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

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Abstract
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.

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