A somitic contribution to the apical ectodermal ridge is essential for fin formation

Abstract

The transition from fins to limbs was an important terrestrial adaptation, but how this crucial evolutionary shift arose developmentally is unknown. Current models focus on the distinct roles of the apical ectodermal ridge (AER) and the signaling molecules that it secretes during limb and fin outgrowth. In contrast to the limb AER, the AER of the fin rapidly transitions into the apical fold and in the process shuts off AER-derived signals that stimulate proliferation of the precursors of the appendicular skeleton. The differing fates of the AER during fish and tetrapod development have led to the speculation that fin-fold formation was one of the evolutionary hurdles to the AER-dependent expansion of the fin mesenchyme required to generate the increased appendicular structure evident within limbs. Consequently, a heterochronic shift in the AER-to-apical-fold transition has been postulated to be crucial for limb evolution. The ability to test this model has been hampered by a lack of understanding of the mechanisms controlling apical fold induction. Here we show that invasion by cells of a newly identified somite-derived lineage into the AER in zebrafish regulates apical fold induction. Ablation of these cells inhibits apical fold formation, prolongs AER activity and increases the amount of fin bud mesenchyme, suggesting that these cells could provide the timing mechanism proposed in Thorogood's clock model of the fin-to-limb transition. We further demonstrate that apical-fold inducing cells are progressively lost during gnathostome evolution;the absence of such cells within the tetrapod limb suggests that their loss may have been a necessary prelude to the attainment of limb-like structures in Devonian sarcopterygian fish.

A contractile acto-myosin belt promotes growth anisotropy during the early stages of pectoral fin development in zebrafish

“…The fin field is specified by the expression of the transcription factor Tbx5a in the lateral plate mesoderm (LPM) at 18 hours post fertilisation (hpf) [2]. Most studies on the pectoral fin are focused on the late stages of its development (after 36 hpf or later), when the fin is already morphologically distinct [3,4]. However, little is known about the mechanisms that control the establishment of fin shape during the early stages of its development.…”

“…Live imaging showed an influx of cells into the fin field from the posterior region corresponding to the 4 th somite area between 32 and 46 hpf (Movies 1 and 2). Somitic mesodermal cells migrate into the fin field from the area corresponding to 4 th -7 th somites, and some contribute to the apical ectodermal ridge (AER) [4]. These invading somitic cells do not express EGFP.…”

“…Fin growth occurs through a combination of cell proliferation in the lateral plate mesoderm, which contributes to fin mesenchyme, and cell migration from the areas of the 4 th and 7 th somites, which contribute to both fin mesenchyme and to the apical ectodermal ridge, the AER [4,19]. Oriented cell motility and divisions contribute to shaping the limb bud in the mouse and the pectoral fin in zebrafish [5,19,20].…”

Latent developmental potential to form limb-like skeletal structures in zebrafish

“…Previous studies have tested if reduction of the fin fold can result in limb-like characteristics in developing zebrafish fins. Experimental perturbation of the fin fold during early development leads to an enlargement of the endoskeletal disc in larval zebrafish, suggesting that the fin fold represses disc growth (29, 30) . However, similar to the effect of reducing hox13 paralog function (11), subsequent elaboration of the adult fin endoskeleton is generally negligible, or has not been assessed, in the different manipulations.…”

“…Whole-mount immunolabeling of actinodin collagen filaments and DAPI counter-labeling was carried out as previously described by Lalonde and Akimenko (2018) (29, 30) .…”