Developmental morphometry of the vagus nerve in the opossum

Developmental morphology of the vagus nerve has been described for placental but not marsupial mammals such as the opossum which undergo even greater postnatal maturation. In the present study, the cervical vagus nerve of opossums from the seventh postnatal day to the adult were compared after 3% glutaraldehyde immersion, postfixation in osmium tetroxide and staining with uranyl acetate and lead citrate. At 7 days of age, axons were unmyelinated. Only 4 myelinated axons were present at the 25th postnatal day and by 50 days of age only 11% of adult values were reached. The number of Schwann cells increased from 105 at age 7 days up to 640 in the adult. The number of myelin lamellae surrounding the largest axons increased from 9 at 25 days up to 102 +/- 9 at adulthood. In the newborn kitten and rabbit, the number of myelinated vagal fibers comprised 10% (n = 326) and 18% (n = 653), respectively, of that reached in the adult animal. Comparable values are not obtained in the opossum until the 50th postnatal day.