Early postnatal development of entopeduncular neurons and their neostriatal connections

The early postnatal development of dendrites and axodendritic connectivity in the entopeduncular nucleus of kittens was studied by light and electron microscopy. We focused on the fine structure of neuropil and the organization of synaptic elements at several ages during the first weeks of life. We were able to define, in newborn to 1-week-old kittens, several of the distinct types of nerve endings seen previously by us in adult animals. They were found singly or in small clusters widely spaced along surfaces of dendrites and cell bodies. Most endings were small, contained 45 to 50-nm synaptic vesicles, and formed symmetrical synaptic junctions. Fewer large endings, filled with 30- to 35-nm vesicles, formed asymmetrical synapses. By the end of the second week, most dendrites and much of the cell body surfaces were ensheathed by these endings. Astroglial elements apposed nonsynaptic surfaces and enveloped arrays of axodendritic synapses. We used Fink and Heimer staining of degenerating axons to show that much of this early connectivity originated in the neostriatum. These morphologic studies support physiologic data that striopallidal pathways are functional at birth and suggest that many of the age-related changes in response parameters reflect a quantitative increase in axodendritic connectivity. To assess this notion, we initiated a computer-assisted study of Golgi-impregnated neurons and measured dendritic growth during this period. Preliminary data suggested dendritic fields are extensive in 3-day-old kittens (mean total dendritic length of 2800 μm) and individual dendrites radiate for long distances and branch sparsely (mean total branch length of 700 μm). By 18 days of age, these parameters had increased by about 40% to 4800 and 1200 μm, respectively.