Abstract
Hypoglossal motor neurons (XII MNs) innervate tongue muscles important in breathing, suckling and vocalization. Morphological properties of 103 XII MNs were studied using Neurobiotin™ filling in transverse brainstem slices from C57/Bl6 mice (n = 34) from embryonic day (E) 17 to postnatal day (P) 28. XII MNs from areas thought to innervate different tongue muscles showed similar morphology in most, but not all, features. Morphological properties of XII MNs were established prior to birth, not differing between E17–18 and P0. MN somatic volume gradually increased for the first 2 weeks post-birth. The complexity of dendritic branching and dendrite length of XII MNs increased throughout development (E17–P28). MNs in the ventromedial XII motor nucleus, likely to innervate the genioglossus, frequently (42 %) had dendrites crossing to the contralateral side at all ages, but their number declined with postnatal development. Unexpectedly, putative dendritic spines were found in all XII MNs at all ages, and were primarily localized to XII MN somata and primary dendrites at E18–P4, increased in distal dendrites by P5–P8, and were later predominantly found in distal dendrites. Dye-coupling between XII MNs was common from E18 to P7, but declined strongly with maturation after P7. Axon collaterals were found in 20 % (6 of 28) of XII MNs with filled axons; collaterals terminated widely outside and, in one case, within the XII motor nucleus. These results reveal new morphological features of mouse XII MNs, and suggest that dendritic projection patterns, spine density and distribution, and dye-coupling patterns show specific developmental changes in mice.
Highlights
Hypoglossal (XII) motor neurons (MNs) are located in bilaterally paired XII motor nuclei in the dorsomedial medulla oblongata, and control the activity of several extrinsic and intrinsic tongue muscles that take part in a number of basic and important motor functions, including mastication, swallowing, suckling, vocalization and respiration (Fregosi 2011; Aldes 1995; Altschuler et al 1994; Sokoloff 1993; Berger et al 1995)
Dendritic processes were classified as spines only if they were no greater than 6 lm in length and no greater than 0.8 lm in cross-sectional diameter (Fogarty et al 2013), to be consistent with other morphometric spinal parameters describing dendritic spines as being less than 0.8 lm3 (Harris and Kater 1994; Harris 1999)
We report for the first time axon collaterals in XII MNs, as *20 % of XII MNs with filled axons had axon collaterals which arborized extensively in the ventrolateral medulla oblongata, and in one instance, within the XII motor nucleus
Summary
Hypoglossal (XII) motor neurons (MNs) are located in bilaterally paired XII motor nuclei in the dorsomedial medulla oblongata, and control the activity of several extrinsic and intrinsic tongue muscles that take part in a number of basic and important motor functions, including mastication, swallowing, suckling, vocalization and respiration (Fregosi 2011; Aldes 1995; Altschuler et al 1994; Sokoloff 1993; Berger et al 1995). Understanding how XII MN morphology contributes to the generation and control of XII motor activity is critical, both for comprehending these functions and for a better understanding of how dysfunction or death of XII MNs contributes to several diseases. These include amyotrophic lateral sclerosis (DePaul et al 1988; van Zundert et al 2008), obstructive sleep apnea (Dempsey et al 2010), Rett syndrome (Voituron et al 2010) and sudden infant death syndrome (Kinney 2009). Previous studies have shown that dendritic structures impose a fundamental constraint on the location and integration of synaptic inputs (Rall 1959; Hausser et al 2000).
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