VAGAL NERVE MATURATION IN THE FETAL LAMB - AN ULTRASTRUCTURAL AND MORPHOMETRIC STUDY

The maturation of the left vagal nerve was studied in the fetal lamb b y transmission electron microscopy and by computer-assisted morphometr y of sections of the entire nerve at seven gestational ages between 79 and 145 days (term is 147 days) and in the adult ewe. The number of u nmyelinated axons per Schwann cell progressively decreased from 25 to 55 at 79 days to 1 to 5 at near-term. Unmyelinated axons of various si zes were enclosed within a single Schwann cell at all ages, but the me an axonal diameter increased in inverse relation to the number of unmy elinated axons. A few Schwann cells enclosed two myelinated axons, but in most instances myelination did not begin until a 1:1 ratio was ach ieved; some single axons with a Schwann cell remained unmyelinated in the adult. Myelinated fibers were rare at 79 days but myelination prog ressed rapidly thereafter until the adult ratio of myelinated: unmyeli nated fibers was reached at about 100 days; myelinated axons were not uniformly distributed. The myelin sheaths and axons of small fibers pr ogressively increased in diameter in late gestation, but new large fib ers were not added. Early myelinating fibers and immature unmyelinated axons contained more microtubules than neurofilaments; neurofilaments predominated in mature axons with or without myelin. Cross-linkages b etween neurofilaments were already evident by 79 days. Maturation of t he vagal nerve thus occurs first by an increase in number of myelinate d fibers and then by an increase in the size of each fiber in this fix ed population. The bimodal distribution in the size histogram of myeli nated fibers is not achieved until 134 days gestation and correlates w ell. with physiological maturation of respiratory patterns. (C) 1993 W iley-Liss, Inc.