There is much variation in the form and function of vertebrate hearts. At one extreme sits the two-chambered, flow-through hearts of fish while at the other end sits the highly efficient four-chambered hearts of birds and mammals that create the complete separation of pulmonary (lung) and circulatory (systemic) systems. Understanding the relationships between heart performance and animal physiology has long fascinated biologists. But more recently, new lines of investigation have also began dissecting the developmental origins of cardiac variation to better understand the ways in which this critical organ has evolved. Several recent research papers have used lizards and snakes – most importantly, anoles – as their centerpiece in the hope of finding new clues about heart evolution and the origin of the fully divided ventricle. These studies fill an important gap in our knowledge of comparative heart development. Prior to this research the study of squamate heart development had lagged well behind species from other vertebrate lineages, sitting idly for over 100 years.
The first of these papers, Koshiba-Takeuchi et al. (2009), compares ventricle development among the green anole, red-eared slider turtle and the laboratory mouse, focusing on early formation of the ventricle. More recently Jensen et al. (2013) compared later stages of ventricle expansion among the green and brown anoles, corn snake, and the Philippine sailfin lizard. The nitty-gritty details of heart development can be found in the original papers, but together these studies illustrate well that squamate heart development proceeds along a similar trajectory to other more widely studied species, with the exception of the septum that separates left and right ventricles. This research suggests that lizards could potentially provide novel insights into ventricular malformations observed in humans and is an area where additional research could be quite valuable.
Developing anole heart visualized with OPT from Koshiba-Takeuchi et al. (2009)
Beyond the novel insights into heart development and evolution that these studies provide, they also demonstrate how emerging imaging techniques can revolutionize the way we study developmental processes in non-model organisms. (Other three-dimensional imaging techniques have been discussed in the pages of AA previously) To visualize the anatomy of the developing heart Koshiba-Takeuchi et al. (2009) uses a technique known as optical projection tomography or OPT. OPT is an ideal 3D visualization technique for specimens 10m to 1cm in approximate size. In regards to developmental studies, OPT has a distinct advantage over previously discussed techniques (e.g., μCT) in that it can image specimens stained for particular molecules or cell types and can also combine scans from stains of different wavelengths to create a high resolution image of developing tissues in the broader context of the intact embryo.
Developmental stages 5, 9, and adult of the anole heart visualized with MRI. Explore in more detail in the supplementary materials of Jensen et al. (2013).
Jensen et al. (2013) use Microscopic magnetic resonance imaging (μMRI) to build an atlas of the developing squamate heart. This technique is typically more expensive than other imaging options and does not provide images at the same resolution as μCT (25μm versus <10μm), but does provide the opportunity to visualize untreated embryos that can potentially be re-used in subsequent experiments. Regardless, I highly recommend that readers take a few minutes to explore the interactive PDF of 3D embryological heart models that is provided with this article. (In other supplementary materials you can also observe the beating heart of a corn snake embryo. To put it plainly, these are really cool!)
A more thorough review of 3D visualization techniques that are appropriate for small specimens is available through Cold Spring Harbor.
- Short Faces, Two Faces, No Faces: Lizards Heads Are Susceptible to Embryonic Thermal Stress - December 15, 2021
- The Super Sticky Super Power of Lizards: a New Outreach Activity for Grade-Schoolers - April 9, 2018
- Updates on the Development of Anolis as a “Model Clade” of Integrative Analyses of Anatomical Evolution - September 4, 2017
Rui Ferreira
Hello. Congratulations for your work. I’m not a lizard “specialist” and I’m needing some advising and orientation. As far as I Know some lizards respond to an attack by playing death, they are suppose to have constant 3 chambers, and the heart / lung innervated only by the parasympathetic. Others have an active defense / attack, these are supposed to have a more developed inter ventricular septum that at higher rates produces a separation of the pulmonary circulation from the systemic circulation. These are also supposed to have a sympathetic innervation of the heart / lungs. Is this right? If so could you please provide me the reference papers (that I find hard to find… maybe because those affirmations aren’t correct). Thank you for your time, best regards Rui