Developmental changes in the spatial pattern of mesencephalic trigeminal nucleus (Mes5) neuron populations in the developing chick optic tectum

The chick optic tectum developmental stages. A dynamic table based on temporal- and spatial-dependent histogenetic changes: A structural, morphometric and immunocytochemical analysis

Development is often described as temporal sequences of developmental stages (DSs). When tables of DS are defined exclusively in the time domain they cannot discriminate histogenetic differences between different positions along a spatial reference axis. We introduce a table of DSs for the developing chick optic tectum (OT) based on time- and space-dependent changes in quantitative morphometric parameters, qualitative histogenetic features and immunocytochemical pattern of several developmentally active molecules (Notch1, Hes5, NeuroD1, β-III-Tubulin, synaptotagmin-I and neurofilament-M). Seven DSs and four transitional stages were defined from ED2 to ED12, when the basic OT cortical organization is established, along a spatial developmental gradient axis extending between a zone of maximal and a zone of minimal development. The table of DSs reveals that DSs do not only progress as a function of time but also display a spatially organized propagation along the developmental gradient axis. The complex and dynamic character of the OT development is documented by the fact that several DSs are simultaneously present at any ED or any embryonic stage. The table of DSs allows interpreting how developmental cell behaviors are temporally and spatially organized and explains how different DSs appear as a function of both time and space. The table of DSs provides a reference system to characterize the OT corticogenesis and to reliably compare observations made in different specimens.

Hes1 and Hes5 regulate the development of the cranial and spinal nerve systems

The basic helix-loop-helix genes Hes1 and Hes5, known Notch effectors, regulate the maintenance of neural stem cells and the development of the central nervous system (CNS). In the absence of Hes1 and Hes5, the size, shape and cytoarchitecture of the CNS are severely disorganized, but the development of the peripheral nervous system remains to be analyzed. Here, we found that in Hes1;Hes5 double-mutant mice, the cranial and spinal nerve systems are also severely disorganized. In these mutant mice, axonal projections from the mesencephalic neurons to the trigeminal (V) ganglion become aberrant and the proximal parts of the glossopharyngeal (IX) and vagus (X) nerves are fused. The hypoglossal (XII) nerve is also formed poorly. Furthermore, the dorsal root ganglia are fused with the spinal cord, and the dorsal and ventral roots of the spinal nerves are lacking in many segments. These results indicate that Hes1 and Hes5 play an important role in the formation of the cranial and spinal nerve systems.

A role forfoxd3andsox10in the differentiation of gonadotropin-releasing hormone (GnRH) cells in the zebrafishDanio rerio

Gonadotropin-releasing hormone (GnRH) is found in a wide range of vertebrate tissues, including the nervous system. In general, GnRH has two functions: endocrine, acting as a releasing hormone; and neuromodulatory,affecting neural activity in the peripheral and central nervous system. The best understood population of GnRH cells is that of the hypothalamus, which is essential for reproduction. Less well understood are the populations of GnRH cells found in the terminal nerve and midbrain, which appear to be neuromodulatory in function. The GnRH-containing cells of the midbrain are proposed to arise from the mesencephalic region of the neural tube. Previously, we showed that neuromodulatory GnRH cells of the terminal nerve arise from cranial neural crest. To test the hypothesis that neuromodulatory GnRH cells of the midbrain also arise from neural crest, we used gene knockdown experiments in zebrafish to disrupt neural crest development. We demonstrate that decrement of the function of foxd3 and/or sox10, two genes important for the development and specification of neural crest, resulted in a reduction and/or loss of GnRH cells of the midbrain, as well as a reduction in the number of terminal nerve GnRH cells. Therefore, our data support a neural crest origin for midbrain GnRH cells. Additionally, we demonstrate that knockdown of kallmann gene function resulted in the loss of endocrine GnRH cells of the hypothalamus, but not of neuromodulatory GnRH cells of the midbrain and terminal nerve, thus providing additional evidence for separate pathways controlling the development of neuromodulatory and endocrine GnRH cells.

Mesencephalic Trigeminal Nucleus Development Is Dependent on Krox-20 Expression

Krox-20, a C2H2-type zinc-finger transcription factor, plays an important role in rhombomere development. This study reveals that the Krox-20 null mutation impacts the development of mesencephalic trigeminal (Me5) neurons, a cell group traditionally thought to emerge from the mesencephalon. Based on cell counting studies, we show that Krox-20 null mutants have twice as many Me5 neurons relative to wildtypes at E15, but by birth have half the number of Me5 cells as wildtypes. TUNEL studies reveal a period of increased apoptosis from E17-P0 in mutants. The mutation does not result in differences in Me5 cell size, morphology, gene expression or peripheral projection patterns between genotypes, as demonstrated by retrograde tracing and Brn3a immunohistochemistry. The data suggest that Krox-20 regulates the period and extent of Me5 apoptosis, impacting the final number of Me5 neurons. The loss of Me5 in Krox-20-/- mice may highlight species-specific differences in the origin of these cells.