Evolutionary Conservation of the Early Axon Scaffold in the Vertebrate Brain

Abstract

The early axon scaffold is the first axonal structure to appear in the rostral brain of vertebrates, paving the way for later, more complex connections. Several early axon scaffold components are conserved between all vertebrates; most notably two main ventral longitudinal tracts, the tract of the postoptic commissure and the medial longitudinal fascicle. While the overall structure is remarkably similar, differences both in the organization and the development of the early tracts are apparent. This review will bring together extensive data from the last 25 years in different vertebrates and for the first time, the timing and anatomy of these early tracts have been directly compared. Representatives of major vertebrate clades, including cat shark, Xenopus, chick, and mouse embryos, will be compared using immunohistochemistry staining based on previous results. There is still confusion over the nomenclature and homology of these tracts which this review will aim to address. The discussion here is relevant both for understanding the evolution of the early axon scaffold and for future studies into the molecular regulation of its formation.

Regulation of downstream neuronal genes by proneural transcription factors during initial neurogenesis in the vertebrate brain

“…Despite the fact that Nlhh1 , Tagln3, and Stmn2 are considered pan-neuronal markers they have, to some extent, specific expression at the level of the first neurons establishing the early axon scaffold tracts in the amniote brain [55]. We show that each of these neuronal populations have a specific combination of these neurogenic markers during differentiation (Table 2).…”

“…Most importantly, with a few exceptions, the expression pattern of these neuronal markers is highly conserved between chick and mouse (Table 2). Still, it is surprising to see that Nhlh1 and Tagln3 are not expressed in the mouse nmesV until after the first neurons differentiated at E8.5 [55], whereas Nhlh1 and Tagln3 are early markers for post-mitotic neurons in the chick [44]. Further analysis will be required to determine the function of this discrepancy as ultimately these neuronal populations express the same genes in both the chick and mouse brains.…”

“…This is an evolutionary conserved structure, formed from longitudinal, transversal and commissural axon tracts that act as a scaffold for the guidance of later axons [12, 55, 57, 59]. Each tract is formed from a small neuronal population, including the nucleus of the medial longitudinal fascicle (nMLF), the nucleus of the tract of the postoptic commissure (nTPOC), the nucleus of the mammillotegmental tract (nMTT), the nucleus of the tract of the posterior commissure (nTPC) and the nucleus of the descending tract of the mesencephalic nucleus of the trigeminal nerve (nmesV) (see Table 1 for abbreviations).…”

The emergence of mesencephalic trigeminal neurons

“…MTN neurons are generated close to the dorsal midline and in many jawed vertebrates they first appear in the posterior midbrain, close to the isthmus, before spreading anteriorly [13, 15]. Thus, their formation may be controlled by the isthmus and the dorsal midline, and involve signals secreted by these territories.…”

“…They are generated either side of the dorsal midline forming first posteriorly close to the isthmus but then later across the anteroposterior extent of the midbrain [13, 15]. Indeed, the production of these cells occurs over a protracted period, between stages (HH) 14 and 25 in chick [7, 13, 16].…”