The Molecular Metamorphosis of Experimental Embryology (original) (raw)
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I attempt to characterize the relationship of classical experimental embryology (CEE) and molecular developmental biology and compare it to the much-discussed case of classical genetics. I first show that CEE had some causal knowledge and hence was able to answer specific why?-questions. A paradigm was provided by the case of eye induction, perhaps CEE’s greatest success. The case of the famous Spemann organizer is more difficult. I argue that before the advent of molecular biology, knowledge of its causal role in development was very limited. As a result, there was no functional definition of the concept of organizer. I argue that, like the classical gene concept, it is best view as an operational concept. This means that an account of reduction such as Kim’s functional reduction cannot work in these cases. Nonetheless, again like in the classical gene case, the operational concepts of CEE played an important heuristic role in the discovery of molecules involved in morphogenesis and cell differentiation. This was made possible by what I call inter-level investigative practices. These are practices that combine experimental manipulations from two (or more) different levels. I conclude that the two sciences are more closely related via their experimental practices than by any inter-level explanatory relations.
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M c L a r e n , A. (1976a). Mammalian Chimaeras. Cambridge University Press. M c L a r e n , A. (19766). Growth from fertilization to birth in the mouse. In Embryogenesis in Mammals: Ciba Fdn Symp. 40 (new series) (ed. K. Elliott & M. O'Connor), pp. 47-51. Amsterdam: Elsevier. M i n t z , B. (1962). Formation of genotypically mosaic mouse embryos. Aw. Zool. 2, 432 (Abstr. 310). M i n t z , B. (1965). Experimental genetic mosaicism in the mouse. In Preimplantation Stages of Pregnancy: Ciba Fdn Symp. (ed. G. E. W. Wolstenholme & M. O'Connor), pp. 194-207. We s t , J. D. (1 9 8 2). X chromosome expression during mouse embryogenesis. In Genetic Control of Gamete Production andFunction (ed. P. G. Crosignani & B. L. Rubin), pp. 4 9-9 1. New York: Academic Press. W e s t , J. D. (1984). Cell markers. In Chimaeras in Developmental Biology (ed. N. L e Douarin & A. McLaren), pp. 39-67. New York: Academic Press. W e s t , J. D ., P a p a i o a n n o u , V. E ., F r e l s , W. I., & C h a p m a n , V. M. (1978). Preferential expression of the maternally derived X chromosome in extraembryonic tissues of the mouse. In Genetic Mosaics and Chimeras in Mammals (ed. L. B. Russell), pp. 361-377. New York: Plenum Press. W h i t t e n , W. K. (1978). Combinatorial and computer analysis of random mosaics. In Genetic Mosaics and Chimeras in Mammals (ed. L. B. Russell), pp. 445-463. New York: Plenum Press. ZIMMERMAN, J. (1975). The initiation of melanogenesis in the chick retinal pigment epithelium.
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The word morphogenesis when used strictly should mean the molding of cells and tissues into definite shapes" C.H. Waddington, 1956. A central mission of developmental biology as a science is to understand how organisms develop from simple fertilized eggs into complex animals and plants of diverse shapes and elaborate internal architecture. This quote from Waddington coincides with the establishment of Developmental Biology as a journal. In celebration of the journal's 50 th anniversary we present the following series of review articles, which address the problem of morphogenesis. Macroscopic changes in tissue or organ structure result from coordinated changes in the arrangements and shapes of cells. To this end, the pre-molecular era, and the first two decades of this journal, were dominated by studies of descriptive and experimental embryology. The processes that are responsible for the development of animals and plants can be roughly divided into three main categories: cell communication, differentiation and morphogenesis. Morphogenesis is derived from the Greek words meaning the emergence (γενντση-gennisi) of shape (μορϕη-morphi). It is therefore only fitting that the study of morphogenesis dates back to an ancient Greek philosopher, Aristotle, who recognized that the egg of an animal had the "potential" to influence its final form. Over the last three decades, genetic analyses have elucidated the central signaling pathways directing cell communication and differentiation and shown them to be evolutionarily conserved. This acquisition of information can be viewed as a description of the parts. Indeed many key factors regulating processes like cell division, fate determination and differentiation are encoded by a relatively small number of conserved gene families. Together with an additional level of regulation, afforded by antagonists, activators, as well as posttranscriptional or posttranslational modifiers, a framework of genes and mechanisms controlling development have emerged. In this framework, a cell type results from the activity of multiple genes, and genetic pathways are viewed as parallel information pipelines that converge on the regulatory regions of specific genes. By contrast, elucidation of the molecular genetic mechanisms of morphogenesis has proven more challenging. Forward genetic approaches in D. melanogaster, C. elegans, plants and