Advances in Bioscience and Biotechnology, 2013, 4, 785-797 ABB doi:10.4236/abb.2013.47103 Published Online July 2013 (http://www.scirp.org/journal/abb/ ) Selective and constructive mechanisms contribute to neural circuit formation in the barrel cortex of the developing rat Eileen Uribe-Querol 1* , Eduardo Martínez-Martínez 2 , Luis Rodrigo Hernández 2 , Patricia Padilla Cortés 3 , Horacio Merchant-Larios 2 , Gabriel Gutiérrez-Ospina 2 1 División de Estudios de Posgrado e Investigación, Facultad de Odontología, Universidad Nacional Autónoma de México, Mexico City, Mexico 2 Cell Biology and Physiology Department, Instituto de Investigaciones Biomédicas, Universidad Nacional Autónoma de México, Mexico City, Mexico 3 Unidad de HPLC, Instituto de Investigaciones Biomédicas, Universidad Nacional Autónoma de México, Mexico City, Mexico Email: * euquerol@comunidad.unam.mx Received 26 May 2013; revised 28 June 2013; accepted 7 July 2013 Copyright © 2013 Eileen Uribe-Querol et al. This is an open access article distributed under the Creative Commons Attribution Li- cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT The cellular strategy leading to formation of neuronal circuits in the rodent barrel cortex is still a matter of controversy. Both selective and constructive mecha- nisms have been proposed. The selective mechanism involves an overproduction of neuronal processes and synapses followed by activity dependent pruning. Conversely, a constructive mechanism would increase the number of axons, dendrites, and synapses during development to match functionality. In order to dis- cern the contributions of these two mechanisms in establishing a neuronal circuit in the somatosensory cortex, morphometric analysis of dendritic and ax- onal arbor growth was performed. Also, the number of synapses was followed by electron microscopy during the first month of life. We observed that ax- onal and dendritic arbors retracted distal branches, and elongated proximal branches, resulting in in- creased arbor complexity. This neuronal remodeling was accompanied by the steady increase in the num- ber of synapses within barrel hollows. Similarly, the content of molecular markers for dendrites, axons and synapses also increased during this period. Fi- nally, cytochrome oxidase activity rose with age in barrels indicating that the arbors became more com- plex while synapse density and metabolic demands increased. Our results support the simultaneous use of both selective and constructive mechanisms in es- tablishing the barrel cortex circuitry. Keywords: Barrel Cortex; Synaptogenesis; Axonal Growth; Dendritic Growth; Thalamocortical Pathway 1. INTRODUCTION During the ontogeny of the nervous system, axonal and dendritic branches, as well as synapses, are thought to be formed in numbers that far exceed the amount required by the mature brain to function properly. The predomi- nant theory for neuronal circuit formation is the selective mechanism, which states that a period of overproduction of neuronal processes and synapses is followed by the elimination of a significant fraction of them. This process of elimination is driven by competition. Axons or den- drites compete for synaptic partners that may provide neurotrophic factors in sufficient amount to consolidate synapses and retain their partners [1]. Axonal and den- dritic branches that establish synaptic-like contacts with incorrect partners are eliminated because they do not re- ceive enough trophic support. Even though the notion described above constitutes the dominant view about how the developing brain sculpts its circuitry, increasing anatomical evidence supports that the selective mecha- nism is not a universal rule. Indeed, anatomical evidence gathered during the development of different brain struc- tures indicates that axons may grow towards and within their targets by preferentially adding branches with a high degree of precision until they attain their adult size [2]. Similarly, multiple reports also support the notion that the dendritic arbors of some classes of neurons also develop preferentially by adding individual branches until they reach their adult configuration [3]. The rodent primary somatosensory cortex (S1) processes tactile in- formation and contains a representation of the body formed by modules called barrels. Each barrel contains an anatomical map that represents collections of mech- anosensory receptors distributed throughout the body. * Corresponding author. Published Online July 2013 in SciRes. http://www.scirp.org/journal/abb