Monkey chimeras: what they are, and why they’re cool
Last week, the Oregon National Primate Research Center announced that it had successfully fostered the birth of three baby chimera monkeys–Roku, Hex, and Chimero. The accompanying paper was published online in advance for the January 20th issue of Cell. (See here.) Producing mouse chimeras has allowed scientists many insights into development, medicine, and cell biology–but it had not yet been produced in a primate. Although mice are useful little creatures, they cannot simulate every last detail of the human body. Chimeras would be even more useful for medical science if they were primates.
But just what are chimeras? And why has it taken so long to make a primate chimera?
In Greek mythology, the Chimera was a fire-breathing monster–part goat, part snake, and part lion–borne of a half-woman half-snake mother, Echidna, and a part-viper part-dragon father, Typhon. Nifty, right? But in science, a chimera is a single animal formed from fusing at least two separate zygotes or blastocysts. That means at least two sperm and two eggs have all genetically contributed to one, fully-functioning, chromosomally normal individual. It might just be me–but I find this even more compelling than any mythological lion-goat-snake concoction.
How is a chimera created in the laboratory? A classic chimeric mouse is created from two different cell lines bred in the lab. Let’s assume that one cell line makes mice with white fur, and the other cell line makes mice with black fur. A biologist can take embryonic stem (ES) cells from a pregnant white mouse, and mix them with the developing ES cells from a pregnant black mouse. It’s important that they’re stem cells from very early in development–these cells are pluripotent, or capable of dividing and differentiating into every type of cell that the organism might need. The fused cells sort themselves out and form one coherent blastocyst, which is then implanted into a foster mother.
As a result, a foster mother with white fur will give birth to mice that have a patchwork, or even stripes, of black and white fur. Voila! An individual mouse made from two different sets of parents.
Either way, the end result, once the beginnings of the chimera have been fused together, it will be implanted back into a mouse and allowed to grow. The end result? A stripey, black-and-white zebra-type chimera!
But, as the authors of the Cell paper describe, attempts to use this method with primates were unsuccessful. Pluripotent cells, while powerful, are unable to form any tissues in the placenta, which separates from the inner mass of pluripotent cells very early in development. The ultimately powerful cells, which can form the placenta and any tissues of the body, are dubbed totipotent.
The scientists at OHSU found that they could only create a successfully developing chimera when they merged several developing embryos together in very, very early development. We’re talking embryos that are 4-cells big. They have only divided twice since fertilization.
Their paper has many, many implications–but one of them is that primate development is critically different from mouse development. Cells begin to differentiate much earlier than we had assumed, given our data from the mouse, and this might mean that there are even greater developmental differences some weeks or months later. I find this idea enchanting. Learning all this sort of information in my Developmental Biology course, I felt a sort of kinship with all forms of life, knowing that from very, very humble beginnings arise incredibly complex cells, tissues, and organisms. But now scientists might be able to begin unraveling how the (debatably) more complex primates develop–although, of course, we encounter huge ethical questions with how to proceed.
Anyway, it will be many years before we hear anything else about primate chimeras, since this discovery has barely been made. But I’ll be keeping tabs on the exciting developments.
Featured image credit: Oregon Health and Science University
All images from Wikimedia Commons.
Works cited: Tachibana, M., Sparman, M., Ramsey, C., Ma, H., Lee, H., Penedo, M.C.T., Mitalipov, S. Generation of chimeric rhesus monkeys. Cell (2012), doi:10.1016/j.cell.2011.12.007.