Postnatal dendritic development in motoneurons: evaluation by a Monte Carlo technique.

A Monte Carlo method was used to quantitate the dendritic characteristics of horseradish peroxidase-injected lumbar motoneurons (MNs) from kittens 44 to 73 days old (during the postnatal interval when motor cortical input/output linkages mature) and compared with those from adults. All of the 6 MNs analyzed were within the same range of somal sizes (40-61 micron) and dendritic domain volumes. However, two-dimensional distributions of dendritic diameter versus distance from the soma revealed that in the youngest MNs' (44 and 51 days) most dendritic processes (82 and 84%) were less than 2 micron in diameter, and had no dendrites greater than 5 micron. The adult MNs had large dendrites (20 and 24% were greater than 5 mu), while the kittens of intermediate age (66 and 73 days) exhibited both adult-like (6 and 13% greater than 5 micron) and immature (48 and 73% less than 2 micron) characteristics. "Hit probabilities" calculated from the Monte Carlo data suggest that immature MNs' dendrites may present denser targets per unit volume for ingrowing growth cones than adult MNs, and may facilitate the probability of contact with appropriate afferent axons.