Mammalian inner ear and fish lateral line sensory hair cells (HCs) detect fluid motion to transduce environmental signals. Actively maintained ionic homeostasis of the mammalian inner ear endolymph is essential for HC function. In contrast, fish lateral line HCs are exposed to the fluctuating ionic composition of the aqueous environment. Using lineage labeling, in vivo time-lapse imaging and scRNA-seq, we discovered highly motile skin-derived cells that invade mature mechanosensory organs of the zebrafish lateral line and differentiate into Neuromast-associated (Nm) ionocytes. This invasion is adaptive as it is triggered by environmental fluctuations. Our discovery of Nm ionocytes challenges the notion of an entirely placodally derived lateral line and identifies Nm ionocytes as likely regulators of HC function possibly by modulating the ionic microenvironment. Nm ionocytes provide an experimentally accessible in vivo system to study cell invasion and migration, as well as the physiological adaptation of vertebrate organs to changing environmental conditions.

The accessibility of the lateral line system and its amenability to long-term in vivo imaging transformed the developing lateral line into a powerful model system to study fundamental morphogenetic events, such as guided migration, proliferation, cell shape changes, organ formation, organ deposition, cell specification and differentiation. In addition, the lateral line is not only amenable to live imaging during migration stages but also during postembryonic events such as sensory organ tissue homeostasis and regeneration. The robust regenerative capabilities of the mature, mechanosensory lateral line hair cells, which are homologous to inner ear hair cells and the ease with which they can be imaged, have brought zebrafish into the spotlight as a model to develop tools to treat human deafness. In this chapter, we describe protocols for long-term in vivo confocal imaging of the developing and regenerating lateral line.

Loss of sensory hair cells leads to deafness and balance deficiencies. In contrast to mammalian hair cells, zebrafish ear and lateral line hair cells regenerate from poorly characterized support cells. Equally ill-defined is the gene regulatory network underlying the progression of support cells to differentiated hair cells. scRNA-Seq of lateral line organs uncovered five different support cell types, including quiescent and activated stem cells. Ordering of support cells along a developmental trajectory identified self-renewing cells and genes required for hair cell differentiation. scRNA-Seq analyses of fgf3 mutants, in which hair cell regeneration is increased, demonstrates that Fgf and Notch signaling inhibit proliferation of support cells in parallel by inhibiting Wnt signaling. Our scRNA-Seq analyses set the foundation for mechanistic studies of sensory organ regeneration and is crucial for identifying factors to trigger hair cell production in mammals. The data is searchable and publicly accessible via a web-based interface.

Systema Naturae is one of the major works of the Swedish botanist, zoologist, and physician Carl Linnaeus (1707–1778) and introduced the Linnaean taxonomy. Although the system, now known as binomial nomenclature, was partially developed by Gaspard and Johann Bauhin 200 years earlier, Linnaeus was the first to use it consistently throughout his book. The first edition was published in 1735. The full title of the 10th edition [1758], which was the most important one left image below), was “Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis” or translated: “System of nature through the three kingdoms of nature, according to classes, orders, genera and species, with characters, differences, synonyms, places”. The left illustration below shows the front page of the 10th edition, the right illustration, page 194, introducing the chapter on the class Amphibia. The first amphibian species described by Linnaeus in this 10th edition were Rana arborea (Hyla arborea), Rana boans (Boana boans), Rana ocellata (Osteopilus ocellatus), Rana paradoxa (Pseudis paradoxa), and Rana typhonia (Trachycephalus typhonius). Unfortunately, and for the despair of many herpetologists, since the publication of the Systema Naturae, the nomenclature has been subjected to many changes, sometimes irrational and un­necessary (see also Appendix 1 of the present monograph).