[How is the sense of smell connected? Cellular and molecular mechanisms guiding the development of the synaptic connections from the nose to the cortex (I)].

The physiological particularities that occur during the development of the olfactory system make it one of the most fascinating parts of the central nervous system and one of models that has been most widely studied in order to understand the mechanisms related with axonal growth and guidance towards the right targets. A variety of mechanisms are known, some mediated by contact (laminins, cell adhesion molecules, ephrins, etc.) and others that are secreted (semaphorins, slits, growth factors, etc.), to play diverse roles in establishing the synaptic interactions among the olfactory epithelium, the olfactory bulb and the olfactory cortex. In relation to this, other specific mechanisms for this system have also been proposed, including the incredible family of close to 1000 different olfactory receptors. In recent years, different reviews have focused on the partial elements of this system, especially on the mechanisms involved in the formation of the olfactory nerve. However, no detailed review of those related with the development of the connections between the different olfactory structures (epithelium, bulb and cortex) has been put forward to date. In this first part of the review, we address this topic from the following perspective: the different cellular and molecular mechanisms that guide the formation of the olfactory nerve and the lateral olfactory tract.