Ability of developing epithelia to attract neurite outgrowth in culture is not correlated with the appearance of laminin

Using a new technique for organ explants that facilitates the visualization of developing epithelia, we tested the abilities of salivary gland and lung rudiments to support de novo axonal outgrowth from the embryonic submandibular ganglion. We confirmed that salivary gland epithelia, but not lung epithelia, are able to support axonal outgrowth. This neurite outgrowth is also supported by salivary gland epithelia that have been lightly fixed with paraformaldehyde. When given a choice of both salivary gland and lung epithelia as a substrate for axonal outgrowth, the submandibular ganglion neurons showed an absolute preference for the salivary gland. Immunohistochemical localization of laminin was performed on whole mounts of developing epithelia after growing neurites were localized with a histochemical stain for esterase. Areas of lung epithelium devoid of any neurite outgrowth contained substantial immunoreactivity for laminin. In addition, Western blot analyses of extracts of embryonic lung and salivary gland indicate that the same amount of laminin or more is present on a per protein basis in the lung as in the salivary gland. An antiserum directed against laminin and a monoclonal antibody that blocks axonal regeneration on basal laminae in vitro (INO) were unable to block the outgrowth of axons over fixed epithelia. This suggests that molecules other than laminin are responsible for the preferential growth of axons over the salivary gland epithelia.