Editorial highlights

Every organism is a model organism for understanding development, evolution, disease, and regeneration, and we have only begun to scratch the surface of the interdisciplinary genetic, molecular, cellular, and developmental mechanisms that regulate these biological processes. These “Highlights” denote exciting advances recently reported in Developmental Dynamics that illustrate the complex dynamics of developmental biology.

Zebrafish Neurogenesis. “Plexina4 and cell survival in the developing zebrafish hindbrain” by Zachary Nurcombe, Carrie Lynn Hehr, and Sarah McFarlane; DevDyn 252:11, pp. 1323–1337, https://doi.org/10.1002/dvdy.633. Apoptosis plays a fundamental role in normal embryo development and tissue homeostasis. For example, during nervous system development, neurons are initially overproduced but then pruned through programmed cell death. Plexins (PLXNs) are transmembrane receptors for a class of guidance proteins known as Semaphorins (SEMAs) and control the directed movements of migrating neurons and extending axons. But what about proliferation? plxna4 is dynamically expressed within the developing embryonic brain, including the hindbrain, and in this paper CRISPR mediated gene editing of zebrafish demonstrates that plxna4 plays a critical role in neuronal survival during zebrafish hindbrain development. This has implications for neurodegenerative disorders since PLXNA4 is implicated in Alzheimer’s Disease.

Retina Development “A framework to identify functional interactors that contribute to disrupted early retinal development in Vsx2 ocular retardation J mice” by Amanda Leung, Mahesh Rao, Nathan Raju, Minh Chung, Allison Klinger, DiAnna Rowe, Xiaodong Li, and Edward Levine; DevDyn 252:11, pp. 1338–1362, https://doi.org/10.1002/dvdy.629. The Visual System Homeobox 2 (Vsx2) gene is initially expressed in retinal progenitor cells where it regulates retinal neurogenesis before ultimately resolving to bipolar cells and Müller glia. Mutations in VSX2 in humans cause bilateral congenital microphthalmia, disrupted retinal architecture, and lifelong blindness. The recessive loss-of-function ocular retardation J allele encodes a premature stop codon in the mouse Vsx2 homeodomain. Combining in vivo and ex vivo testing with transcriptome analysis revealed interactions between Vsx2, Mitf, RXR, and gamma-Secretase activities during retinal development, and provides a platform that could be adapted to other gene mutations with complex phenotypes.

Cartilage Development and Evolution “Common features of cartilage maturation are not conserved in an amphibian model” by Jason Nguyen, Patsy Gómez-Picos, Yiwen Liu, Katie Ovens, and B. Frank Eames; DevDyn 252:11, pp. 1375–1390, https://doi.org/10.1002/dvdy.594. Cartilage is a flexible connective tissue made up of specialized cells called chondrocytes. It is replaced by bone in many parts of the body through a process called endochondral ossification. Much of our knowledge about skeletal development has come from studies in chicken and mouse models in which cartilage maturation during endochondral ossification is a gradual, progressive event. This study tested the hypothesis that the standard process of cartilage maturation is conserved in the amphibian X. tropicalis. Although long bone chondrogenesis followed a relatively conserved maturation process, larval head cartilages, such as the ceratohyal, exhibit dramatic differences in timing and gene expression from the standard process of cartilage maturation. These findings in amphibians have both evolutionary and health significance.