Epithelial mitochondria-rich cells and associated innervation in adult and developing zebrafish

Abstract

Studies of ion regulation by mitochondria-rich cells (MRCs) of transport epithelia in fish have revealed many processes by which ion homeostasis is achieved. However, the control of these mechanisms and, particularly, the extent of nervous system involvement are not completely understood. We characterized the potential innervation of MRCs in various gill and extrabranchial tissues involved in ion transport in the model vertebrate the zebrafish. Confocal and conventional microscopy of whole-mount preparations were combined with immunofluorescence techniques to label MRCs with antibodies against a subunit of the enzyme Na(+)/K(+)-ATPase and nerve fibers with a zebrafish neuronal marker, zn-12. MRCs of the gill filaments were identified by their morphology and migration out to the lamellae in response to ion-poor water acclimation. Gill MRCs were intimately associated with nerve fibers originating from outside the filaments. MRCs of the opercular epithelium resembled those of the gill and were also located adjacent to nerve fibers. Mitochondria-rich "pseudobranch cells" were identified in the pseudobranch by immunofluorescence and labeling of dissociated cells with the mitochondrial marker DASPEI. Pseudobranch MRCs resembled gill MRCs and received innervation from a dense network of nerve fibers. In larvae, MRCs were distributed across the surface of the skin. These cells were situated among a dense network of varicose nerve fibers, and some MRCs of the skin displayed extensive cytoplasmic processes. Evidence is presented suggestive of widespread association of MRCs with the nervous system in transport epithelia and the neural control of MRC-mediated ion regulation in teleost fish.

New developments on gill innervation: insights from a model vertebrate

“…These cells are characterized by apical localization of the enzyme Na + /K + -ATPase and an extensive cytoplasmic tubular network ( Fig. 6B; Laurent and Dunel-Erb, 1984;Quinn et al, 2003;Jonz and Nurse, 2006). Early studies investigating the function of the pseudobranch suggested that it may act as a gland, a sensory organ involved in O 2 chemoreception, or take part in osmoregulation (Laurent and Rouzeau, 1972;Laurent and Dunel-Erb, 1984).…”

“…There is evidence that the teleost pseudobranch receives afferent innervation (Laurent and Rouzeau, 1972;Laurent and Dunel-Erb, 1984), but similar nerve fibres in zebrafish are immunopositive for the synaptic vesicle protein SV2 (Jonz and Nurse, 2003), suggesting a neurosecretory or efferent role of these fibres. Moreover, since innervation of PBCs occurs apically, rather than basolaterally (Laurent and Dunel-Erb, 1984;Jonz and Nurse, 2006), the physiological significance of these cells would appear to be limited to internally oriented functions that may take place between the basolateral membrane and the vasculature, such as hormone secretion or chemical sensing of the blood. While the precise function of the pseudobranch and its component cells is currently speculative, further study of the nature of PBC innervation may reveal significant clues about the role of this potentially important organ in fish.…”

A Time Differential Staining Technique Coupled with Full Bilateral Gill Denervation to Study Ionocytes in Fish

“…Studies by Jonz and Nurse (2003) on zebrafish (Danio rerio) gill innervation showed that the origin of innervation is extrinsic (cell body of nerve fiber is extrinsic to the gill) as well as intrinsic (cell body of nerve fiber is intrinsic to the gill). The same authors also demonstrated that branchial ICs are extrinsically innervated 7 .…”

“…The gills, and presumably the branchial ICs, are innervated by the IX and X cranial nerves 7,[23][24][25] . These nerves carry both efferent and afferent inputs to and from the gill, respectively.…”

“…ICs are the functional unit for ionic regulation in fish and are found on the epithelial surfaces of the gill filaments and lamellae 4,[6][7][8]10 . Although a variety of subtypes have been described that possess unique features, many of the ICs are characterized by a high density of mitochondria (thus they are also known as mitochondrion-rich cells) and/or an abundance of the enzyme Na + /K + ATPase (NKA).…”