Embryology - Biology 104, Spring 2006 - Albert Harris and Corey Johnson   OUTLINE OF FIFTH LECTURE: Jan 25, 2006, by Corey Johnson Historical background of embryological thought and study Note: Memorization of dates is not required. That's just for your reference. If you're a history buff you might notice how some of these ideas fit into (and were influenced by) the fabric of contemporary thought, but that's well beyond the scope of this course. Historical overview (more will be covered as we study various aspects of development) Embryology went through several stages in its own 'ontogeny.' Latter steps in the historical progression of the field do not necessarily displace the former. There're still elements of each of these in embryology today. 1. Speculative 2. Descriptive - Comparative 3. Experimental - Chemical/biochemical - Molecular/genetic The anatomical approach, called descriptive embryology or developmental anatomy, began in antiquity. The earliest written description extends back to at least to India in 1600 BC. There are some written descriptions of the development of the human from fertilization through the first month - though not entirely accurate. How are the parts of an embryo formed (preformed or newly formed each time)? Aristotle (4th centrury BC) - speculated as to how an embryo forms an adult: Epigenesis - de novo production of structures, favored be Aristotle Preformation - unraveling of pre-existing structures, was and remained prevalent - Dark ages - black hole of embryology Ovists vs. spermists - The spermists believed that the sperm contained the ingredients for life and the egg merely provides suitable soil. The ovists believed the opposite. -observation of parthenogenesis swayed in favor of the ovists Malpighi (1st to observe capillaries) - showed very detailed drawings of chick development - remained a preformationist even though he couldn't see the miniature chicken parts under the microscope. This shows how one's paradigm can overshadow experimental evidence. The biology never lies; interpretations may vary. Caspar Friedrich Wolff (1759) - like Malpighi showed clearly the development of the chick convincingly demonstrating epigeneis Schleiden and Schwann (1839) "Cell Theory" preformationism dies, as it eventually becomes acknowledged that there are limits to the size of components of organisms How can development be understood in light of evolution? Darwin's "Origin of Species" (1859) stimulated the study of comparative embryology Ernst Haeckel was a powerful force in uniting the study of embryology and evolutionary biology Believed that the lineage of life on earth was linear rather than branched (more like a stick than a tree) The biogenic law; Recapitulation theory. "ontogeny recapitulates phylogeny" Haeckel suggested that the embryology of a species involves successive stages of adult ancestors. So human development has a fish stage, an amphibian stage, etc. Haeckel believed that the cause of embryonic development was its phylogeny. As you might notice, this is a lot like Lamarckian evolution: In recapitulation, ancestral lineage cause development; in Lamarckian evolution, the acquired characteristics of ancestors cause the morphological characteristics of offspring (giraffe's neck got longer because members of its lineage stretched them... a weightlifter's child will have huge muscles... etc.). Haeckel, for all the flack that he received(s) from anti-Darwinists, never promoted Darwinian evolution. He was in fact, a Lamarckian. The mechanism of change across different animal phyla was the terminal addition of novel structures or condensation, the compression of adult stages of ancestors. So it may not look like a mammal has a reptile stage, but that stage has been compressed so you can't see it. These aren't the droids you're looking for (ignore if star wars-impaired). Haeckel's career seemed to be characterized by a lot of hand-waving and conjecture. He was forever on the "dinner-circuit" touting his ideas... he was publicly popular, but not nearly so revered by his scientific peers. biography of Haeckel Karl Ernst von Baer disagreed with Haeckel's assertions Embryonic form, not adult) of ancestors may be "recapitulated"-in some sense Species-specific features appear later, general features appear earlier specificity emerges from generality Thought of evolution as a branching tree, which Darwin found intriguing biography of von Baer More recently a paper has disputed the idea of a phylotypic stage (as shown in Haeckel's drawings and a concept widely assumed to be true today) - It is suggested by the authors that a true phylotypic stage does not exist. No one can agree what stage it actually is because species are a lot more variable than many admit. link to paper You may wish to peruse the pictures. The introduction will explain how people tend to think about the phylotypic stage. Also see the detailed images from lecture of Haeckel's drawings and the paper's updated version. What does this tell us? Rule #1: embryologists generalize too much. Are the Xenopus frog, chicken, and mouse typical of their respective phylogenetic classifications? Are they even typical of vertebrates? Do vertebrates have a typical development among them? We tend to point out similarities among groups but we have no idea if these similarities are consistent across all vertebrates or if the similarities are biologically relevant at all. The above article suggests that we've been too willing to accept similarities and ignore differences. Maybe? Experimental biology - what governs embryonic development? Vitalism - position that a vital force is responsible for instructing embryonic development Mechanism - position that natural laws can explain development Wilhelm Roux (1890s). Roux was the father of field of developmental mechanics, the manipulation of and intervention in embryonic development... though he (like Darwin) didn't live in a vacuum. Wilhelm His laid the groundwork for the field in his criticism of Haeckels lack of logical thought (history can't cause a frog to form a gastrula... a "machine" was at work within the embryo. Roux: - Postulated that complex embryonic development could be understood as the summation of smaller, simpler processes subject to natural laws - At the 2-cell stage he took a hot needle and destroyed one of two frog blastomeres. To his satisfaction, the remaining cell continued to divide and gave rise to half a frog embryo - half of the frog-machine had been destroyed and the remaining machine carried out its operations as normal. Hans Driesch (1890s) repeated the experiment with 2 or 4-cell sea urchin embryos. When he separated them, 2 or 4 (smaller) sea urchin larvae developed. - Driesch wondered how a machine could be divided and yet remain what it was. He surmised that a machine that kept dividing would become so small as to be useless - He abandoned the mechanist cause and became a vitalist, suggesting that a vital force, "entelechy," was the explanation for developmental processes. The causal mechanism of development is a 'force' unexplainable by natural laws... there is no machine inside the cells that is responsible for making changes happen. More later on these guys! Really, Roux's experiment would have achieved similar results to Driesch's if he had completely separated the blastomeres. Driesch had demonstrated regulative development which we'll discuss Friday... see regulation (first lecture). But then... Discovery of DNA - it became clear that the machine was encoded in a self-replicating blueprint. It duplicated before it divided. In the 1900s it became common to use chemical and eventually biochemical means to manipulate and understand development. The application of cellular and molecular techniques to embryology characterizes Developmental Biology. DB emphasizes molecular mechanisms of development is nowadays almost synonymous with embryology but includes the study of the simplest of multicellular organisms to man. It encompasses everything from gametogenesis, to fertilization, to embryogenesis, to development in childhood, and to aging and disease.

 

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