Barrelettes—architectonic vibrissal representations in the brainstem trigeminal complex of the mouse. II. Normal post‐natal development

Abstract

Vibrissal representations in the brainstem trigeminal complex (BTC) of rodents are manifested as architectural sub‐units called barrelettes. The development of barrelettes was studied by using Nissl staining, cytochrome oxidase histochemistry, and Golgi‐impregnation methods. On the day of birth (PND‐1), barrelettes are manifested as longitudinal, histochemical cylinders in sub‐nuclei principalis, interpolaris and caudalis of the BTC. One day later (PND‐2), fully formed histochemical barrelette formations are seen in the three sub‐nuclei. The development of cytoarchitectural barrelettes lags behind histochemical barrelettes by about two days. Between PND‐2 and PND‐3, longitudinal cytoarchitectonic cylinders begin to appear. By PND‐3, BTC neurons segregate into five rows of barrelettes in the coronal plane. Segmentation of rows into individual barrelettes begins on PND‐4, and complete cytoarchitectonic barrelette formations are seen by PND‐5. Golgi‐impregnation shows that on the day of birth, primary afferent terminals and dendritic arbors of second‐order trigeminal neurons within the BTC are short and poorly ramified. Over the next five post‐natal days, lengthening of these processes as well as elaboration into secondary and tertiary branches take place. Growth of these processes continues for two additional weeks, contributing to the increase in barrelette neuropils (hollows). As the neuropils expand, neuronal somata are pushed toward barrelette sides. Morphometric measurements show that there is a relatively constant rate of growth of barrelettes over the first three post‐natal weeks. The growth rate of the barrelette formations is identical to that of BTC as a whole. Thus, at the time of birth, the volume of neural tissue in the brainstem allotted to vibrissae is fixed relative to that allotted to other sensory receptors. Several features of the early development of barrelettes are identified: (1) Chemoarchitectural barrelettes appear before cytoarchitectural barrelettes, suggesting that terminal arbors of primary trigeminal afferents are organized before their target neurons form barrelettes. (2) Early cytoarchitecture is manifested in the form of unsegmented rows, suggesting that rough, row‐based topological maps are first formed, which are then fine‐tuned into individual sub‐units. Recent evidence shows that other vibrissal representations—thalamic barreloids and cortical barrels—also follow these “afferent‐before‐target” and “row‐before‐individual units” sequences of development. This gradual, afferent‐dependent fine‐tuning of topological organization is analogous to similar events during the early development of the visual system, and may be a general feature of developing sensory systems. In the visual system, sensory input plays an important role in fine‐tuning centers in the brain; the mechanisms by which barrelettes are organized and fine‐tuned are still unknown. 1993 Wiley‐Liss, Inc.