L. Chimelli et al., DEVELOPMENT OF CORTICAL FIBERS INTO THE GRACILE NUCLEUS OF THE RAT - AN ULTRASTRUCTURAL-STUDY, Developmental neuroscience, 16(5-6), 1994, pp. 260-266
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.