Concepts

What is embryonic development?
Figure 1. Fertilized zygote and unfertilized egg of the purple sea urchin Strongylocentrotus purpuratus (20x magnification). © embryos-and-evolution.com
Figure 2. Top image: Ubiquitous spatial distribution of a fluorescently tagged protein (β-catenin) early in Eucidaris tribuloides development). Bottom image: Localization of the same protein to a particular region of the Eucidaris tribuloides embryo.  © embryos-and-evolution.com
We are all products of development
All of the creatures you see with the unaided eye have undergone the process of development. Adults come from eggs, and eggs come from adults. It is necessarily circular since cells come from cells. The egg is nothing more than a cell. In fact, the largest cell on the planet is an egg--an ostrich egg.

When viewed under the microscope, as seen in Figure 1, eggs of the sea urchin look like simple spheres. But this geometrical beauty belies a profound molecular complexity of which biologists are just beginning to reveal after 100 years of groundbreaking research in developmental biology.

Eggs are molecularly compartmentalized
It is now clear that animal eggs are molecularly compartmentalized, that is to say that molecules like the nucleic acid RNA and proteins like enzymes inhabit particular regions of the egg. This maternal molecular anisotropy, as it's sometimes called, can be described as a loosely-tuned arrangement of biomolecules.

Development starts with a union
Eggs and sperm are reproductive cells that carry one half the genetic complement of normal (somatic) cells. When a sperm cell is lucky enough to meet an egg, fertilization will take place (unless of course another sperm got there first). The union of these two cell types, which brings together DNA in their nuclei, results in a cell that after their union will have the full genetic complement; we call this cell the zygote.

Cells in the embryo inherit different sets of biomolecules
A cycle then begins during which the zygote begins dividing, duplicating its genetic complement and divvying up its biomolecules that are outside the nucleus. As was previously mentioned, an asymmetric localization of biomolecules occurs in the egg, and this has consequences for development of the zygote, as one cell becomes two, two become four, four become eight, and so on. As the number of cells increases in the embryo, the biomolecules that first inhabited the egg become unequally distributed in the cellular progeny of the zygote. An example is seen in the two images shown in Figure 2, which shows a fluorescently-tagged protein that, at first is present in every cell of the embryo, and later in development is seen in only a few cells.*See Note 1

What is a developmental program?
The unequal distribution of biomolecules in early embryonic development sets in motion a developmental program that is encoded in the genome of each species. The spatial arrangement of biomolecules in the egg is also part and parcel of the developmental program since these biomolecules are inherited differentially depending on their location in the egg. But what exactly are we referring to when we talk about "developmental programs"?

Flexible, responsive programs define development
It is important to note that we refer to a developmental program more in the sense of a planned series of future events--like a planned arrangement and sequence of events you would receive at a recital or a wedding--rather than a computer program. As everyone with a little experience writing and editing in a computer language knows, computer programs are finicky, inflexible and rigid. Biological systems are different in an important way. They are extremely robust and capable to responding to their environment, as well as molecular egg-to-egg variation and differences in DNA sequence--the inherited genetic material. These types of systems are quite different from those that we design and write into code everyday. With that said, we still speak of developmental programs and how genes are "wired"; we just have to be aware of the flexibility of this encoded developmental program.

Developmental programs: Like begets like
Figure 3. Children learn a central concept of development from a very early age! From PD Eastman's 'Are You My Mother?'
What do we mean by a 'developmental program'? As seen in Figure 3, we learn something akin to this very early in our childhood. In P. D. Eastman's children's book Are You My Mother?, a distraught, just-hatched birdie gets lost on a farm and cannot find his mother. The little birdie goes around the farm asking all the animals 'Are you my mother?' No, of course not, say the wise cow and other denizens of the farm, How could that be?! What the farm animals are suggesting is couched in the age-old adage that dogs beget dogs, flies beget flies, and birds beget birds.

So why don't dogs beget flies and vice versa? The reason is because dogs are in the dog lineage and flies are in the fly lineage. Their current genetic constitution has encoded within it arrangements for building more dogs and, in the case of flies, building more flies. In their family trees, their recent ancestors produced dogs and flies, and the molecular information for building dogs and flies--a bit like instructions--are specified in the arrangement and historical antecedents of their DNA sequences. What's more, the cellular machinery responsible for building eggs and sperm does a bit of reshuffling every time one of these cells is produced. Thus, each union of egg and sperm produces a unique arrangement of DNA that varies from its siblings. Importantly, this rearrangement event normally does not interfere with the developmental program.

Variation in development comes at different degrees
Each human we come across shares our developmental program, and yet each human we interact with has unique variations of that program. This is because the shared features of the basic program are free to vary more so near the end of the developmental program than near the beginning, as in early development. While the developmental program is free to vary throughout all stages, in general it is more difficult to alter the arrangements and events early in the program than later. Thus, early embryos share more in common from individual to individual than embryos later in development--or in the case of visible characteristics, in the child, teenager or adult.

Why study developmental programs?
What can sea urchin embryos tell us about human development? One goal of studying embryonic programs is to decode the developmental program: To get at its basic features, as well as understanding how the program can vary. To do this, we need access to developmental programs that are easier to manipulate than the human program. However, the system also must be similar in many ways to what we know about human development. Remarkably, the sea urchin is an excellent system for this task. Even more powerful is the insight provided by studying numerous developmental systems. These comparative studies yield details about the fundamental and basic processes of early development and the general strategies that these systems employ to make embryonic development move forward.


maternal anisotropy: spatial asymmetry of molecules in the egg
soma: (greek body) cells of the body other than the reproductive cells
nuclei: (pl. nucleus, from latin inner part) cells of multicellular organisms (eukaryotic cells) are compartmentalized into a nucleus, which contains DNA and other biomolecules, and cytoplasm, which contains biomolecules other than DNA, e.g. RNA and protein
genome: the full complement of DNA particular to each species
Notes & References
1: For further details on this experiment: click here. Ref: Erkenbrack and Davidson (2015) Evolutionary rewiring of gene regulatory network linkages at divergence of the echinoid subclasses. Proc Natl Acad Sci USA. http://dx.doi.org/10.1073/pnas.1509845112
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