AN ORGANOTYPIC SPINAL-CORD DORSAL-ROOT GANGLION SKELETAL-MUSCLE COCULTURE OF EMBRYONIC RAT .1. THE MORPHOLOGICAL CORRELATES OF THE SPINAL REFLEX ARC

The cytoarchitecture of a spinal cord - dorsal root ganglion - skeletal muscle tissue coculture system was investigated at the level of the light microscope using a number of different staining techniques. In these cultures central synapses between dorsal root ganglion (DRG) cells and interneurons in the ventral spinal cord and between DRG cells and motoneurons were visualized by parvalbumin immunostaining and by intracellular horseradish peroxidase (HRP) filling of DRG cells. Skeletal muscle fibres regenerated in vitro first into multinucleated myotubes, and around day 8 in vitro into well differentiated muscle fibres with regular cross-striation. At the same time newly formed motor endplates could be visualized using acetylcholinesterase staining. The axons of motoneurons could be traced retrogradely by local application of HRP to the regenerated muscle fibres. The motor axons sometimes gave off collaterals reminiscent of Renshaw collaterals at about 300-mu-m from the axon hillock. Intracellular filling of motoneurons with HRP revealed that only a minority of the motoneurons within a culture had reached their appropriate target. Comparing the dendrograms of the motoneurons which had innervated muscles to those which had not suggested that motoneurons innervating muscle tissue had more complex dendritic trees and larger somata than those which did not innervate muscle tissue. Peripheral neurites of parvalbumin-immunoreactive DRG cells coiling around regenerated muscle fibres could be demonstrated in these cultures. These probably correspond to that part of the sensory muscle spindle apparatus which developed in vivo. However, only a few of the several hundred DRG cells found in every culture were parvalbumin-immunoreactive, suggesting that the actual number of Ia and II afferents within the population of DRG cells in culture is very small. This study demonstrates that all the neural elements necessary for the segmental spinal reflexes develop and can be maintained for several weeks in vitro.