FINE-STRUCTURE AND DEVELOPMENT OF DORSAL-ROOT GANGLION NEURONS AND SCHWANN-CELLS IN THE NEWBORN OPOSSUM MONODELPHIS-DOMESTICA

The aim of these experiments was to determine the state of maturity of dorsal root ganglia and axons in opossums (Monodelphis domestica) at birth and to assess quantitatively changes that occur in early life. C ounts made of dorsal root ganglion cells at cervical levels showed tha t the numbers were similar in newborn and adult animals, approximately 1,600 per ganglion. In cervical dorsal root ganglia of newborn animal s, division of neuronal precursors cells had ceased. The number of axo ns in cervical dorsal roots was similar in newborn and adult animals ( about 4,500). For each ganglion cell body, approximately three axons w ere counted in the dorsal root. At birth, dorsal roots contained sever al bundles about 30 mu m in diameter consisting of small axons (0.05-2 mu m in diameter). A few non-neural cells were identified as Schwann cell perikarya, each enclosing a number of neurites. Later, marked cha nges occurred in Schwann cells and in their relationship to axons in t he roots. Thus, at 12 days, an increase occurred in the number of Schw ann cells and fibroblasts, and the bundles had enlarged to about 80 mu m with little increase in axon diameter (0.1-2 mu m). By 18 days, the bundles were larger, and myelination had already started. At 23 days, the dorsal root contained more than 500 myelinated axons that could r each 5 mu m in diameter. The adult dorsal root enclosed about 900 myel inated axons. Throughout this time, the relationship between the Schwa nn cells and axons changed. Together, these results indicate that the number of axons and cell bodies of sensory dorsal root ganglia in opos sum do not show major changes after birth. In addition, these results set the stage for quantitative studies of regeneration of dorsal colum n fibers in injured neonatal opossum nervous system. (C) 1998 Wiley-Li ss, Inc.