Emergence of axonal tracts in the developing brain of the turbot (Psetta maxima)

In this study we have investigated the pattern of morphogenesis and axogene sis in the turbot brain during embryonic and early larval stages with immun ohistochemistry using an antibody against acetylated tubulin. The first, im munoreactive elements were detected at 74 h post-fertilization in fibers ru nning in the medial and lateral longitudinal fascicles. Newly positive axon al bundles are progressively added during development forming rostrocaudall y directed tracts. The tract of the postoptic commissure appears at 86 h po st-fertilization located rostrally to the medial longitudinal fascicle. Tog ether, the medial longitudinal fascicle and the tract of the postoptic comm issure constitute a major longitudinal axonal pathway, which is extended ro strally in embryos of 98 h post-fertilization by the supraoptic tract. In t he forebrain, two vertical tracts, the tract of the posterior commissure (a ppearing around 98 h post-fertilization) and the tract of the anterior comm issure (detected at 110 h post-fertilization) project descending axone to t he pre-existing axonal longitudinal pathway. These early tracts are connect ed 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 cl early associated with any axonal bundle, was observed from 98 h post-fertil ization lining the caudal floor of the presumptive hypothalamus. Several hy potheses are proposed to determine the nature of these cells. A comparison of the emergence of the axonal circuitry in turbot and that of other teleos ts reveals significant analogies, suggesting that a common pattern underlie s the establishment of the embryonic tracts in this vertebrate group. The m inor differences observed between different teleost species, associated wit h the absence of some axonal fascicles, is also considered. Copyright (C) 2 001 S. Karger AG. Basel.