Article on how and why some of the key internal organs of the common fruit scale up (or down) as the embryo develops and grows in size. Written for MBI.

Based on: Scaling of internal organs during Drosophila embryonic development

Picture two dogs, a Bloodhound and a Pomeranian, side by side.

At a glance, you can immediately tell their striking difference in size. Looking deeper, they actually possess similar internal profiles, scaled up or down to fit their body sizes. This is an example of biological scaling, also known as allometry. Visually, it appears that the organs are simply sized up or down. The reality is that the processes and circumstances governing scaling are extremely complex.

Take a look at the common fruit fly (Drosophila melanogaster), an organism often used for allometric research. During the early embryonic stages, the embryo is partitioned in a very specific manner. This pattern of partitioning remains the same regardless of the size of the embryo, which is to say that the positions of the partitions adapt and scale according to embryo size. This begs the question of whether organs will behave similarly as well, especially because they each have varying morphologies and developmental strategies, and are drastically altered ultimately in the larval and pupal stages.

Thus, a team of researchers from the Saunders Lab of MBI set out to investigate the scaling of internal organs to embryo size and centred on three critical organs: the heart, ventral nerve cord (analogous to our spinal cord) and hindgut (analogous to the distal part of our colon). Both the heart and the ventral nerve cord (VNC) are largely linear and symmetrical organs that are oriented along the length of the embryo, with the VNC spanning its entirety. Thus, they are suspected to scale along that axis. They deviate in that only the heart has a fixed cell count. On the other hand, the hindgut is an organ with a unique, non-symmetrical shape, making it hard to pinpoint which axes it can possibly scale to, if it does.

Using embryos of both genetically modified and regular fruit flies (which were smaller and larger respectively to provide size variability), the structures and positions of the organs were visualised with fluorescent markers, measured and compared embryo size to see if they correlate.

It was discovered that the internal organs can adapt and scale to changes in embryo size, but the extent and mechanism in which they do vary greatly from organ to organ. The heart was observed to scale precisely with embryo length, and with even greater fidelity at later stages. It is probable that its orientation to the length of the embryo increased its sensitivity to longitudinal changes. As to how it did, smaller-scaled hearts have more sets of narrow cells, altering the overall heart morphology, thereby reducing its size.

Contrary to the heart, the VNC exhibited increasingly lackluster scaling responses to changes in embryo length; A substantial extension in embryo length was met with a minor reduction in VNC length. This suggests that the VNC might have intrinsically adhered to a minimal length that is conserved across all Drosophila embryos, regardless of their sizes. Alternatively, the VNC might have scaled to embryo volume instead, since the volumes across the embryos were similar. The VNC scales by eliminating cells, but in shorter embryos, the spaces between bundles of cells are slightly reduced instead, leading to a more minor length decrease.

The hindgut displayed a different behaviour. In larger, regular embryos, the hindgut demonstrated limited adaptation to embryo size changes. Conversely, there was visibly stronger scaling in the smaller, artificial embryos. This suggests that the hindgut is more responsive towards space-stressed environments, likely with surrounding organs hindering its development and dictating the axes it accordingly scales. It is, however, still unclear exactly how the hindgut morphs.

Such diversified scaling patterns displayed by the varying organs juxtapose greatly with the precise blueprints for development first laid out in the early embryonic phase. Thus, scaling patterns can be controlled by other mechanical and biochemical factors at various time points. This research has reaffirmed the challenges of decoding allometry and development, and the need to establish new biophysical models to elucidate how each organ grows into the shape that we see.