Development of neuronal types and laminar organization in the central nucleus of the inferior colliculus in the cat.

The development of neuronal morphology and laminar organization in the central nucleus of the inferior colliculus has been studied with the different Golgi methods in kittens and cats of 1 day-2 years of age. The different Golgi methods used allowed us to selectively visualize the axonal or dendritic component of the fibrodendritic laminae. The characteristic lamination of the central nucleus defined by the fiber system of the lateral lemniscus is already present at birth. The axonal component of the laminae is constituted by parallel condensations of varicose terminals, myelinated axons, and preterminal fibers, oriented from ventrolateral to dorsomedial. The laminae are smaller in the dorsolateral edge of the nucleus. Neurons are classified mainly on the basis of their dendritic trees and the axonal ramification patterns. Three main types are distinguished: spinous disk-shaped neurons, aspinous to sparsely spinous disk-shaped neurons, and large or giant multipolar neurons. Our results suggest that the basic structures of the central nucleus--neuronal types and lamination of the lemniscal fibers--are already established at birth. The different neuronal types can be distinguished from the first days of life according to the ramification pattern of dendritic and axonal arbors. The characteristics of the different cell types, such as the density and distribution of dendritic spines, and the presence of varicose dendritic branchlets, are recognizable from the second week. At the end of the first month, neurons display an adult-like morphology, although the density of dendritic spines is higher than in the adult. Our morphological data can be related to the development of response properties in the inferior colliculus.