Why are some of us tall, others short? why do we stop growing?

By Jasmin Imran Alsouis

Why are some of us tall, others short? why do we stop growing? and why are we born with seemingly large heads and small limbs, but mature into adults with opposite proportions? In other words, what determines the absolute size of an organism and the relative sizes of its parts? These questions address one of the most basic and intuitive phenomenon that surrounds us: size control. A phenomenon so basic and so intuitive that the relevant questions posed by children differ little from those posed by the mature scientist. The answers however are far from basic: the mechanisms of size control are complex and are challenging to uncover. Despite this, scientists have made significant strides in advancing our understanding of size control, and it was largely a penchant towards simplicity that has made this possible. Relative growth is what I have a penchant for: my latest work has focused on understanding how different parts of a structure grow at different rates to generate patterns of different sizes, and the fruit fly egg was my go-to model system.

We are not first to wonder about the emergence of size differences. As early as the 17th CE, Galileo correctly observed that humans do not grow to their adult size through uniform expansion or dilation of the baby, i.e. through isometric growth. Rather, we achieve our final size and form through relative, or allometric growth: some parts, like our limbs, grow faster than others, like our heads. Furthermore, by examining dinosaur fossils, which were believed to belong to ancient giants, Galileo concluded that with bones this large, those belonging to our alleged giant ancestors would have to have had different proportions, e.g. wider, than our current brethren to support their physique. About 200 years later, D’Arcy Thompson and Julian Huxley joined the discussion. Thompson remarked on the seeming inevitability of relative growth – growth being a phenomenon so complex that uniformity would be an “unlikely and an unusual circumstance” and wrote that during growth, “rates vary, proportions change, and the whole configuration alters accordingly”. Huxley’s ensuing mathematical description, relating the size (mass or length) of an organ at any moment in time to the total size of the organism through a power law qualitatively formalized and generalized the description. Researches have since followed up on this work, however, most work was relatively qualitative and phenomenological, and the mechanisms driving these differential growth patterns remained unclear.

Following in the footsteps of giants, I found myself drawn to this subject, but in need of a simple system to study the mechanisms underlying this complex phenomenon. Precedents existed: a previous study looking the size discrepancy between the Drosophila wings and halteres, which are a second and smaller pair of appendages used for balance and steering, showed that the observed size differences can be explained by differential gene expression. In other words, a gene, called ultrabithorax, expressed in the patch of cells in the larva that give rise to the halteres, but not the ones that give rise to the wings accounted for the size differences of these two structures. Another study showed that the difference in size between the fore and hind wings on butterflies can be accounted for by competition for resources among the two pairs of growing wing appendages. This was best exemplified by surgical experiments that removed the cells that give rise to the hind wings: in such butterflies, a compensatory increase in the size of the forewings was observed. Although these model systems are considerably simpler than whole organisms, they are still relatively complex: both systems have thousands of cells and are non-autonomous subparts of the system.

Enter fruit fly egg chambers. The egg chamber is the developmental unit of the future oocyte, and is a cluster of exactly 16 germline cells that are enveloped by an epithelium. These 16 cells arise from a single founder cell that undergoes four synchronous and incomplete divisions: the cells remain connected through bridges, called ring canals, that allow for intercellular transport and exchange of material. Furthermore, because these divisions are stereotypic, each cell is uniquely identifiable. Although this structure grows by ~4 orders of magnitude throughout oogenesis, it does so non-uniformly and while others have proposed potential mechanisms, the claims remained untested and mathematically unverified. To that end, using 3D imaging of fluorescently labeled egg chambers, we quantified the spatial cell size pattern that emerges within the growing germline cyst. Specifically, we showed that distance to the oocyte on the lineage tree is the primary factor affecting cell size, and as a result, four groups of cell sizes emerge.

To rationalize our experimental observations, we then proposed a simple mathematical model that both correctly predicted the divergent spatial pattern of cell sizes, and that revealed the allometric growth of cells. This result was exciting for three reasons: first, we knew that material is exchanged bi-directionally through the ring canal, but that it is active transport is biased towards the oocyte. By incorporating that into our model and reproducing our experimental results, we showed that unequal distribution of limiting resources is the mechanism driving differential growth. Second, we were able to extract allometry from a small system of 16 cells – a feature usually associated with much larger systems, such as deer antlers or fiddler crab claws.

Finally, our model connected a molecular process to a macroscopic phenotype, namely, two features of growth occurring at vastly different scales.
Our work is not over. We are now in the process of identifying the nature of this limiting resource, and expanding the model to include other potentially critical features of egg chamber growth, such as differential gene expression. Nonetheless, it was great fun to work on this problem in such a beautiful system and to have had the opportunity to address a difficult question using a relatively simple yet rich system: with its 16 cells, it is simpler than most multicellular system, yet more complex than a single cell, and it has brought us one step closer towards understanding an old yet still exciting problem.

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