Postnatal development of cat hind limb motoneurons. II: In vivo morphology of dendritic growth cones and the maturation of dendrite morphology.

The maturation of dendrite morphology was studied by light and electron microscopy in cat spinal alpha-motoneurons intracellularly labeled with horseradish peroxidase. Alpha-motoneurons supplying the triceps surae (TS) and the intrinsic foot sole (SP) muscles were investigated in kittens from birth to 44-46 days of postnatal (d.p.n.) age. At birth, a large number of dendritic branches displayed growth cones, filopodia, and fusiform processes. The growth cones were of lamellipodial and filopodial types, but intermediate forms also occurred. The growth cones shared several morphological features with the neuritic growth cones studied in vitro. It was suggested that the occurrence of different types of growth cones--even in the same dendrite--may reflect their transformation from one type to the other and the level of growth activity could be inferred from the number and form of the growth cones. About 50-70% of the terminal branches in the dendrites of newborn kittens possessed growth cones, filopodia, and/or fusiform processes. The corresponding figure for preterminal branches was 20-30%, with a clear decrease in incidence when approaching the soma. During the period under study, most of these growth-associated processes disappeared from the dendrites so that at 44-46 d.p.n. of age only about 10% of the terminal and less than 1% of the preterminal branches had growth-associated processes. Analysis of the three-dimensional distribution of dendritic branches with such processes disclosed that they were relatively more frequent in the medial, rostral, and caudal dendritic territories. It was concluded that the pattern of distribution and disappearance of growth cones, filopodia, and fusiform processes coincided with postnatal longitudinal dendritic growth and the development of the adult dendritic territories described in a preceding paper (Ulfhake et al., '88). Dendritic growth, with respect to length and caliber, also occurred in the absence of growth cones and filopodia. It is suggested that the important role of these processes may be to act as a steering device in establishing the adult distribution and synaptology of the dendrites. Comparison of TS and SP alpha-motoneuron dendrite morphology at birth and at 22-24 d.p.n. age showed that the SP neurons lagged in the maturation process. Light and electron microscopic observations indicated that postnatally direct contacts might exist between dendrites and fine blood vessels in the neuropil without any interposing glial sheath. The number of such suspected contacts diminished during the period under study, indicating that the glial ensheathment of the blood vessel takes place, in part, postnatally.