Postnatal maturation of neurons in the cat's lateral geniculate nucleus.

The maturation of dendrites in the cat's dorsal lateral geniculate nucleus was studied in Golgi Kopsch preparations of kittens from 3 days to 8 weeks postnatal. During the first postnatal week, more than a month after their birthdate, cells are immature and lack dendrites, bearing only multiple somatic processes or a few short thick extensions. Cells enter an active phase of dendritic extension during the second postnatal week. Growth cone-like structures and filopodia occur at the ends of dendrites and also at dendritic branch points. Assignment to general cell classes based on dendritic disposition is possible only after this period, and characteristic grapelike appendages are obvious after the third week. Mature cells in the lateral geniculate nucleus are not considered spiny, yet spines and hairs are ubiquitous on most cells once dendrites elongate and remain numerous on peripheral dendrites even after the soma and proximal dendrites become smooth, by 4-6 weeks. The decline of spine levels continues after this period. All cells go through a similar but nonsynchronous sequence of maturation. Large cells may mature first, but no correlation was noted between rate of maturation and laminar location or retinal representation. In the second and third postnatal weeks, although the terminal arbors of retinal axons presynaptic to geniculate cells have already attained their final topography and laminar placement, the shape and synaptic relations of axon terminal swellings remain immature (Mason, '82a,b) through the most active phase of dendritic outgrowth. After 3 weeks, both retinal axons and target geniculate cell dendrites finalize the shapes of characteristics appendages and synaptic relations in tandem. Potential interactions between immature axon terminal arbors and dendrite-bare geniculate cells during dendrite outgrowth and subsequent remodeling of structural details are discussed.