DEVELOPMENT OF CORTICAL FIBERS INTO THE GRACILE NUCLEUS OF THE RAT - AN ULTRASTRUCTURAL-STUDY

A study was made on the development of cortical synapses in the gracil e nucleus of rats using degeneration methods. A total of 46 animals, 1 adult and 45 neonates whose ages varied from 1 to 7 days, had the rig ht somatosensory motor cortex destroyed. The survival period varied fr om 1 to 30 days. Identification of axonal terminals in the gracile nuc leus was also achieved by tracing the cortical fibres of 1 adult rat u sing horseradish peroxidase-wheat germ agglutinin (HRP-WGA). Degenerat ing axodendritic and axosomatic terminals that originated from cortica l fibres were seen in the adult animal which survived 2 days. Their or igin was confirmed by the presence of HRP-WGA inside the terminals. Li ght or electron microscopic changes were not seen, and in particular, the gracile nucleus was not smaller than in the control adult animals which survived 30 days or in neonates which survived 8-30 days, consis tent with the small component of cortifugal fibres believed to termina te in secondary sensory nuclei. In neonates that survived a shorter pe riod, terminal degeneration was only seen in cases operated at 4 days and later, indicating that cortical axons do not synapse in the gracil e nucleus until postnatal day 4. Our results provide further support f or the existence of a period in which the fibres approach their target but do not penetrate it to form synaptic junctions during the growth of cortical axons toward their targets, since previous studies have de monstrated that cortical fibres persisting to adulthood decussate comp letely at the level of the medulla at 12-36 h after birth. This may be characteristic of the normal development of neural pathways.