Taking a leaf from the book of cell fate

"Oh God! That one might read the book of fate!"
Henry IV, William Shakespeare

Quail cells (black) 24 hours after transplantation into the chick neural plate. Image first published in Nature Reviews Neuroscience 2, 763–771 (2001)

With all due respect to Shakespeare, let us consider another rich, elegant, and infinitely more knowable book of fate than the one King Henry IV lamented. This book describes the journey of a fertilized egg as it metamorphoses into an embryo, a fetus, a juvenile, and finally an adult. Fate maps are the constellations by which developmental biologists navigate the journey of development, and thanks to pioneering work of early embryologists, this book of fate can be found, at least in rudimentary form, on library bookshelves.

The earliest fate maps — of invertebrate sea creatures for the most part — date back to the 1880s. In 1905, Edwin Conklin published a remarkably comprehensive and insightful collection of ascidian fate maps, presciently observing that substances are segregated to particular cells during development and that this distribution is correlated with the fate of those cells. However, it was probably Walter Vogt who paved the way for modern vertebrate fate-mapping techniques. In 1929, Vogt developed a method of marking groups of cells on the surface of amphibian embryos with dyes, then following the movement of those cells throughout gastrulation. His fate maps have been confirmed and refined by modern microscopy and dye-injection techniques. Variations in the fate maps of different amphibian species were uncovered, and lineage-marking studies by Smith and Malcinski and Lundmark (combined with scanning electon microscopy in Lundmark's case) showed that this could be attributed to variations in the gastrulation mechanism between different species. Amphibian fate maps continue to inform the design and interpretation of modern-day experiments aimed at understanding the molecular mechanisms that regulate developmental movements.

Fate maps relying on vital dyes and radioactive markers were to some extent confounded by the tendency of the marking substances to diffuse, greatly hampering the resolution of the techniques. A milestone was reached in 1969, when Nicole Le Douarin pioneered the use of chick–quail chimaeras for fate mapping. Her experiments are notable for their elegance, the cellular resolution they provided, and the foundation they built for future studies of neural-crest migration, epithelial–mesenchymal interactions and neural-plate formation.

Her studies exploited two characteristics of quails and chickens: first, embryos of the two species develop in similar ways, and second, quail and chicken cells can be distinguished from each other using a simple DNA staining technique. Le Douarin discovered that if she transplanted bits of the quail embryo into the corresponding region of the chick embryo while it was still in the egg, the transplanted tissue would meld perfectly to the host tissue, and normal development would ensue, even for some time after birth. She also noticed that during interphase, quail cells possess a dense patch of nucleolar heterochromatin when stained with Schiff's reagent. By contrast, chick heterochromatin is diffuse and the nucleoli are indistinct. Recognizing that this distinction provided a way of 'marking' the cells, Le Douarin went on to show that quail and chick cells can be distinguished in multiple organ systems, and throughout development to adulthood.

Le Douarin used the staining technique to follow the fate of the transplanted cells during development. In one proof-of-principle experiment, she showed that chimaeric birds can be used to map the fate of neural-crest cells, in what has since become a classic approach for studying this migratory population of embryonic cells. She excised a region of an early embryonic chick neural tube and replaced it with the corresponding region of a quail embryo. After 6 days of development, she sectioned the host embryo and found that the quail neural crest cells had contributed to various tissues, such as chromaffin and pigment cells, and ganglion cells of the peripheral nervous system. She went on show that the chick–quail chimaeras could be useful for studying epithelial–mesenchymal interactions during lung development, and the differentiation of glycogen-producing hepatic cells during liver development.

Fate-mapping techniques have continued to evolve, increasing in their sensitivity, level of resolution and ability to image in real time. Le Douarin could only trace the fate of groups of cells, but modern investigators can mark single cells using retroviruses, fluorescent dyes or genetic markers, and can combine fate mapping with ablation of genes and cells. And now, thanks to the development of a new generation of markers and microscopes, even fate mapping's holy grail — observing the fate of individual cells in real time in living organisms — has become reality.