What has always attracted me to developmental biology is the ability to see the unfolding of pattern—simplicity becomes complexity in a process made up of small steps, comprehensible physical and chemical interactions that build a series of states leading to a mostly robust conclusion. It’s a bit like Conway’s Game of Life in reverse, where we see the patterns and can manipulate them to some degree, but we don’t know the underlying rules, and that’s our job—to puzzle out how it all works.
Another fascinating aspect of development is that all the intricate, precise steps are carried out without agency: everything is explained and explainable in terms of local, autonomous interactions. Genes are switched on in response to activation by proteins not conscious action, domains of expression are refined without an interfering hand nudging them along towards a defined goal. It’s teleonomy, not teleology. We see gorgeously regular structures like the insect compound eye to the right arise out of a smear of cells, and there is no magic involved—it’s wonderfully empowering. We don’t throw up our hands and declare a miracle, but instead science gives us the tools to look deeper and work out (with much effort, admittedly) how seeming miracles occur.
One more compelling aspect of development: it’s reliable, but not rigid. Rather than being simply deterministic, development is built up on stochastic processes—ultimately, it’s all chemistry, and cells changing their states are simply ping-ponging through a field of potential interactions to arrive at an equilibrium state probabilistically. When I’d peel open a grasshopper embryo and look at its ganglia, I’d have an excellent idea of what cells I’d find there, and what they’d be doing…but the fine details would vary every time. I can watch a string of neural crest cells in a zebrafish crawl out of the dorsal midline and stream over generally predictable paths to their destinations, but the actions of an individual melanocyte, for instance, are variable and beautiful to see. We developmental biologists get the best of all situations, a generally predictable pattern coupled to and generated by diversity and variation.
One of the best known examples of chance and regularity in development is the compound eye of insects, shown above, which is as lovely and crystalline as a snowflake, yet is visibly assembled from an apparently homogenous field of cells in the embryo. And looking closer, we discover a combination of very tight precision sprinkled with random variation.
That compound eye is even more precisely organized when you look below the surface. Each facet is a structure called an ommatidium, or little eye, and each has its own collection of cells. It’s ringed with a set of supporting cells, pigment cells, and mechanosensory bristle cells, and capped with four cells that make a lens. The lens focuses light on photoreceptor cells. There are exactly 8 of them, R1-R8, and each has a rod-like organelle called the rhabdomere, in red in the diagram below, which is the actual light-sensing part of the cell.
When you look at a section through the eye the rhabdomeres are visible as dark spots with a characteristic orientation—look at the repeating “” pattern of 7 dots in the electron micrograph to the left (you see 7 instead of 8 because the central pair, R7 and R8, are stacked one on top of the other, so you only see one in a single plane of section). Notice how uniform those rhabdomeric spots are—every one has the same arrangement, and the same orientation, with one consistent difference. All the ommatidia in the top half look like , while every one in the bottom half is flipped upside down (), and if you look carefully, you can probably figure out where the equatorial dividing line falls.
The whole assembly is going to form a visual processing device, so this specific organization is going to contribute to visual acuity. Each ommatidium has a lens that focuses an image of the world on those 8 photoreceptors, and each photoreceptor sees a specific small spot of the visual field. Different, adjacent ommatidia will see the same spot, but it will fall on a different rhabdomere in each one, in a predictable pattern—the diagram below shows in red which rhabdomeres would see the same point in the visual field.
The specificity goes remarkably deep: each of those red rhabdomeres projects an axon into the fly’s brain that find each other and synapse with the same cell in the lamina. The whole thing has an intricacy that makes a human-made watch look like clumsy hackwork. You might even say, if you found a fly eye upon a heath, that you could not imagine how something so beautiful and perfectly arranged and organized could possibly have come into existence without some superhuman engineering.
Except us developmental biologists, of course: the fly eye is an extremely well studied developmental system, and we know that it is assembled by unthinking cellular processes with no design or engineering required.
That splendid organization emerges from an initially uniform sheet of cells, the eye imaginal disk. It arises progressively, too, in tidy order from the posterior edge to the front, so when we look at a slice of the eye, like that below, we actually see all the developmental steps at once, from the earliest, youngest parts at the front of the eye (to the left in this picture), to the progressively more mature pieces as we look towards the right.
The process begins with a wave of synchronized cell divisions sweeping from posterior to anterior. As cells enter the mitotic cycle, they tend to hunker down deep in the epithelial layers, and when all the cells in a region divide at the same time, that region dimples downward. When a whole line of cells divide at the same moment, you get a furrow forming—the morphogenetic furrow. After the cells finish, they pop back up, and the next row of cells to the anterior begin their divisions.
This is easy to understand: the cells in the fly eye are doing The Wave. After the wave has passed, cells form small clusters and signal each other; one cell sets itself apart and begins to differentiate into the R8 cell, and recruits two neighbors to form the R2 and R5 cells. Subsequently, R3 and R4 are drawn in, then R1 and R6, and finally, R7. The cells all have the same orientation to one another (later, the clusters rotate 90° one way in the upper half of the eye and 90° the other way in the lower half, to generate the mirror symmetry). It’s all quite mechanical and reliable, mediated by a small set of genes; one common sort of experiment is to knock out a gene involved in these pathways, and see that some of the later cells fail to form in the absence of a signal, and so we know that genes like Notch and boss and rough are involved in particular steps in the process.
So, a series of specific molecular interactions establish the regularity of many features of the fly eye. It’s not all rigid and deterministic, however, and some features are set up randomly.
In particular, flies have rhabdomeres specialized for color vision, analogous to our system of cones and rods. We have three kinds of cone cells which express different kinds of opsin proteins (the visual pigment) that make them specific to different wavelengths of light, and we also have rods, which are sensitive to a broader range of wavelengths and detect intensity rather than color. In the fly ommatidium, most of the rhabdomeres, R1-R6, the ones arranged around the outside of the structure, are like our rods—they don’t detect color, but just shape and brightness. The central pair, R7 and R8, express specialized opsins that are tuned for different wavelengths. Some (about 30%) of the R6-R8 pairs express an opsion called Rh3 in the R7 cells, and Rh5 in R8; these are called ‘pale’ ommatidia, and are specialized to detect short wavelengths. Most of the remaining ommatidia Rh4 in R7 and Rh6 in R8, the ‘yellow’ ommatidia, and are sensitive to longer wavelengths. Which set of opsins will be expressed in a particular pair of central rhabdomeres is determined randomly, so what you see is a variable salt-and-pepper arrangement of ‘pale’ blue receptors sprinkled among a sea of ‘yellow’ photoreceptors, as shown in the wild type diagram below. (There are also some special pink photoreceptors in the dorsal rim; these are ommatidia where both R7 and R8 express Rh3, in a set of cells used for detecting the plane of polarized light.)
The random distribution of the color receptors is regulated by a particular protein, spineless. The default fate for an R7 receptor is to express the Rh3 (blue) opsin, but the presence of the spineless protein above a certain level is sufficient to induce it to express Rh4 instead. In a mutant in which no spineless is present, the R7 cells all make the blue protein—this would be a kind of color blind fly.
When such a mutant eye is stained for the Rh5 and Rh6 gene products, which are expressed in the R8 receptor, there is a similar result—the eye has gone almost entirely blue.
Complementary experiments with a gain-of-function mutant that turns on excessive spineless expression also has an expected result: the whole eye turns yellow as Rh3 is suppressed and Rh4 is activated, and we get a fly with a different kind of color blindness.
What is happening is that during pupation, the fly turns on a brief pulse of spineless before the expression of the opsin genes; different cells acquire different levels of spineless expression, in red below, and any spineless activity above a threshold leads to that ommatidium forming the Rh4/Rh6 opsin combination and becoming a ‘yellow’ type of receptor. Spineless is acting as a binary switch controlled stochastically to produce a random distribution of cell types.
Although the overall effect is random, there are some specific interactions. The R8 opsin type is correlated with the R7 opsin type, so the first step is a choice made by R7 under the influence of spineless; R8 cells default to expressing Rh7, except that the Rh3-expressing R7 cells can instruct R8 to express Rh5.
I know, it’s all a little bewildering and complicated—intensely complicated with all kinds of interactions between cells. However, when you dig into it and explore the literature, what you find is the successful application of a reductionist program of study, with each piece of the story a fully comprehensible and actually rather simple product of a molecular/cellular interaction. Complexity is the result of simple, repetitive, iterated processes which can yield regularity and chance variation…but at no point are there any events beyond local chemistry and cell biology.
Wernet MF, Mazzoni EO, Çelik A, Duncan DM, Duncan I, Desplan I (2006) Stochastic spineless expression creates the retinal mosaic for colour vision. Nature 440:174-180.