A List of Thirteen Unsolved Problems of Embryology

Albert K Harris
Professor of Biology
UNC-Chapel Hill
Presented in the Embryo Physics Course, April 10, 2013


Problem Number One is embryonic regulation, discovered more than a century ago by Hans Driesch, and never yet solved. Number Two is the consistency and regularity of most anatomical shapes. Number Three is whether any example has been found in which any reaction-diffusion system, such as was proposed by Alan Turing, has ever been proven to control an actual anatomical pattern. Number Four is the molecular mechanism that controls cell differentiation, “turning on” blocks of unlinked genes so as to produce each of our bodies’ 250 differentiated cell types. The Fifth Problem is whether fibroblast traction really is the normal cause of tendon formation, and other cases of collagen fiber realignment and packing. The Sixth Problem is the control of Hox gene formation, and especially the colinearity problem (arrangement of Hox gene DNA on chromosomes in the same spatial order as these genes are transcribed along the anterior-posterior axis of the embryonic body). The Seventh Problem is the cause of somite formation, and how somites control locations of segmentation of sensory and motor nerves, vertebrae, ribs, and other segmental anatomy. The Eighth Problem is how bone mineral is deposited, and how this deposition changes in response to different physical loads. This includes the medical problem of osteoporosis, and also the demineralization of astronauts’ bones. The Ninth Problem is how to explain galvanotaxis of tissue cells, both in cause and function. The Tenth Problem is how to transmit shearing force tangentially through fluid mosaic membranes of cells. The Eleventh Problem is how salamanders can regenerate legs but people cannot. This includes the question whether regeneration is by geometric rearrangement of cells that do not change their differentiated states. The Twelfth Problem is the explanation and cure of autoimmunity, as in multiple sclerosis and lupus, and the Thirteenth Problem is why cancer chemotherapy drugs work as well as they do, considering cancer cells don’t really grow faster but the drugs are mostly designed to kill fast-growing cells. This list is preceded by discussion of a series of five common misconceptions that hold back progress, starting with the mistaken idea that growth itself contributes significantly to organ shape.



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