REDISTRIBUTION OF SCHWANN-CELLS AT THE DEVELOPING PNS-CNS BORDERLINE - AN ULTRASTRUCTURAL AND AUTORADIOGRAPHIC STUDY ON THE S1 DORSAL-ROOT OF THE CAT

In order to test our hypothesis that myelin-forming Schwann cells earl y during development, after having been eliminated from their parent a xons, colonize neighbouring unmyelinated axons, we studied the distrib ution of Schwann cells at the PNS-CNS border in the feline S1 dorsal s pinal root during pre- and postnatal development using electron micros copy and autoradiography. Myelination of axons peripheral to the PNS-C NS border began about 1.5 weeks before birth. The adult distribution o f one-third myelinated and two-thirds unmyelinated axons was noted 3 w eeks after birth. Analysis based on to-scale reconstructions of axon a nd Schwann cell samples from the first 6 postnatal weeks gave the foll owing results. (1) CNS tissue appeared in the proximal part of the roo t around birth and expanded peripherally during the first three postna tal weeks. (2) The number of Schwann cells associated with myelinated axons decreased. (3) The number of Schwann cells associated with unmye linated axons increased. (4) The mitotic activity of the Schwann cells was low at birth and nil after the first postnatal weak. (5) Apoptoti c cell units were virtually absent. (6) Aberrant Schwann cells, i.e. s hort and very short Schwann cells with distorted and degenerating myel in sheaths, were common. (7) The endoneurial space contained numerous Schwannoid cells i.e. solitary cells surrounded by a basal lamina. (8) Cytoplasmic contacts between unmyelinated axons and aberrant Schwann cells or Schwannoid cells were observed. We take these results to supp ort our hypothesis.