Embryology   Biology 441   Vertebrate Embryology, Spring 2017   Albert Harris   April 28, 2017 Final Lecture

Concluding Thoughts Although these topics were covered quickly in the last part of the lecture, you should read the notes carefully and think about the questions raised. Remember that "Star Wars" scrolling screen on the first day of classes? Let's ask ourselves if what it said was true. And also ask what surprised you; What facts could have easily been otherwise? I) Subdivision into differentiated cell types could have occurred all at once, instead of sequentially. All 250 cell types might have differentiated simultaneously. Instead of branching like a railway, with sub-branches, and sub-sub branches. e.g. no germ layers, no subdivisions of germ layers, no somites Cells here, become heart; cells there, become muscle. II) Different branching pathways could be used by different kinds of animals. For example, fish could have used a different branching patterns than salamanders, or frogs, reptiles, mammals birds. At least insects could be different. What logical necessity is there for skin and nerves to share the same subdivision; or for heart and uterus, etc. and what's a connection between those cell types that develop from neural crest: sensory nerve ganglia, post-ganglionic autonomic ganglia, pigmented mesenchymal cells, Schwann cells, skeleton of the face? Do these have something special in common? For example, it was seriously proposed that this connection is biochemical synthesis of melanin and synthesis of adrenaline, which are biochemically related. III) Notice that fundamentally different extra-embryonic membranes occur in teleost fish, versus, reptiles birds and mammals. The enveloping layer is not homologous to the chorion or amnion; The yolk syncytial layer is not homologous to the yolk sac. But these do serve similar functions. IV) Notice superficial difference between invagination versus forming solid rod and cavitating. Not just difference between teleost fish versus mammals and amphibia, but also difference between anterior and posterior of bird neural tube. From the point of view of cell motility, invagination and cavitation are much less that our intuition. V) Locations of eye formation of vertebrates versus eyes of insects are controlled by very similar transcription factors. (Which was unexpected based on the optics of camera eyes being so different from compound eyes of arthropods) Likewise, anticipate different transcription factors wnt, distal-less, hox genes So little evolutionary change in transcription factors, versus such big change in what organs these proteins control. The major families of transcription factors, discovered in the genetic screens in Drosophila, also control development in vertebrates. VI) Active cell rearrangement & migration creates many/most anatomical patterns. (Unlike higher plants, where each cell type differentiates at the same location where it will remain during life). That includes plant gametes; plants don't have primordial germ cells. VII) Regeneration of salamander limbs is by rearrangement and reshaping of skeletal and muscle cells (also skin, pigment nerves, blood vessels). (Not by "archeocytes" or undifferentiated stem cells; and NOT by real dedifferentiation and switching between differentiated cell types. (which are what most embryologists had assumed). Notice the parallel history of H. V. Wilson trying to explain cell sorting by switching from one cell type to another, based on location; or if not that, then archeocytes. Notice that many theories of regeneration and embryonic development assume switching from one differentiated cell type to another, according to location. Turing's reaction-diffusion system and Wolpert's "French Flag Theory" both assume that anatomical patterns are created by spatial control of differentiation, and ignore rearrangements of cells (like H.V. Wilson did). VIII) A consensus is now developing among researchers on embryonic stem cells that mechanical forces, and resistance to elastic forces are normal parts of the system for controlling cell differentiation Richard Gordon, Antone Jacobson first to propose this, in the 1970s. James Murray, George Oster & I invented a mathematical system for compresssion of mesenchyme to stimulate cartilage differentiation; analogous to Turing, but used forces & cell population densities of crawling cells instead of "A" & "B" etc and differences of diffusion rates. Compression -> Skeleton (which resists compression) Tension -> tendons, ligaments, muscles (which resist tension) To emphasize this idea: imagine if we had a cell type that resisted twisting; then differentiation of twist-resistant cells would be stimulated by twisting. Ossification stimulated by stresses on bone: strengthening of muscles in response to exercise, these types of phenomena occur in embryonic structure development. Notice the analogy to homeostasis: getting hot induces sweating; getting cold induces shivering. (Analogy to plants sprouting when pruning induces sprouting.) The newest breakthrough is this direction is that stem cell differentiation can be changed by differences in elasticity of rubber or gel substrata. IX) What causes the near-irreversibility of cell differentiation? Changes in chromatin itself? Self-perpetuation processes, like cytosine methylation? Now learning how to make stem cells, analogous to "rooting" plant cuttings. X) Dolly the sheep, in contrast, was not a surprise to embryologists. Already, for decades, since 1952, researchers had been producing animals by nuclear transplantation. (Briggs and King, 1952), Gurdon in the 1970s. All the cells have all the genes. A few exceptions are the surprises. " Imprinting " abnormal development occurs if two male pronuclei, or two female pronuclei are transplanted into the same oocyte. XI) Surprisingly little use of growth to create anatomical shapes. Mitosis doesn't push. The real physical forces inside developing embryos are water pressure, osmotic pressure, electro-osmotic XII) Scalars, vectors, tensors, & other engineering concepts are needed by embryologists, but are seldom mentioned in any biology textbooks. XIII) Symmetry concepts are also very important for explaining patterns: spherical symmetry, axial symmetry, reflection, rotation, displacement, dilation (scaling) "...if there is something you can do to it, such that it looks the same afterward as it did before." Weyl's definition of curvature. Using this definition allows one to invent many new kinds and combinations of symmetry. For example, certain embryology textbooks might be regarded as being symmetrical with respect to transcription factors. Every 100 pages, replace wnt with notch, etc. Curie's principle: Symmetries of causes produce the same combinations of symmetries in the results. Why the spherical shapes of most oocytes & early stages of development? Are they recapitulating spherical early life forms? Or do their internal pressures and tensions have spherical symmetry? Also notice that reduced symmetry ("symmetry breaking") gets produced by random fluctuation --> reaction diffusion Point of sperm entry direction of gravity rotational symmetry of flagellar basal body XIV) P = CT + ct If genes can control P, T and t then they control C and c P = pressure difference; C = curvature in one direction, c is curvature in the direction perpendicular to the direction of C. T and t are tensions in the directions of C and c which means controlling shape. D'Arcy Thompson mistakenly used a similar equation: P = T (1/R = 1/r) "R" and '"r" are radii of curvature: R=1/C r=1/c Thompson treats tension as if it were a scalar, not capable of varying with direction. That greatly decreased the number of geometric shapes he could explain in terms of the combinations of forces that cause them. a quote from D'Arcy Thompson with my illustration Differential geometry is the appropriate math for explaining embryology, because it defines long-range shapes in terms of local properties, especially rates of angular changes of tangent directions, per unit distance along a surface (Which is the definition of what?) XV): Primordial germ cells: Why don't oocytes and sperm differentiate from the cells of the germinal ridges? Is it to protect them from inductive signals that would stimulate them to differentiate irreversibly? By what experiments could you prove or disprove that explanation? XVI): Bioassays: How almost every important biological chemical got discovered But notice some "blind-spots" of bioassays: Lack of specificity. Multiple Alternative Signals, any one of which can give a positive result. Think of other reasons why a bioassay might either fail to identify an actual chemical, or give a false positive, in the sense of seeming to discover some substance that doesn't actually exist. XVII): Genetic screens: Becoming the more powerful "discovery machinery" Find mutant animals that are abnormal in some particular properties. Identify all the different genes that affect that process. Which of these genes code for enzymes, which code for transcription factors, signaling proteins, receptor proteins, structural proteins, motor proteins. If auxin and serotonin hadn't been discovered by bioassays, what sort of genetic screens would have discovered the same set of phenomena? (Plus the receptor proteins, the transport method, how roots are stimulated, etc. A major "blind-spot" of genetic screens, is lethal mutations. If even the smallest change in a protein causes the animal to die, then will you be able to study that protein genetically? But temperature sensitive mutation can often get around that problem. Consider the many advantages of using one of the model organisms: Flies, mice, zebra-fish etc. XVIII) Regeneration of legs: Why only salamanders? Also frog legs before these are completed. Also human finger-tips, especially in childhood. Nobody understands why. Discovering the reasons will be a big step toward discovering how to stimulate regeneration of human fingers, hands, legs or arms. Conversely, can we hope to control regeneration until we understand these fundamental aspects of its mechanism. Repair of scratches in children's skin is very rapid. Someday you will go blackberry-picking with some young children; notice that both you and they get scratches, and that three days later your scratches are still healing, but the children's scratches have completely healed in 1 or 2 days. XIX) Oncogenes: Interesting that there are so few, only around a hundred. Also interesting that cancer results more from over-activity than under-activity Chain of signals causing growth      Sis, receptor proteins, ras, src, myc, bcl 2 XX) Can knowledge of oncogenes be used to invent better chemotherapy? Maybe a chemical that gets converted into a poison when it is phosphorylated? How to kill just those cells whose Ras proteins can't convert ras-GTP to ras-GDP? The latter could cure a fifth of cancers; the answer will seem easy, once somebody figures out how to do it. The goal is to kill the abnormal cells, not to inhibit their abnormal properties. XXI) How do current chemotherapy drugs selectively kill cancer cells? Why should faster-growing cells be selectively killed by drugs that block some stage of growth (e.g. DNA synthesis, mitotic cell division)? True explanation that loss of growth control at check-points allows damaged chromosomes to pass check-points, mitosis, DNA synthesis. What kills is not that they are fast, and not that the drugs inhibit their growth. (but the opposite) because even though damaged, cancer cells don't stop growing. XXII) Which are better? * Drugs that fix abnormalities of cancer cells, & make them more nearly normal. * Drugs that selectively kill those cells with cancerous abnormalities. (Getting rid of the cancer cells) Gleevec...? Inhibitors of angiogenesis...? XXIII) Why have the later stages of drug synthesis and testing been handed over to pharmaceutical companies? Why doesn't insulin cost a thousand dollars per day? Why doesn't snake venom anti-toxin cost a year's salary? Why don't vaccines cost as much as a house? Could such prices have been charged? Why weren't they? Did people just not imagine the possibility of charging such high prices? Can we predict that, from now on, ALL major new medical treatments will cost thousand$? (Like the diphosphonate anti-osteoporosis drugs) Would such large financial incentives have caused such treatments to be discovered sooner, or better versions to be developed? Have discoveries been missed, of these levels of importance, for lack of large prices. For example, would cures already have been found for cancer and lupus, if even larger profits could be made; or if there were more research funding? Who discovers which part The existence of auxin? Darwin. The chemical structure of auxin? Dutch Professors. Synthesis of auxin analogs, like 2-4 D? US Professors. How to synthesize tons of 2-4 D? Chemical companies. Who discovered uracil ribo-phosphate structure and methods of synthesis? Who discovered that chemical analogs sometimes cure diseases? Who discovered how to legalize patenting of chemicals, and for special uses? Who invented the idea of persuading reporters that India is breaking patent laws. because they continue to apply the traditional laws against patenting chemicals? --------------------- Non-Hogkins lymphoma has been cured by isolating the antibody binding sites of lymphoma B-cells, injecting these into young mice, and isolating monoclonal antibodies specific for killing that particular person's cancerous lymphocytes. "But this method was not developed because treatments would be prohibitively expensive" Translation? A treatment that actually costs a hundred thousand dollars, isn't worth it; because the sale price could only be ten times the cost of manufacture. (not 50 times) But a treatment for which you can charge a hundred thousand dollars, but only actually costs a few hundred dollars, is profitable enough to be developed and sold. If the new thousand dollar per pill hepatitis drug actually cost five hundred dollars per pill to make, (half the sale price) then would it be "prohibitively expensive"? Is this going to be the "New Normal" of medical research? Thousand dollar pills! Cures for cancer abandoned because of not being profitable enough. Pasteur, Fleming. Waksman, and Salk could each have been multi-billionaires! Each richer than Gates; Why didn't they? Why doesn't the news discuss reasons? Revised from the 1960s folk-song: "All My Trials, Lord, Soon Be Over" If living was a thing that money could buy, Well, you know, the rich would get richer, and the un-insured would die... The end.

 

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