Pattern in nature

Functional and structural aspects in biology

(aboriginal art - wikimedia commons)

The pattern is the concept that best combines functional and structural aspects in biology. Patterns in nature are visibly regular forms 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 mathematically modeled. Natural patterns include symmetries, trees, spirals, meanders, waves, foams, weaves, cracks, and stripes. For example, the bronchial tree in the lungs of an animal follows the same branched pattern as the branches of a tree, as well as its roots.

The stripes of a zebra are similar to the stripes on the wings of a butterfly:

The multiple spirals intersecting in the cross-section of a red cabbage resemble the shape of a shell:

A pattern is formed through the so-called pattern formation, i.e. a development process through which the cells involved 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 mental experiment, and take a snapshot of a pattern: we appreciate its structural aspects and their direct aesthetic implications. If, on the other hand, we make time flow, and observe the living pattern through the underlying biological processes, we appreciate its functional aspects and evolution, from its formation to the maintenance of its homeostasis.

The question arises as to how complex patterns can be generated during development without the expert eye of a designer or architect directing the work on a cellular level, or the inspired brush of an illustrator sketching the complexities. When we study the morphogenesis (i. e. the generation of shape) of a pattern we talk about emerging 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 simple, single events. Rather, patterns are generated without the need for external coordination, but thanks to a dense network of functional interactions between and within cells, which step by step self-organize into semi-regular structures, with the above-mentioned peculiarities of form and function.

An adult organism, as well as a pattern, is composed of a multitude of cells organized in complex architectures. Among these, some specialize to perform a given function, others another one, and working together with the respective specializations they give rise to system operation. At the beginning of an organism's development process, 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 non-specialized cells emerges the concert of functions that constitutes the pattern?

The answer lies in the local interactions between the cells. In fact, from the very beginning, cells begin to sketch a preference for one specialization or another. If a cell, even by chance, finds itself to have moved even a little towards a certain specialization, this changes its way of communicating with neighboring cells, i.e. it sends molecular messages related to this slight change of identity. The nearby cells, receiving different messages, also change, i.e. they in turn take a small step towards 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 intervals: sometimes the cells only affect the neighbors they are physically in contact with.

Other times, during morphogenesis, some cells take on the role of inducers of a pattern, and their functional identity at that stage causes them to begin to release a very interesting signal: a morphogen. This spreads in the extracellular space, resulting in a very strong signal for the cells which are closer to the source, and weaker for those further away. The cells are able to translate this different degree of signal intensity into different responses so that the nearby cells will take a step towards one specialization, the distant ones towards another. To make this mechanism robust, mechanisms mediated by direct contact between distant and neighboring cells also contribute.

Moreover, these small differentiation steps (cell differentiation is the process of progressive functional specialization in cells) are interspersed, during development, with phases of proliferation, migration, and programmed death of cells, which contribute to shaping the architectural landscape of the pattern. In this example, different concentrations of morphogen determine the different fate 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 also admire their surprising regularity and functionality. Under this lens we can ask ourselves: why during the evolution different species have 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 in the development of new technologies. Sometimes we try to extract mathematical constants that could 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 objective is to artificially replicate the same characteristics of the natural pattern, to obtain the same functionality.

Why draw inspiration, inspiration, or even copy biology in today's arts and techniques?

In the next posts, we will try to answer these questions in a precise manner.

Roberta Bardini is a researcher in computational Biology and systems. She currently works at the Sysbio Group, Polytechnic of Turin, where she obtained her PhD. She deals with the development of multicellular organisms, and their enhancement in the business environment.

(All images from Wikimedia commons)

#biology #pattern #nature #science #research

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