Journal of Neuroscience Methods 165 (2007) 18–24 Recurrent laryngeal nerve post-natal development in rats Nara Terezinha Montali Silva Lima a , Val´ eria Paula Sassoli Fazan b , Jos´ e Fernando Colafˆ emina a , Amilton Antunes Barreira c,∗ a Department of Ophthalmology and Otolaryngology, Medical School of Ribeir˜ ao Preto, University of S˜ ao Paulo, 14049-900, Ribeir ˜ ao Preto, S˜ ao Paulo, Brazil b Department of Surgery and Anatomy, Medical School of Ribeir˜ ao Preto, University of S˜ ao Paulo, 14049-900, Ribeir ˜ ao Preto, S˜ ao Paulo, Brazil c Department of Neurology, Medical School of Ribeir˜ ao Preto, University of S˜ ao Paulo, Av. Bandeirantes 3900, Monte Alegre, 14049-900, Ribeir˜ ao Preto, S˜ ao Paulo, Brazil Received 16 March 2007; received in revised form 13 May 2007; accepted 17 May 2007 Abstract Studies on the normal pattern of development and post-natal maturation of the recurrent laryngeal nerve in rats are scanty. The aim of the present study was to investigate the normal microscopic aspects of the recurrent laryngeal nerve myelinated fibers in the post-natal developing rat, with special attention to longitudinal morphology and lateral symmetry. Fifteen male rats were divided into experimental groups according to age 20, 55, 76, 150 and 250 days. A female group aged between 76 and 150 days was also used. Right and left RLN were studied by light microscopy at proximal, medium and distal segments and morphometric data comparisons were made between sides, segments, ages and gender. Our results showed that the left recurrent laryngeal nerves were significantly longer than the right in all ages studied and this difference increased with ageing. There was a slight decrease in the myelinated fiber number, according to proximal to distal gradient, but a significant decrease was observed only on older animals (ages 55 (both sides), 76 (left side), and 150 (left side) days). This difference was also observed for female rats (left side). No differences between ages were observed. There was an age-dependent difference on ranges and histograms form (younger animals showed a unimodal histogram while older animals showed a bimodal one), with no significant differences between segments or sides, in all groups studied. Also, no differences between males and females of the same age were observed. In conclusion, the RLN alters its morphology due to development and ageing and the present study describe normal patterns of the recurrent laryngeal nerve development in rats that can be useful for a better understanding of pathological alterations on experimental neuropathies involving the laryngeal nerves. © 2007 Elsevier B.V. All rights reserved. Keywords: Recurrent laryngeal nerve; Rat; Development; Morphometry 1. Introduction Despite the importance of the recurrent laryngeal nerve (RLN) in the maintenance of laryngeal function in terms of both respiration and phonation, studies on the normal pattern of devel- opment and post-natal maturation of this nerve in rats are scanty. Peripheral nerve function is significantly affected by maturation and aging. However, knowledge regarding differences between the nerves of adult and aged animals has been based on compar- isons of only two experimental groups, and it has been pointed out that there is a need for multiple time points in maturation and ∗ Corresponding author. Tel.: +55 16 3602 3307; fax: +55 16 3602 3307. E-mail address: aabarrei@fmrp.usp.br (A.A. Barreira). aging studies (Colleman et al., 1990; Jeronimo et al., 2005). Rats described as adult have weights that vary between about 180 and 1000 g or more (Saitua and Alvarez, 1988), with no information about the animals’ ages, which might lead to confusion. On the other hand, investigations of the maturation of a nerve in very young animals might indicate the period of most rapid post-natal differentiation of this nerve. It is well known that degenerative changes in peripheral nerves occur first in the distal portion of the fibers (Groover-Johnson and Spencer, 1981; Ceballos et al., 1999), indicating the need for longitudinal studies in experimen- tal neuropathies. Despite the fact that it is well known that the RLN is asymmetric in length, because of its different origin on both sides, a longitudinal study of the RLN in rat is not available. Studies on the post-natal development of laryngeal reflexes, including the study of laryngeal nerves myelination and con- 0165-0270/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jneumeth.2007.05.012