This Chick Has Heart - Evolution News & Views

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This Chick Has Heart

One of the many eye-catching moments in the new Illustra film Flight: The Genius of Birds comes in the sequence showing development from egg to chick. We recommended a clip of that earlier. The narrator says,

On day two, the bird’s spinal cord, ears and eyes take shape, its vascular and circulatory systems are established, and its heart beats for the first time.

That tiny little heartbeat exemplifies what Timothy Standish says later in the sequence:

What you’re seeing is a mechanism at work; information being translated into a physical product at each step along the way; machines doing jobs - it’s absolutely incredible, absolutely incredible.

To show how incredible, consider a recent paper in Science by three cardiologists at UC San Francisco that explored what takes place when those heart cells begin to beat. As Ann Gauger points out in the film:

You’ve got to pay attention to the details. You have genes being switched on and off, interactions, cell communication... it’s an elaborate dance. It’s like a ballet taking place on a stage with thousands of cast members, all of them doing everything they’re supposed to on cue, in the right order, in the right sequence, in the right time.

The authors of the paper explain what some of those genes are, and how cell communication goes on well before the first heartbeat. Talk about details! You can skim over this paragraph to get the point:

Electrophysiological studies (2-4) have mapped cells that initiate cardiac APs to the inflow region at heart tube, looping, and septation stages. However, recent evidence indicates that as the heart matures, it continually expands, with cells being added to both the inflow and outflow segments [reviewed in (5)]. To determine which, if any, of the previously identified developmental pacing centers give rise to the mature SAN, we used optical mapping to image the AP initiation site and propagation pattern in embryonic chick hearts (4). Coincident with the heart’s first contractions at stage 10 [St10; Hamburger and Hamilton staging (6)], the AP initiation site was preferentially associated with the left posterior inflow segment of the heart [Fig. 1, A (red region) and F]. Left-sided pacing remained dominant through the process of dextral looping (Fig. 1, B and F). By late heart looping (St18), the AP initiation site shifted to the ventral surface of the right inflow, juxtaposed to and outside of the forming atria (Fig. 1C). From this stage on, all hearts displayed a right-sided AP initiation site (Fig. 1, D to F; see also movies S1 to S5).

Here are scientists, after a century of looking into chick embryos, still trying to figure out the choreography of this amazing ballet. Paul Nelson describes it in the Illustra video clip: these cells are moving and heading toward the places in the embryo where they’re going to develop, they’re also changing what they’re going to do eventually. They’re committing themselves, in most cases irreversibly, to particular functional roles.

One of the things the UCSF team discovered is that cell signals begin to commit “pacemaker cells” (PC) to a certain location well before they develop “action potentials” (AP) that begin to beat autonomously. Those beats then tune up the rest of the orchestra that will join the developing heart. Unexpectedly, the pacemaker cells locate to a “tertiary heart field” apart from the primary and secondary mesoderm where they were thought to go. Skim this quote:

These data suggest that by St8 [stage 8], PC fate is already established in the Nkx2.5- and Isl1-negative lateral plate mesoderm. To determine the spatial restrictions of PC specification, we isolated mesoderm directly adjacent to the PC precursors from the presumptive atria, atrioventricular junction, and proepicardium (see Fig. 3C). Only the PC region displayed elevated phase 4 depolarization and a high rate of AP production (Fig. 3D and fig. S7). These findings suggest that PC fate is specified in a highly restricted subdomain of the right lateral plate mesoderm by St8, and that the initial events dictating the functional divergence of PC fate from the adjacent working myocardium must occur before this stage.

Additionally, our data indicate that a large region of mesoderm outside of the primary and secondary heart fields is already specified into working myocardial and PC fates by St8. To distinguish this mesodermal subdomain from the more classically defined heart fields, we refer to it as a tertiary heart field in chick. Although the precise boundaries of the primary and secondary heart fields remain controversial (18), our high-resolution fate mapping reveals the distribution and boundaries of several subtypes of cardiac precursors within the tertiary heart field. (Emphasis added.)

That word “specified”... does it sound familiar? The authors use it 11 times in the paper. They developed a map of cell fates relating to the developing heart, and speak of their “fate map information.” And there’s no question that a developing check embryo is complex. There it is; complex specified information, a hallmark of intelligent design. It’s not surprising evolution was never mentioned in the paper.

This glimpse into the details at one stage of a chick embryo, which is only one episode in Illustra’s new film, should underscore how powerful an argument for intelligent design it makes.