Cartilage on the move: Cartilage lineage tracing during tadpole metamorphosis

Abstract

The reorganization of cranial cartilages during tadpole metamorphosis is a set of complex processes. The fates of larval cartilage-forming cells (chondrocytes) and sources of adult chondrocytes are largely unknown. Individual larval cranial cartilages may either degenerate or remodel, while many adult cartilages appear to form de novo during metamorphosis. Determining the extent to which adult chondrocytes/cartilages are derived from larval chondrocytes during metamorphosis requires new techniques in chondrocyte lineage tracing. We have developed two transgenic systems to label cartilage cells throughout the body with fluorescent proteins. One system strongly labels early tadpole cartilages only. The other system inducibly labels forming cartilages at any developmental stage. We examined cartilages of the skull (viscero- and neurocranium), and identified larval cartilages that either resorb or remodel into adult cartilages. Our data show that the adult otic capsules, tecti anterius and posterius, hyale, and portions of Meckel’s cartilage are derived from larval chondrocytes. Our data also suggest that most adult cartilages form de novo, though we cannot rule out the potential for extreme larval chondrocyte proliferation or de- and re-differentiation, which could dilute our fluorescent protein signal. The transgenic lineage tracing strategies developed here are the first examples of inducible, skeleton-specific, lineage tracing in Xenopus.

Deconstructing cartilage shape and size into contributions from embryogenesis, metamorphosis, and tadpole and frog growth

“…This study in contrast focuses on the individual cartilages of a skull region throughout life, and separates their shape trajectories into phases of pre-and post-metamorphic growth and metamorphosis. Quantifying shape trajectories in this manner provides a framework for investigating how cartilage growth and metamorphic remodeling are controlled at the level of individual cell behaviors (Rose, 2009;Slater et al 2009;Kerney et al 2012) and affected by mutations and exogenously applied hormones, teratogens and environmental toxins (Huang et al 1999;Schreiber et al 2001;Das et al 2002;Du Preez et al 2008;Kerney et al 2012;Vandenberg et al 2012). Further, comparing multiple cartilages with similar embryonic origins and different functions throughout life reveals how skeletal shape trajectories can be modified by selection on a complex life history Alberch, 1987Alberch, , 1989 and at the same time constrained by the cellular and histological properties of cartilage tissue (Rose, 2009).…”

Regulation of thyroid hormone-induced development in vivo by thyroid hormone transporters and cytosolic binding proteins

“…We used a tail injection assay involving a standard form of GFP whose protein lasts for weeks in vivo (Kerney et al, 2012) to determine the effect of THT or CTHBP overexpression on metamorphic change, i.e., TH-dependent tail muscle cell disappearance. The loss of GFP signal from a muscle cell within 24 hrs is considered strong evidence of cell death Nakajima et al, 2012).…”

The importance of cartilage to amphibian development and evolution

“…Both pathways require that larval cartilages have relatively little matrix and undergo cell division to produce smaller cells that can either be rearranged to reshape the larval cartilage or undergo morphogenesis to form a new cartilage within the resorbing larval cartilage. In contrast to Kerney et al, (2012b), histological description and careful morphometric analyses of the Meckels cartilage and ceratohyal in Xenopus (Rose, 2009(Rose, , 2015 suggest that these cartilages exemplify these two pathways.…”

“…Other new elements or cartilage additions that appear to arise in this way include the frog tympanic ring and columella, the cells for which appear to derive from the perichondrium of a resorbing part of the palatoquadrate (Barry, 1956, van der Westhuizen, 1961; the posterior hook and lateral plate-like additions to salamander ceratohyals; the posterior radial cartilages of salamanders and the two or three paired processes of the hyoglossal skeleton in frogs. Efforts to label the larval portions of adult cartilages in Xenopus suggest that both ends of the adult Meckels cartilage are also added on in this manner (Kerney et al, 2012b). Whether progenitor cells are always recruited from the perichondria of resorbing cartilages or can also derive from uncondensed populations of former neural crest cells is unclear.…”