The expression of the calcium‐binding protein calretinin was analysed by immunohistochemistry techniques in the retina of turbot (Psetta maxima) from embryonic to juvenile stages. Calretinin immunoreactivity was first detected in retinae from newly hatched larvae, in which the anlage of the inner plexiform layer and a subset of amacrine and ganglion cells displayed a faint immunolabelling. First appearance of photoreceptors during larval life coincided with an increase in the intensity of the labelling. During subsequent larval development, the expression of calretinin affected distinctive retinal components. The inner plexiform layer, optic fiber layer, and a population of amacrine and ganglion cells were invariably labelled. Occasional bipolar cells were labelled at the end of the larval period. By metamorphosis, calretinin is sequentially expressed in horizontal cells, and bipolar immunoreactive cells become numerous. The pattern of calretinin immunoreactivity of the inner plexiform layer changes from the larval to juvenile period. In all cases, calretinin immunoreactivity exhibited variations between the peripheral retina, which contains the most recently differentiated retinal components, and the remainder of the differentiated retina. Our results suggest that the progressive expression of calretinin in the turbot retina appears associated with some degree of neuronal differentiation. Once the definitive pattern of calretinin immunoreactivity is established in the turbot retina, both similarities and differences with the calretinin location in the retina of other vertebrates can be demonstrated. J. Comp. Neurol. 406:425–432, 1999. © 1999 Wiley‐Liss, Inc.

The spatial and temporal pattern of GABA-expression in the brains of zebrafish (Danio rerio) embryos was studied by means of immunohistochemical techniques. GABA is said to exert neurotrophic actions in the early regulation of the differentiation of the central nervous system. In early stages GABAergic cells form distinct clusters throughout the CNS. As development progresses, more GABAergic clusters appear, and a pattern of GABAergic axonal projections is well defined. Although there is a corresponding pattern of distribution and appearance of GABA-expression in the brain of different teleosts, further studies are needed to establish its role during early morphogenesis of the CNS of vertebrates.

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.

The morphogenesis and cell proliferation in the retina of turbot (Psetta maxima, Pleuronectiformes: Teleostei) from embryo through metamorphosis have been examined by using proliferating cell nuclear antigen (PCNA)-immunohistochemistry and general histological procedures. In the embryonic retina, cell proliferation and spatial cell reorganization form the anlage of the pigment epithelium and neural retina. Neurogenesis begins around hatching in the temporal retina, dorsal to the optic nerve exit, and then a wave of cell differentiation spreads to the nasal retina to yield a laminated retina by the end of the prolarval turbot stage. Germinal zones in the differentiated retina persist as a rim at the retinal margin, as well as surrounding the optic fissure in premetamorphic and metamorphic turbot larvae. In these zones, progenitor cells with different morphologies show a similar spatial arrangement, which suggests that they have a similar retinogenic potential. During metamorphosis, asymmetric proliferative activity in turbot germinal zones is associated with a marked expansion of the retinal tissue. Scattered stem cells in the laminated retina, related to the lineage of rod photoreceptors, were also observed both in large premetamorphic larvae and metamorphic turbots. The proliferative activity of these cells increases considerably during metamorphosis, when rod photoreceptors become morphologically differentiated.

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