Postnatal differentiation of "presynaptic dendrites" in the lateral geniculate nucleus of the rhesus monkey

The most characteristic synaptic arrangement in the LGN is the triadic unit, in which a retinal terminal is presynaptic to a principal cell (P cell) and to a Golgi interneuron (I cell) dendrite, which contains synaptic vesicles and is in turn presynaptic to the same P-cell element. The ontogenetic differentiation of these "presynaptic dendrites" was studied in monkey LGN by standard and quantitative electron microscopy. The dendrites and axonal arborization of I cells are well developed in the newborn monkey. Scattered synaptic vesicles are present in the dendrites, but these profiles exhibit only postsynaptic sites. The dendrodendritic synapse of the triadic arrangement is missing, although contacts between P-cell and I-cell dendrites can be observed. Conversely, the I-cell axons in the newborn establish numerous synapses with dendrites and perikarya of P cells. At about 2 weeks of age, presynaptic sites appear in the I-cell dendrites, resulting in the formation of synaptic triads. Parallel to the development of "axonal" properties in the dendrites, the number of true I-cell axonal profiles decreases sharply. These transformations become progressively more frequent with age, and beyond 8 weeks the LGN ultraarchitectonics approaches that of the mature animal. The percentage of the surface of interneuron processes occupied by presynaptic sites is similar at all developmental stages at about the 2.6% level. The relative contribution of presynaptic dendrites and of axons changes, however, so that the actual length of contacts in the mature monkey is only one-half that of the newborn for the axonal sites, and over 50 times longer for the dendritic sites. The correlation of these findings with electrophysiologic and behavioral references suggests that some but not all axonal functions may be taken over by the dendrites, that the possible inhibitory phasing of P-cell discharge would appear only after the second postnatal week, and that the triadic arrangement may not be indispensable for brightness or total luminous flux discrimination but could be required for more complex forms of visually guided behavior.