Occipital somites guide motor axons of the accessory nerve in the avian embryo

BDNF and NGF signals originating from sensory ganglia promote cranial motor axon growth

After exiting the hindbrain, branchial motor axons reach their targets in association with sensory ganglia. The trigeminal ganglion has been shown to promote motor axon growth from rhombomeres 2/3 and 4/5, but it is unknown whether this effect is ganglion specific and through which signals it is mediated. Here, we addressed these questions by co-cultures of ventral rhombomere 8 explants with cranial and spinal sensory ganglia in a collagen gel matrix. Our results show that all cranial sensory ganglia and even a trunk dorsal root ganglion can promote motor axon growth and that ganglia isolated from older embryos had a stronger effect on the axonal growth than younger ones. We found that brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) are necessary and sufficient for this effect. Altogether, our results demonstrate that the promoting effect of sensory ganglia on cranial motor axon growth is stage dependent, but not ganglion specific and is mediated by BDNF and NGF signals.

The unique axon trajectory of the accessory nerve is determined by intrinsic properties of the neural tube in the avian embryo

The accessory nerve is a cranial nerve, composed of only motor axons, which control neck muscles. Its axons ascend many segments along the lateral surface of the cervical spinal cord and hindbrain. At the level of the first somite, they pass ventrally through the somitic mesoderm into the periphery. The factors governing the unique root trajectory are unknown. Ablation experiments at the accessory nerve outlet points have shown that somites do not regulate the trajectory of the accessory nerve fibres. Factors from the neural tube that may control the longitudinal pathfinding of the accessory nerve fibres were tested by heterotopic transplantations of an occipital neural tube to the cervical and thoracic level. These transplantations resulted in a typical accessory nerve trajectory in the cervical and thoracic spinal cord. In contrast, cervical neural tube grafts were unable to give rise to the typical accessory nerve root pattern when transplanted to occipital level. Our results show that the formation of the unique axon root pattern of the accessory nerve is an intrinsic property of the neural tube.

The dermomyotome ventrolateral lip is essential for the hypaxial myotome formation

Background

The myotome is the primitive skeletal muscle that forms within the embryonic metameric body wall. It can be subdivided into an epaxial and hypaxial domain. It has been shown that the formation of the epaxial myotome requires the dorsomedial lip of the dermomyotome (DML). Although the ventrolateral lip (VLL) of the dermomyotome is believed to be required for the formation of the hypaxial myotome, experimentally evidence for this statement still needs to be provided. Provision of such data would enable the resolution of a debate regarding the formation of the hypaxial dermomyotome. Two mechanisms have been proposed for this tissue. The first proposes that the intermediate dermomyotome undergoes cellular expansion thereby pushing the ventral lateral lip in a lateral direction (translocation). In contrast, the alternative view holds that the ventral lateral lip grows laterally.

Results

Using time lapse confocal microscopy, we observed that the GFP-labelled ventrolateral lip (VLL) of the dermomyotome grows rather than translocates in a lateral direction. The necessity of the VLL for lateral extension of the myotome was addressed by ablation studies. We found that the hypaxial myotome did not form after VLL ablation. In contrast, the removal of an intermediate portion of the dermomyotome had very little effect of the hypaxial myotome. These results demonstrate that the VLL is required for the formation of the hypaxial myotome.

Conclusion

Our study demonstrates that the dermomyotome ventrolateral lip is essential for the hypaxial myotome formation and supports the lip extension model. Therefore, despite being under independent signalling controls, both the dorsomedial and ventrolateral lip fulfil the same function, i.e. they extend into adjacent regions permitting the growth of the myotome.