What is neural development?
Neural development comprises the processes that generate, shape, and reshape the nervous system, from the earliest stages of embryogenesis to the final years of life. The study of neural development draws on both neuroscience and developmental biology to describe the cellular and molecular mechanisms by which complex nervous systems emerge.
Neural development in zebrafish
The zebrafish has come to be an excellent model organism for examining neural development for a number of reasons:
- It has a simple brain structure homologous to those of other vertebrates, including mammals
- Rapid embryonic development
- The zebrafish embryo is transparent, giving scientists a unique opportunity to study development
The emerging field of neural optogenetics allows the initial development and coalescence of neurons, as well as the functional organization of complete neural networks, to be visualized and investigated. Specific studies focus on the development of the nervous system in zebrafish on a variety of levels, examples of which will be explored below. Much of the genetics that underlie the development of the vertebrate nervous system is conserved, making the zebrafish an invaluable model for human neural diseases.
Gongal et al. attempts to identify the role of H6 homeobox (HMX) genes in early neural development, using the zebrafish as a model. Mutations in HMX genes have been linked to neural mis-patterning and neural tube closure defects in humans. Both of these affected developments are highly dependent on correct levels of retinoic acid (RA). While RA is critical in anterior-posterior patterning, the neural tube is patterned dorso-ventrally by Sonic Hedgehog (Shh). This study is motivated by the suspected relationship between HMX genes and these two critical morphogens.
One specific mammalian HMX gene, HMX1, has a paralog in zebrafish which is strongly expressed in early development: hmx4. Two morpholinos (MOs) against hmx4 were injected into zebrafish embryos. Independently, each MO knockdown induced identical phenotypes. The MO-knockdown of hmx4 resulted in a narrowing of the eye field, failure of the neural tube to close, small ears, and loss of the pectoral fins. An assay on neural patterning exhibited a significant reduction in the number of vagal motor neurons present in the embryos. Real-time quantitative PCR was used to evaluate the presence of RA synthesis genes and Shh signal transducers in hmx4-depleted embryos.
It was concluded that RA and SHH are critical in correct neural forebrain patterning during zebrafish development. Furthermore, the results of this experiment identify hmx4 as a requirement for transcription of the RA synthesis gene, and in turn regulation of gli3, a pathway critical to SHH transduction. One interesting outcome of this study was the discovery that RA treatment was able to rescue forebrain morphology, Shh signaling, and gli3 transcription in hmx4-depleted embryos.
Axon pathfinding is the precise process by which neurons send out axons to reach their correct targets. Networks of growing axons navigate in response to guidance cues in the embryonic stage of neural development. Brusegan et al. focuses particularly on motoneuron axon pathfinding in which axons are extended toward skeletal muscles and form synaptic contacts. Motoneuron axon pathfinding begins quickly after fertilization in zebrafish and spontaneous muscle contractions are exhibited after only 18-19 hours post fertilization.
A variety of stimuli are responsible for axon pathfinding in vertebrates. One such gene, Coiled-Coil-Domain Containing 80 (Ccdc80) has only recently been implicated in neural development when its expression was recorded in the notochord, muscle pioneers, and adaxial cells of zebrafish, all of which are responsible for axon guidance. Brusegan et al. aims to further investigate the role of Ccdc80 in axon pathfinding, utilizing two zebrafish homologs, Ccdc80 and Ccdc80-like 1 (Ccdc80-l1).
Antisense oligonucleotide morpholinos were used to knockdown ccdc80-l1 during zebrafish development. These zebrafish exhibited normal early development such as segmentation and gastrulation, however, motility issues were observed. The ccdc80-l1 morphant embryos exhibited abnormal escaping behavior with tactile stimulation often resulting in body contractions and circling behavior. Examination of somitogenesis and myogenesis markers eliminated the possibility of defects in adaxial cells, muscle pioneers, and musculature formation. However, znp1- and zn-5-immunohistochemistry of the trunk region revealed multiple axonal pathfinding impairments. Disorganization of both dorsal and ventral motoneurons resulted in mis-orientation, over-branching, and in some cases even axon stalling. Interestingly, the morphant phenotype was dose-dependent, with a lower dose of morpholinos resulting in ventral axon migration impairment alone. Dorsal axons alone were never affected.
Gongal et al. and Brusegan et al. illustrate the variety of neural development topics that can be investigated using the zebrafish as a model. Each article was well-written and gave sufficient background knowledge in order to understand the remaining of the article. Both articles also did a great job of explaining the methods and noting the controls. Brusegan et al. performed extensive preliminary research to confirm the results of other papers, which led their conclusions to be more convincing. Gongal et al. could have strengthened their credibility in this way.
1. Wikipedia. Neural development. http://en.wikipedia.org/wiki/Neural_development
2. McLean DL, Fetcho JR. 2011. Movement, technology and discovery in the zebrafish. Current Opinion in Neurobiology 21:110-5
3. Gongal PA, March LD, Holly VL, Pillay LM, Berry-Wynne KM, et al. 2011. Hmx4 regulates Sonic hedgehog signaling through control of retinoic acid synthesis during forebrain patterning. Developmental Biology 355:55-64
4. Brusegan C, Pistocchi A, Frassine A, Della Noce I, Schepis F, Cotelli F. 2012. ccdc80-l1 Is Involved in Axon Pathfinding of Zebrafish Motoneurons. PLoS ONE 7:e31851