Postnatal development of the corticothalamic projections from the visual association cortex to the lateralis...

The postnatal development of the corticothalamic projection from the lateral suprasylvian cortex (LS) to the lateral medialis-suprageniculate nucleus (LM-Sg) of the cat thalamus was assessed by means of the anterograde tracer biocytin. In the adult, two types of corticothalamic fibers were found: type I established a network of fine fibers present throughout the LM-Sg, it was characterized by a linear sequence of small (less than 0.5 microm in diameter), single terminal boutons making contact mainly with thin dendrites and/or dendritic spines. Type II, found less frequently, gave off short, side branches near axon terminals and formed clusters of 5-10 large terminal boutons (0.5-1.5 microm in diameter), making contact predominately with medium-sized dendrites and/or vesicle-containing profiles, forming a synaptic glomerulus. At birth (P0), anterogradely-labeled fibers were found in the LM-Sg as in adults. In the early postnatal period (until P6) as well as around the time of eye-opening (P7-P10) to P21, neonatal fibers were largely unbranched many of them having axons tipped with growth cones. Axon terminals containing synaptic vesicles were rarely observed but when present these exhibited considerable variation in their morphological appearance of synapses. Thus, it was not possible to categorize them into the two types of axons which characterize the adult. After P25, terminal swellings bearing a close resemblance to those of type II fibers begin to appear. In this way, the main two corticothalamic fiber types could be identified. These findings demonstrate that significant postnatal changes occur in the synaptology of corticothalamic fibers in the LM-Sg, particularly with the maturation of type II fibers.