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 Xe nopus [40,43]. HNK-1 recognizes a carbohydrate bound to subsets of a n umber of cell and extracellular matrix (ECM) adhesion molecules, inclu ding those of the tenascin/cytotactin family. Both HNK-1 and Tenascin (TN) antigens have been implicated in neural development. Here we desc ribe and compare expression patterns of the two antigens during develo pment of the myotomal neuromuscular system. HNK-1 marks peripheral mot or axon processes, including their fine branches at neuromuscular junc tions (NMJs), in a pattern that persists throughout development. NMJs of myotomal muscle are located on either side of the intermyotomal spa ces (IMS). From the beginning, TN-immunoreactivity (TN-IR) appears pre dominantly at the NS, but not on peripheral nerves. Expression of both antigens is graded along the anteroposterior axis, with tenascin appe aring first. At the ultrastructural level, HNK1-immunoreactivity appea rs on motor axons and at the neuromuscular junction on pre-and postsyn aptic membranes. In contrast, TN-IR appears ubiquitously in the ECM Of the intermyotomal junction, but not in association with axons or syna ptic components. The respective distributions of the two antigens duri ng 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 my otomes.