Postnatal development of lamina III/IV nonpyramidal neurons in rabbit auditory cortex: Quantitative and spatial analyses of Golgi‐impregnated material
Authors: McMullen Nathaniel T. , Goldberger Bruce , Glaser Edmund M. 
Published: January 2005

We have studied the postnatal development of lamina III/IV spine‐free nonpyramidal neurons in the auditory cortex of the New Zealand white rabbit. The morphology and dendritic branching pattern of single cells impregnated with a Golgi‐Cox variant were analyzed with the aid of camera lucida drawings and three‐dimensional reconstructions obtained with a computer microscope. Sample sizes of 20 neurons were obtained at birth (day 0), postnatal day (PD) 3, 6, 9, 12, 15, 21, and 30 days of age. Normative data were also available from PD‐60 and young adult rabbits studied previously. At birth, lamina II‐IV have not yet emerged from the cortical plate; immature nonpyramidal neurons at the bottom of the cortical plate (presumptive layer IV) are characterized by short, vertically oriented dendrites. Growth‐cone‐like structures are present along the shafts and at the tips of the dendrites. At birth, soma area and total dendritic length are, respectively, 34 and 10% of adult values. The cortical plate acquires a trilaminar appearance at PD‐3. The six‐layered cortex is present by PD‐6. During the first postnatal week dendritic length increases fourfold and is accompanied by a significant increase in both terminal and preterminal dendritic growth cones. At the onset of hearing at PD‐6, there is a significant proliferation of dendrites and branches to 144 and 200% of adult levels, respectively. These supernumerary dendrites are rapidly lost during the second postnatal week, at which time the somata and dendrites become covered with spines. The loss of higher‐order dendrites occurs more gradually; the number of dendritic branches is still 116% of adult values at PD‐30. Spine density peaks between days PD‐12 and PD‐15 and then gradually diminishes until the cells are sparsely spined or spine free by PD‐30. Total dendritic length increases in a linear fashion up to PD‐15, at which time it is 80% of adult values. An analysis of terminal and intermediate branches demonstrated that the increase in total dendritic length after PD‐6 is due entirely to the growth of terminal dendrites. Total dendritic length attains adult levels by PD‐30. Spatial analyses revealed that a vertical orientation of dendrites is present at birth. Associated with the onset of hearing at PD‐6, there is an explosive elaboration of dendrites toward the pial surface. This asymmetry in dendritic orientation peaks at PD‐15 and gradually recedes because of the loss of supernumerary branches. The relatively symmetric vertical orientation of dendrites is achieved by PD‐30. In contrast, the significant tangential orientation of mature neurons is not yet present at PD‐30. Its appearance after PD‐30 may be the result of the selective loss of some terminal dendrites and continued growth of surviving dendrites along the y‐axis. The growth spurt exhibited by developing nonpyramidal cells at the onset of hearing suggests that the appearance of patterned acoustic activity has a stimulatory influence on dendritic branching. The elaboration and resultant loss of supernumerary dendritic branches and spines may reflect competitive interactions among afferent fiber systems and the maturation of cortical synaptic circuitry.