Differential distributions of HNK-1 and tenascin immunoreactivity during innervation of myotomal muscle in Xenopus.

The HNK-1 antibody labels developing neurons and their processes in Xenopus [40,43]. HNK-1 recognizes a carbohydrate bound to subsets of a number of cell and extracellular matrix (ECM) adhesion molecules, including those of the tenascin/cytotactin family. Both HNK-1 and Tenascin (TN) antigens have been implicated in neural development. Here we describe and compare expression patterns of the two antigens during development of the myotomal neuromuscular system. HNK-1 marks peripheral motor axon processes, including their fine branches at neuromuscular junctions (NMJs), in a pattern that persists throughout development. NMJs of myotomal muscle are located on either side of the intermyotomal spaces (IMS). From the beginning, TN-immunoreactivity (TN-IR) appears predominantly at the IMS, but not on peripheral nerves. Expression of both antigens is graded along the anteroposterior axis, with tenascin appearing first. At the ultrastructural level, HNK1-immunoreactivity appears on motor axons and at the neuromuscular junction on pre- and postsynaptic membranes. In contrast, TN-IR appears ubiquitously in the ECM of the intermyotomal junction, but not in association with axons or synaptic components. The respective distributions of the two antigens during myotomal innervation indicate that tenascin does not bear the HNK-1 antigen in Xenopus embryos and larvae, but that both antigens occupy positions consistent with roles in the establishment of NMJs at the myotomes.