The functional and structural aspects in biology
(aboriginal art - wikimedia commons)
The pattern is the concept that best combines the functional and structural aspects in biology. In nature, they are visibly regular forms that are found in the natural world.
These patterns are repeated in different contexts (for example, in two different animal species, or between animals and plants) and can sometimes be modeled mathematically. Natural patterns include symmetries, trees, spirals, meanders, waves, foams, tiles, cracks, and stripes. For example, the bronchial tree in the lungs of an animal follows the same branching pattern as the branches of a tree, as well as its roots.
The stripes of a zebra are similar to the stripes on a butterfly's wings:
The multiple spirals that intersect in the cross-section of a red cabbage recall the shape of a shell:
A pattern is formed through so-called pattern formation, that is, a developmental process through which the involved cells acquire different identities, depending on their relative spatial position within the embryo. Pattern formation ensures that tissues and organs develop in the correct position and orientation within the body.
Let's try a thought experiment, and take a snapshot of a pattern:
we appreciate its structural aspects, and their direct aesthetic implications. If instead we let time flow, and observe the pattern living through the underlying biological processes, we appreciate its functional aspects and evolution, from its formation to the maintenance of its homeostasis. It is in fact natural to wonder how complex patterns can arise, during development, without the expert eye of a designer or architect directing the work at the cellular level, or the inspired brush of an illustrator sketching its complexities.
When studying morphogenesis (i.e. the generation of form) of a pattern, we speak of emergent properties, that is, the characteristics of the system that are recognizable, and reappear every time the process is repeated, but which are not directly dictated by single simple events. Rather, patterns arise without the need for external coordination, but thanks to a dense network of functional interactions between cells and within them, which step by step self-organize into semi-regular structures, with the aforementioned peculiarities of form and function.
An adult organism, just like a pattern, is composed of a multitude of cells organized into complex architectures. Among these, some specialize to perform a given function, others another, and by working together with their respective specializations they give rise to a system functioning. At the beginning of a developmental process of an organism, it is composed of a few so-called stem cells, that is, not yet specialized, and able to give rise to all the cell types of the adult organism. How is it possible that from these few unspecialized cells the concert of functionalities that constitutes the pattern emerges?
The answer lies in the local interactions between cells. In fact, from the very beginning, cells begin to sketch a preference for one specialization or another. If a cell, even by chance, happens to have moved even slightly toward a certain specialization, this changes its way of communicating with neighboring cells, that is, it sends molecular messages related to this slight change of identity. The neighboring cells, receiving different messages, also change, that is, in turn, take a small step toward a functional specialization. So in turn they will send a different message to their neighbors, and so on. This mechanism can take place at various distance intervals: sometimes cells influence only the neighbors with which they are physically in contact.
Other times, during morphogenesis, some cells take on the role of pattern inducers, and their functional identity at that stage means they begin to release a very interesting signal: a morphogen. This spreads in the extracellular space, resulting in a very strong signal for the cells closest to the source, and weaker for those further away. Cells are able to translate this different degree of signal intensity into different responses, so that nearby cells take a step toward one specialization, those further away toward another. To make this mechanism robust, mechanisms mediated by direct contact between distant and nearby cells also contribute.
Moreover, these small differentiative steps (cellular differentiation is the process of progressively approaching a functional specialization in cells) are interspersed, during development, with phases of proliferation, migration, and programmed cell death, which contribute to shaping the architectural landscape of the forming pattern. In this example, different concentrations of morphogen determine different fates for different parts of the Drosophila embryo, the fruit fly, resulting in a striped pattern:
In addition to appreciating the beauty of natural patterns, we can admire their surprising regularities and functionalities. Through this lens we can ask ourselves: why during evolution have different species developed similar patterns, sometimes even at very different spatial scales?
The principles behind the aesthetics of natural patterns have been extracted and reused in the visual arts as well as in design and the development of new technologies. Sometimes people try to extract mathematical constants that can be applied de novo in the construction of images far from the world of biology, such as the golden ratio in paintings and photographs. Other times the goal is to artificially replicate the same characteristics of the natural pattern, to obtain the same functionality.
Why take inspiration from, or even copy, biology in today's arts and techniques?
The next posts will try to answer these questions in detail.
Roberta Bardini is a researcher in the field of computational and systems biology. She currently works at the Sysbio Group, Politecnico di Torino, where she obtained her PhD. She deals with the development of multicellular organisms, and their valorization in the entrepreneurial field.
(All images from Wikimedia commons)
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