborn, anterior regions of the skeleton show a robust cartilaginous skeleton with traces of ossification in the humerus and clavicle, while posterior regions show fewer cartilaginous skeletal elements and no ossification (Martin and Mackay, 2003;Weisbecker et al, 2008;Sears, 2009;Keyte and Smith, 2010). The muscles in the fore-and hindlimb also exhibit this anterior-posterior gradient in degree of maturation and the muscles of the hindlimb region shows little evidence of motor innervation (Cabana, 2000;Etherington et al, 2016).…”