DEVELOPMENT OF THE DENDRITIC FIELDS OF LAYER-3 PYRAMIDAL CELLS IN THEKITTENS VISUAL-CORTEX

The cat's visual cortex is immature at birth and undergoes extensive p ostnatal development. For example, cells of layers 2 and 3 do not comp lete migration until about 3 weeks after birth. Despite the importance of dendritic growth for synaptic and functional development, there ha ve been few studies of dendritic development in the cat's visual corte x to correlate with numerous studies of functional and synaptic develo pment. Accordingly, we used the Golgi method to study the development of the dendrites of layer 3 pyramidal cells in the visual cortex of a series of cats ranging in age from 2 days to 3 years. Blocks of visual cortex were impregnated by the Golgi-Kopsch method and sectioned in t he tangential plane. Layer 3 pyramidal cells were drawn with a camera lucida and analyzed by Shell diagrams and vector addition. In kittens <1 week old, these cells were very immature, with only an apical dendr ite and no basal dendrites. Basal dendrites appeared during the second week. By 2 weeks, all of the basal dendrites had emerged from the som a, but they had few branches and were tipped with growth cones. By 4 w eeks, they had finished branching but continued to grow in length unti l, by 5 weeks, they reached their adult size. Examination of the basal dendritic fields in the tangential plane revealed that the dendritic fields were more elongated at 2 weeks than at later ages, perhaps beca use of their smaller size. The distribution of dendritic field orienta tions was uniform at all ages except 3 and 4 weeks, when there was a p reponderance of fields oriented in the rostrocaudal direction. Because dendritic growth and branching occurred very rapidly over a period th at precedes and overlaps with the peak periods of synaptogenesis and o f sensitivity to the effects of early visual experience, they may depe nd on afferent visual activity. The early emergence of primary dendrit es, however, suggests that this process is independent of afferent act ivity. The coincident timing of dendritic branching with the presence of dendritic growth cones suggests that branching may occur at growth cones. (C) 1994 Wiley-Liss, Inc.