Development of Y‐axon innervation of cortical area 18 in the cat.

1. Geniculocortical Y‐axons (n = 38) in the optic radiations of 4‐5‐week‐old kittens (n = 20) and adult cats (n = 18) were studied both physiologically and morphologically. Axons were recorded from intracellularly and subsequently filled ionophoretically with horseradish peroxidase (HRP). The HRP filled the axons' terminal arborizations in visual cortex (particularly well for those innervating area 18). Fourteen axons appeared to be completely filled with HRP (n = 8 in kitten, n = 6 in adult) and served as the basis for the quantitative analysis of the terminal arborizations reported in this study. 2. The distribution and correspondence of the axonal boutons to presynaptic elements in cortical layer 4A was analysed at both the light and electron microscope level using computerized three‐dimensional analysis and serial section reconstruction, respectively. Compared to adult axons, the boutons of the kitten axons were smaller (means = 0.75 vs. 1.75 microns length, P less than 0.001) and more densely spaced both along individual axon branches (means = 6.60 vs. 11.20 microns interbouton interval, P less than 0.001) and between neighbouring branches of the same axon (means = 4.7 vs. 6.4 microns nearest‐neighbour distance, P less than 0.01). 3. Most kitten boutons made a single Gray's type 1 synapse on a cortical neurone, unlike adult boutons which usually contacted two or more postsynaptic targets. Both kitten and adult axons had dendritic spines as their major target. Occasionally, HRP reaction‐product was observed in cortical neurones postsynaptic to the labelled geniculocortical axon, which gave some estimate of the number of synaptic contacts between a single geniculocortical axon and target cell (about five). 4. The kitten Y‐axons innervated the visual cortex in a pattern similar to that of the adult, with the richest terminal branching and bouton density in layer 4A with some additional boutons distributed in layers 3, 4B and 6. The extent of the terminal arborizations primarily in layer 4A (as measured in surface views) of kitten Y‐axons in area 18 was significantly less than that of adult Y‐axons in area 18 (means = 0.9 mm2 vs. means = 1.2 mm2, P = 0.04). 5. We conclude that between 4 and 5 postnatal weeks and 1 year, geniculocortical Y‐axons projecting to cortical area 18 undergo four major changes. These include a reduction in synaptic bouton density (both in three‐dimensional space and along individual branches), a concomitant moderate expansion in the surface area of cortex innervated, an increase in bouton size and an increase in the number of synaptic contacts made by each bouton. A general proportional growth of the individual axons' terminal arborization together with fusion and/or separation of neighbouring boutons is sufficient to explain this development.