Thickness of the basal lamina at the frog neuromuscular junction.

The Schwann cell basal lamina at the frog neuromuscular junction is much thicker than previously supposed. Its thickness has been demonstrated by modifying the fixation: the nerve muscle preparation is fixed in situ without prior exposure to Ringer solution and the osmolarity of the glutaraldehyde fixative is increased to 285 mOsm. With this procedure the basal lamina is seen to be up to 1 micron thick around the Schwann cells of the nerve terminal and the preterminal axon. The thickness of the basal lamina surrounding the Schwann cells of the myelinated portion of the axons within the perineural sheath (20 nm) is not changed by this modification of fixation. The present results indicate that in addition to the ability to form myelin, other properties of the Schwann cells differ inside and outside the perineurial sheath.