3-DIMENSIONAL VISUALIZATION OF THE DISTRIBUTION, GROWTH, AND REGENERATION OF MONOAMINERGIC NEURONS IN WHOLE MOUNTS OF IMMATURE MAMMALIAN CNS

At birth, the opossum, Monodelphis domestica, corresponds roughly to a 14-day-old mouse embryo. The aim of these experiments was to compare the distribution of monoaminergic neurons in the two preparations duri ng development and to follow their regeneration after injury. Procedur es that allowed antibody staining to be visible in transparent whole m ounts of the entire central nervous system (CNS) were devised. Neurons throughout the brain and spinal cord were stained for tyrosine hydrox ylase (TH) and for serotonin (5-HT). At birth, patterns of monoaminerg ic cells in opossum CNS resembled those found in 14-day mouse embryos and other eutherian mammals. By postnatal day 5, immunoreactive cell b odies were clustered in appropriate regions of the midbrain and hindbr ain, and numerous axons were already present throughout the spinal cor d. Differences found in the opossum were the earlier presence of TH ne urons in the olfactory bulb and of 5-HT neuronal perikarya in the spin al cord. Most, if not all, monoaminergic neurons in opossum were alrea dy postmitotic at birth. To study regeneration, crushes were made in c ervical cords in culture. By 5 days, 8% of all TH-labeled axons and 14 % of serotonergic axons had grown beyond lesions. Distal segments of m onoaminergic axons degenerated. In CNS preparations from opossums olde r than 11 days, no regeneration of monoaminergic fibers occurred. Isol ated embryonic mouse CNS also showed regeneration across spinal cord l esions, providing the possibility of using knockout and transgenic ani mals. Our procedures for whole-mount observation of identified cell bo dies and their axons obviates the need for serial reconstructions and allows direct comparison of events occurring during development and re generation. (C) 1998 Wiley-Liss, Inc.