Emergence of Axonal Tracts in the Developing Brain of the Turbot <i>(Psetta maxima)</i>

Abstract

In this study we have investigated the pattern of morphogenesis and axogenesis in the turbot brain during embryonic and early larval stages with immunohistochemistry using an antibody against acetylated tubulin. The first immunoreactive elements were detected at 74 h post-fertilization in fibers running in the medial and lateral longitudinal fascicles. Newly positive axonal bundles are progressively added during development forming rostrocaudally directed tracts. The tract of the postoptic commissure appears at 86 h post-fertilization located rostrally to the medial longitudinal fascicle. Together, the medial longitudinal fascicle and the tract of the postoptic commissure constitute a major longitudinal axonal pathway, which is extended rostrally in embryos of 98 h post-fertilization by the supraoptic tract. In the forebrain, two vertical tracts, the tract of the posterior commissure (appearing around 98 h post-fertilization) and the tract of the anterior commissure (detected at 110 h post-fertilization) project descending axons to the pre-existing axonal longitudinal pathway. These early tracts are connected by four associated commissures (ventral tegmental, postoptic, posterior and anterior commissure). Some groups of labeled cell bodies are identified either as the origin of the embryonic tracts or contributing axons to the axonal pathways. Additionally, a conspicuous cluster of large cells, not clearly associated with any axonal bundle, was observed from 98 h post-fertilization lining the caudal floor of the presumptive hypothalamus. Several hypotheses are proposed to determine the nature of these cells. A comparison of the emergence of the axonal circuitry in turbot and that of other teleosts reveals significant analogies, suggesting that a common pattern underlies the establishment of the embryonic tracts in this vertebrate group. The minor differences observed between different teleost species, associated with the absence of some axonal fascicles, is also considered.

Molecular mechanisms in the formation of the medial longitudinal fascicle

“…1990), and has since been studied in a variety of non‐amniote (e.g. Hartenstein, 1993; Doldan et al. 2000) and amniote species (e.g.…”

Axonogenesis in the medaka embryonic brain

“…T-6793, Sigma Chemical, St. Louis, MO) was used to immunohistochemically stain early axons and young somata of neurons. This antibody has been successfully used in embryos of various animals including teleost fishes (Chitnis and Wilson et al, 1990;Hartenstein, 1993;Kuratani and Horigome, 2000;Doldan et al, 2000). A monoclonal antibody, HNK-1 or CD57 (leu-7, clone HNK-1, No.…”

Evolution and development of interhemispheric connections in the vertebrate forebrain

“…Notably, these early systems of protein gradient production not only instruct overall brain area patterning (Shimogori and Grove, 2005; O'Leary et al, 2007; Assimacopoulos et al, 2012), but also serve as guidance cues for growing axons (Charron et al, 2003; Walshe and Mason, 2003; Tole et al, 2006; Zou and Lyuksyutova, 2007; Toyama et al, 2013). Similarly, as described in more detail below, the spatial locations of these organizing centers broadly coincide with regions of commissural axon crossing, such as the post-optic commissure and posterior commissure, which are the first commissures to form during vertebrate development (Figure 1B; Herrick, 1937; Kuratani et al, 1998; Doldan et al, 2000; Barreiro-Iglesias et al, 2008). Thus, the conservation of these early mechanisms of forebrain development across vertebrate species suggest that area patterning and cell-specification functions may have been co-opted for axon guidance and commissural circuit formation.…”