How a single fertilized egg gives rise to the human form is one of the great mysteries of biological science. Each zygote must generate distinct cell types in a spatially and temporally controlled manner, ultimately assembling complex organs with specific structures and functions. For centuries, our understanding of this remarkable process was largely descriptive, starting with Aristotle’s account of chicken (Gallus gallus) embryogenesis in Generation of Animals. The scientific tradition of embryological observation has since expanded to include several invertebrate and vertebrate model organisms, such as the sea urchin (Strongylocentrotus purpuratus, Lytechinus variegates, and other species), the worm (Caenorhabditis elegans), the fruitfly (Drosophila melanogaster), the fish (Danio rerio and Oryzias latipes), the frog (Xenopus laevis and Xenopus tropicalis), and the mouse (Mus musculus). While these metazoans appear dissimilar in adult structures and forms, their embryos share some common features with the developing human fetus. For example, embryonic cells in these organisms segregate during a process called gastrulation to form three “germ” layers (the ectoderm, the mesoderm, and the endoderm) and become asymmetrically patterned with respect to each body axis (e.g. anterior-posterior, dorsal-ventral, and left-right). The differentiation of each germ layer into specialized tissues is also grossly conserved, with epidermis and nerves arising from the ectoderm, muscle from the mesoderm, and the digestive system from the endoderm.
Using these model systems, embryologists have interrogated the origins of multicellular pattern and function through a “perturb and observe” paradigm. Perturbation strategies have varied with each organism, according to their amenability to available technologies. Prior to the advent of modern genetics and molecular biology, embryological manipulations were primarily limited to surgical procedures and the labeling of certain cell populations with visible dyes, favoring the study of embryos that develop ex utero and are readily dissected. At the turn of the 20th century, cell dissociation and transplantation experiments with sea urchin embryos helped establish fundamental concepts such as regulative development, inductive interactions between cells, and the existence of morphogen gradients. Subsequent transplantation studies using frog embryos led to the discovery of cellular “organizing” domains, named the Nieuwkoop center and Spemann organizer, that establish the dorsal-ventral axis during gastrulation, and surgical procedures on chick embryos revealed specific tissue structures that regulate limb patterning. Other insights were obtained from spontaneous mutants with embryonic defects, such as the antennapedia fruitfly which has ectopic legs on its head rather than antennae and the talpid chicken which has extra toes and craniofacial abnormalities.
Elucidation of the genetic code and development of molecular biology techniques transformed embryology from a descriptive science to one deeply immersed in molecular mechanism. Large-scale mutagenesis screens pioneered by Nüsslein-Volhard and Wieschaus in the 1980s yielded several hundred fruitfly mutants with distinct developmental abnormalities,1
and the positional cloning of these mutated genes has yielded many of the key molecules that regulate embryonic patterning. What has emerged from these and subsequent studies is that a relatively limited number of signaling mechanisms—such as the Hedgehog (Hh), Wnt, transforming growth factor-β (TGF-β), bone morphogenetic protein (BMP), fibroblast growth factor (FGF), and Notch pathways—are used iteratively throughout development to coordinate cell proliferation, migration, and differentiation. We now know that Wnt pathway activation in a subset of cells within the frog blastula (the embryo prior to gastrulation) establishes the Nieuwkoop center, which in turn induces the Spemann organizer. The organizer then secretes inhibitors of BMP signaling to promote dorsal cell fates. Ectopic expression of Wnt proteins or BMP antagonists in the frog embryo can consequently induce a secondary body axis, recapitulating the phenotypes observed upon organizer transplantation. Similarly, digit identity in the developing limb bud is controlled by a gradient of Hh pathway activation established by a posterior domain of Hh ligand-secreting cells, and ectopic Hh protein expression in the nascent chick wing can cause dramatic mirror-image duplications of this structure.
The recent completion of multiple genome sequencing projects promises to further revolutionize our understanding of embryonic development, since we now have a comprehensive parts list for Nature’s genetic circuitry. Determining how these genes orchestrate embryogenesis at the molecular and systems levels is the challenge that lies ahead, and realizing this goal will require the expertise of multiple scientific disciplines. Genetic approaches will certainly continue to make significant contributions to this effort, building upon current technologies for controlling embryonic gene function, generating mutant and transgenic organisms, and analyzing spatiotemporal changes in gene expression levels. Computational approaches will be necessary to understand how diverse developmental signaling mechanisms integrate to create specific morphogenetic outcomes. While currently underrepresented in this initiative, chemists have an important role to play in embryological research as well. Our ability to synthesize novel compounds can create new ways to “perturb and observe” embryological processes, circumventing the limitations of Nature’s building blocks. In addition, our intuition about chemical structure and reactivity can bring a unique perspective to the molecular mechanisms of embryogenesis. This tutorial review will summarize areas in which chemical concepts and technologies have advanced our knowledge of embryonic patterning, in the hope that these examples will guide and inspire other chemists to explore developmental biology. We will first focus on chemical methods that can alter embryonic gene expression or function with spatiotemporal control, enabling the interrogation of complex patterning mechanisms. Complementary tools for visualizing the molecular and cellular processes that constitute embryogenesis will then be discussed, as well as future research opportunities at the “chemical embryology” interface.