ORGANIZATION OF THE PROXIMAL, ORBITAL SEGMENT OF THE INFRAORBITAL NERVE AT MULTIPLE INTERVALS AFTER AXOTOMY AT BIRTH - A QUANTITATIVE ELECTRON-MICROSCOPIC STUDY IN RAT

Although much is known of the central consequences of infraorbital ner ve (ION) transection at birth, little is known about the effects of th is lesion on the organization of the ION itself. To advance our unders tanding of how deafferentation alters the developing trigeminal neurax is, 19 newborn rats were subjected to left ION section and perfused 1, 2, 4, 7, 17, or 90 days later. Left IONs were removed in the orbit pr oximal to the nerve injury site, and axon numbers, types, and fascicul ation patterns were assessed with light and electron microscopic metho ds. Complete axon counts demonstrated that the axotomized ION containe d an average (+/-SD) of 13,945 +/- 10,335, 14,112 +/- 3,501, 16,531 +/ - 1,904, 9,045 +/- 1,465, 7,018 +/- 4,212, and 8,672 +/- 1,030 axons a t the above-listed ages, respectively. These values are well below the 33,059 axons in the normal adult ION (Jacquin et al. [1984] Brain Res . 290:131-135) and the 42,219 axons in the newborn ION (Renehan and Rh oades [1984] Brain Res. 322:369-373). The axotomized ION also containe d lower than normal percentages of myelinated axons (26.7% +/- 6.3% on postnatal day 90 vs. 59.7% +/- 6.2% in normal adults). Unmyelinated f ibers constituted the vast majority of the remaining fiber types; dege nerating fibers never accounted for > 1.6% of all the axons. The numbe r of fascicles making up the axotomized ION overlapped significantly w ith those found in the normal newborn and adult ION. We conclude that 1) extensive, though variable, axon elimination occurs proximally with in one day of the lesion; 2) the 74% reduction in fiber number seen at 90 days is not reliably achieved until postnatal day 7; 3) the higher than normal proportion of unmyelinated axons in the injured ION may u nderly many of the known effects of neonatal ION injury on the develop ing whisker-barrel neuraxis; 4) gross changes in ION fasciculation pat terns are not prerequisite to injury-induced pattern alterations in th e developing trigeminal system. (C) 1993 Wiley-Liss, Inc.