Early stages of facial nerve regeneration through silicone chambers in the rabbit.

The early stages of rabbit peripheral facial nerve regeneration [N = 16], over a 10-mm transectional gap, were analyzed at 1, 3, and 5 weeks of buccal nerve entubation in silicone chambers prefilled with saline. Normal nerve pooled data were obtained in nine nerves. Chronologic morphologic and morphometic light and electron microscopic computer-captured data reveal that the regeneration process can be subdivided into four stages: 1. The establishment of an acellular intergap matrix; 2. the ingrowth of mesodermally derived cells (macrophages, fibroblasts, endothelial cells, etc.) to form an intergap scaffolding; 3. the ingrowth of ectodermally derived cells (neurons, Schwann's cells, etc.) to reconstitute the transected peripheral nerve; and 4. neural maturation by progressive axonal enlargement, myelination, and compartmentalization. The first two stages occur during the first week, the third stage during the third week, and the fourth stage from the fifth week onwards of neural entubation. Morphometic analyses indicate that total neuroregenerates increase in number from 303 +/- 286 to 2693 +/- 1334 (+/- values are standard deviations) and myelination of axons increases from 12% to 48% during the third to fifth week of entubation. The thickness of the myelin (g-ratio) increases from 0.89 +/- .01 to 0.74 +/- .06 (SD) during the same period of time. At the end of the fifth week of entubation, the midchambers contain, on the average, 1279 +/- 980 myelinated axons (30% of normal counts) and 483 (+/- 11.2%) axons cross the chambers to innervate the distal transected neural stump (12.5%). Specific target-organ (quadratus labii superioris muscle) activity can be recaptured with a smaller number (12.5%) of regenerating myelinated axons from the buccal division of the facial nerve during the fifth week of entubation.