Changes in somatodendritic morphometry of rat oculomotor nucleus motoneurons during postnatal development

Abstract This work investigates the somatodendritic shaping of rat oculomotor nucleus motoneurons (Mns) during postnatal development. The Mns were functionally identified in slice preparation, intracellularly injected with neurobiotin, and three‐dimensionally reconstructed. Most of the Mns (∼85%) were multipolar and the rest (∼15%) bipolar. Forty multipolar Mns were studied and grouped as follows: 1–5, 6–10, 11–15, and 21–30 postnatal days. Two phases were distinguished during postnatal development (P1–P10 and P11–P30). During the first phase, there was a progressive increase in the dendritic complexity; e.g., the number of terminals per neuron increased from 26.3 (P1–P5) to 47.7 (P6–P10) and membrane somatodendritic area from 11,289.9 μm 2 (P1–P5) to 19,235.8 μm 2 (P6–P10). In addition, a few cases of tracer coupling were observed. During the second phase, dendritic elongation took place; e.g., the maximum dendritic length increased from 486.7 μm (P6–P10) to 729.5 μm in adult Mns, with a simplification of dendritic complexity to values near those for the newborn, and a slow, progressive increase in membrane area from 19,235.8 μm 2 (P6–P10) to 24,700.3 μm 2 (P21–P30), while the somatic area remained constant. In conclusion, the electrophysiological changes reported in these Mns with maturation (Carrascal et al. [2006] Neuroscience 140:1223–1237) cannot be fully explained by morphometric variations; the dendritic elongation and increase in dendritic area are features shared with other pools of Mns, whereas changes in dendritic complexity depend on each population; the first phase paralleled the establishment of vestibular circuitry and the second paralleled eyelid opening. J. Comp. Neurol. 514:189–202, 2009. © 2009 Wiley‐Liss, Inc.