ERRATIC DEPOSITION OF AGRIN DURING THE FORMATION OF XENOPUS NEUROMUSCULAR-JUNCTIONS IN CULTURE

In order to disclose the mechanisms that regulate synapse development we compared the distributions of agrin, acetylcholine receptors (AChR) and a basal lamina heparan sulfate proteoglycan (HSPG) in sections an d cultures prepared from Xenopus laevis and Ambystoma mexicanum embryo s. While agrin, AChR and HSPG may accumulate almost synchronously at s ynapses in vivo, agrin deposition usually lagged well behind the other synaptic markers during development in culture, and was not detectabl e at many differentiated junctions. Agrin deposition at nerve-muscle c ontacts in culture also appeared to require the presence of other syna ptic components. A similarly variable deposition occurred on noninnerv ated myocytes, where agrin again collected near sites of HSPG and AChR accumulation on some cells. Profuse agrin accretion occurred consiste ntly, however, within the extracellular matrices of surrounding epithe lial cells derived from both myotomes and neural tubes. In cocultures of Ambystoma neurons and Xenopus myocytes Ambystoma agrin collected at some chimeric neuromuscular junctions, but also accumulated on noninn ervated myocytes and in the extracellular matrices of salamander neuro endothelial cells. Based upon these observations we conclude that (a) focal agrin deposition is not required for synaptic differentiation on Xenopus myocytes and (b) agrin may be one of several muscle basal lam ina components that stem mainly from the secreted products of nearby e pithelial cells. (C) 1995 Academic Press, Inc.