Morphogenetic effects of 9-cis-retinoic acid on the regenerating limbs of the axolotl

In Vivo Imaging of Newt Lens Regeneration: Novel Insights Into the Regeneration Process

Purpose

To establish optical coherence tomography (OCT) as an in vivo imaging modality for investigating the process of newt lens regeneration.

Methods

Spectral-domain OCT was employed for in vivo imaging of the newt lens regeneration process. A total of 37 newts were lentectomized and followed by OCT imaging over the course of 60 to 80 days. Histological images were obtained at several time points to compare with the corresponding OCT images. Volume measurements were also acquired.

Results

OCT can identify the key features observed in corresponding histological images based on the scattering differences from various eye tissues, such as the cornea, intact and regenerated lens, and the iris. Lens volume measurements from three-dimensional OCT images showed that the regenerating lens size increased linearly until 60 days post-lentectomy.

Conclusions

Using OCT imaging, we were able to track the entire process of newt lens regeneration in vivo for the first time. Three-dimensional OCT images allowed us to volumetrically quantify and visualize the dynamic spatial relationships between tissues during the regeneration process. Our results establish OCT as anin vivo imaging modality to track/analyze the entire lens regeneration process from the same animal.

Translational Relevance

Lens regeneration in newts represents a unique example of vertebrate tissue plasticity. Investigating the cellular and morphological events that govern this extraordinary process in vivo will advance our understanding and shed light on developing new therapies to treat blinding disorders in higher vertebrates.

Signaling during lens regeneration

The newt is one of the few organisms that is able to undergo lens regeneration as an adult. This review will examine the signaling pathways that are involved in this amazing phenomenon. In addition to outlining the current research involved in elucidating the key signaling molecules in lens regeneration, we will also highlight some of the similarities and differences between lens regeneration and development.

Gap junctions in the limb regeneration blastema of the axolotl,Ambystoma mexicanum, are not distributed uniformly and are regulated by retinoic acid

Gap junctions are thought to play a role in pattern formation during limb development and regeneration by controlling the movement of small regulatory molecules between cells. An anteroposterior gradient of gap junctional communication that is higher posteriorly has been reported in the developing chick limb bud. In both the developing chick limb bud and the amphibian regenerating limb, an anteroposterior retinoic acid gradient is present, and this is also higher posteriorly. On the basis of these observations, we decided to examine the role of gap junctional communication in the regenerating amphibian limb. Gap junctions were observed in both the axolotl, Ambystoma mexicanum, limb regeneration blastema and cardiac tissue (as a positive control), using immunohistochemical labelling and laser scanning confocal microscopy. The scrape-loading/dye transfer technique for tracing the movement of a gap junction permeable dye, Lucifer yellow, showed that in blastemal epidermis there were nonuniform distributions of gap junctions in both the dorsoventral and anteroposterior axes of the blastema. Retinoic acid was found to increase gap junctional permeability in blastemal epidermis 48 h after injection and in blastemal mesenchyme 76 h after injection. The potential role of gap junctions during pattern formation in limb regeneration is discussed based on these results.

Gene expression during amphibian limb regeneration

Limb regeneration in adult urodeles is an important phenomenon that poses fundamental questions both in biology and in medicine. In this review, we focus on recent advances in the characterization of the regeneration blastema at cellular and molecular levels and on the current understanding of the molecular basis of limb regeneration and its relationship to development. In particular, we discuss (i) the spatiotemporal distribution of genes and gene products in the mesenchyme and wound epidermis of the regenerating limb, (ii) how growth is controlled in the regeneration blastema, and (iii) molecules that are likely to be involved in patterning the regenerating limb such as homeobox genes and retinoids.