Geniculocortical projection to layer I of area 17 in kittens: orthograde and retrograde HRP studies.

Using the orthograde and retrograde HRP methods, the distribution of geniculocortical (G-C) afferent terminals in area 17 was examined in kittens and an adult cat. Following HRP injection in the entire dorsal lateral geniculate nucleus (LGNd), afferent terminals were labeled in layer I and deeper layers. The terminals in layer I in young kittens (less than 2 weeks old) were densely distributed throughout layer I, whereas in old kittens (over 1 month old) they were sparsely distributed in the outer half of layer I. When the HRP injection site was confined to the A laminae (lamina A and lamina A1), a few HRP-labeled terminals in layer I were seen in young kittens but not in older kittens or an adult cat. Following HRP injection in layer I of area 17 in young kittens, relatively large neurons were labeled retrogradely. The labeled neurons were located chiefly in the C complex and partly in the A laminae. The present experiments revealed that the G-C afferents to layer I of area 17 in young kittens originate from relatively large neurons which are located chiefly in the C complex and partly in the A laminae and that these afferents give dense terminals covering the entire layer I. These findings contrast with those obtained in adult cats, in which the G-C afferents to layer I originate exclusively from small neurons in the C complex and give sparse terminals to the outer half of layer I.