Development of the kitten substantia nigra: a rapid Golgi study of the early postnatal period

The rapid Golgi method was used to describe the morphological maturation of substantia nigra (SN) neurons and the surrounding afferent axons. Observations were made from 25 kittens grouped at 1-3, 7-10, 18-24, and 40-55 days of age. Although variability in dendritic development among neurons is seen at each age, a common maturational sequence can be defined. The dendritic shafts at 1-3 and 7-10 days exhibit prominent varicosities and thin filiform processes along their shafts, and growth cones at their tips. The dendrites at 18-24 days are longer, thicker, and have more regular contours proximally, while varicosities and filiform processes persist distally. Neurons in this age group display shorter, spine-like processes although adult cells are known to lack typical spines. By 40-55 days, most dendritic surfaces are smooth with only scattered appendages distally. Computer-assisted measurements of dendritic growth in pars compacta neurons demonstrate a 42% increase in dendritic lengths. The number of dendrites per neuron and the number of branches per dendrite do not change between the youngest and oldest age groups. The afferent connectivity in the kitten SN is en passant in character. Two axonal types are identified at all ages. Most prominent is a thin fiber with irregularly spaced varicosities and terminal expansions 1 micron in diameter. These swellings correspond, most likely, to the type I terminals described ultrastructurally and shown to be neostriatal in origin. The second axonal type is thicker, with branches that end in clusters of enlargements, 1-2 microns in diameter. The origin of this second type is unknown.