Give the timeline of development biologist ,and their contributions in the field of development biology

Krishan k jakhad

Hans Spemann, Nobel laureate of 1935, is one of the most remarkable biologists of the 20th century and the founder of modern experimental embryology (Developmental biology – or ‘embryology’ , formally formed an independent discipline in the 1970s). Developmental biology is the study of the process by which animals and plants grow and develop. Much of developmental biology research in recent decades has focused on the use of a small number of model organisms. Observations of embryos and embryonic stages were made and recorded in antiquity (e.g., Aristotle, fourth century BC) and with increasing attention in recent centuries (e.g., Malphigi in the 1600s, Wolff in the 1700s, and von Baer in the early 1800s). However, it was only in the late nineteenth century that scientists pursued a detailed description of the embryonic stages of a variety of vertebrates and invertebrates, aided by the then-recent improvements in light microscopy and in staining methods and stimulated by Darwin's proposals that the study of ontogeny (i.e., the animal's embryonic development) holds clues to phylogeny (i.e., its evolutionary origin). Among the highlights during the period of 1880-1940 were the detailed anatomical descriptions of developmental stages of embryos, including the first atlas of human embryos, reconstructed from microscopic sections, published by W. His, Sr., in 1880-1885. In vertebrate embryology, these descriptions revealed the organogenesis of the heart, kidney, limbs, central nervous system (CNS), and eyes. Developmental-fate mapping studies revealed the embryonic sites of the origin of cells of the organs and the rearrangements of groups of cells in morphogenesis. The stages of development were found to include, in reverse order, cytodifferentiation, organogenesis, morphogenesis (gastrulation and neurulation), rapid cleavage, fertilization, and gametogenesis. By the 1940s, anatomical descriptions of the embryos of related animals were integrated into coherent evolutionary schemes, taught in comparative embryology classes, revealing, for example, the modification of the gill slits of jawless fish to the jaw of jawed fish and further modification to the middle ear of mammals. Also, by this time, Haeckel's oversimplified scheme had been abandoned, namely, that ontogeny merely recapitulates phylogeny. Experimental embryology also began in the late 1800s. In experimental studies, which mostly involved techniques of cell and tissue transplantation and removal, the central role of cytoplasmic localizations and cell-lineage-restricted developmental fates was recognized in the development of certain invertebrates by the early 1900s. In vertebrate development, the importance of inductions (also called tissue interactions) was recognized in the 1920s, following the stunning organizer transplantation experiments by Spemann and Mangold (1924) on newt embryos. By the 1950s, inductions had been found in every stage and place in the vertebrate embryo, for example, in all the kinds of organogenesis. Vertebrate development, including that of mammals, had become comprehensible as a branching succession of inductive interactions among neighboring members of an increasingly large number of different cell groups of the embryo. Developmental mechanisms, as understood even in the 1970s, were descriptions of the movements and interactions of cells or groups of cells. They were cellular- or tissue-level mechanisms. The all-important “inducers” were materials of unknown composition released by one cell group and received by another group. Consequently, the recipient cells took a path of development different from the one that would have been taken if they were unexposed. The progression or momentum of development also was recognized: that the individual events of interactions and responses are time-critical, and that certain subsequent aspects of development never occur if one event is prevented. Molecular mechanisms, however, were not understood at that time. Embryologists encountered the limits of the field in the 1940-1970 period, as they tried to discover the chemical nature of inducers and the responses of cells to them. The basic information and methods of biochemistry, molecular biology, cell biology, and genetics were not yet available to analyze cell-cell signaling and transcriptional regulation in embryos. In light of discouraging results, some embryologists considered that the organizer concept was faulty and that inducers were an experimental artifact (see later discussion for recent successes in understanding inductions). Although Morgan and other early geneticists had proposed that inducers and cytoplasmic localizations elicit specific gene expression and that development was in large part a problem of ever-changing patterns of gene expression (Morgan 1934), the means were not at hand to pursue those insights. Roux, Spemann, and Harrison had outlined plausible lines of inquiry into determination and morphogenesis in the early part of the twentieth century; however, the means were also not available to pursue those questions at that time. To many scientists in the 1940-1970 period, the study of development seemed messy and intractable. Researchers turned to more informative subjects such as the new molecular genetics of bacteria and phages (viruses that infect bacteria). From those inquiries came new insights in the 1950-1965 period on the nature of the gene and the code and the processes of replication, transcription, translation, enzyme induction, and enzyme repression. For example, it was only in 1961 that Monod and Jacob described gene regulation in bacteria in terms of promoters, operators, and repressor proteins (Monod and Jacob 1961). Those authors immediately saw the relevance to animal development. All of their insights made possible the invention of techniques for gene isolation and amplification, for in vitro expression of genes, for genome analysis, and, thereafter, for the new developmental biology. With so little molecular information about developmental processes, there was scarcely any understanding of the action of developmental toxicants. For example, Wilson (1973) in his book Environment and Birth Defects could only raise the following possibilities for connections between inductions and developmental defects: Observations of embryos and embryonic stages were made and recorded in antiquity (e.g., Aristotle, fourth century BC) and with increasing attention in recent centuries (e.g., Malphigi in the 1600s, Wolff in the 1700s, and von Baer in the early 1800s). However, it was only in the late nineteenth century that scientists pursued a detailed description of the embryonic stages of a variety of vertebrates and invertebrates, aided by the then-recent improvements in light microscopy and in staining methods and stimulated by Darwin's proposals that the study of ontogeny (i.e., the animal's embryonic development) holds clues to phylogeny (i.e., its evolutionary origin). Among the highlights during the period of 1880-1940 were the detailed anatomical descriptions of developmental stages of embryos, including the first atlas of human embryos, reconstructed from microscopic sections, published by W. His, Sr., in 1880-1885. In vertebrate embryology, these descriptions revealed the organogenesis of the heart, kidney, limbs, central nervous system (CNS), and eyes. Developmental-fate mapping studies revealed the embryonic sites of the origin of cells of the organs and the rearrangements of groups of cells in morphogenesis. The stages of development were found to include, in reverse order, cytodifferentiation, organogenesis, morphogenesis (gastrulation and neurulation), rapid cleavage, fertilization, and gametogenesis. By the 1940s, anatomical descriptions of the embryos of related animals were integrated into coherent evolutionary schemes, taught in comparative embryology classes, revealing, for example, the modification of the gill slits of jawless fish to the jaw of jawed fish and further modification to the middle ear of mammals. Also, by this time, Haeckel's oversimplified scheme had been abandoned, namely, that ontogeny merely recapitulates phylogeny. Experimental embryology also began in the late 1800s. In experimental studies, which mostly involved techniques of cell and tissue transplantation and removal, the central role of cytoplasmic localizations and cell-lineage-restricted developmental fates was recognized in the development of certain invertebrates by the early 1900s. In vertebrate development, the importance of inductions (also called tissue interactions) was recognized in the 1920s, following the stunning organizer transplantation experiments by Spemann and Mangold (1924) on newt embryos. By the 1950s, inductions had been found in every stage and place in the vertebrate embryo, for example, in all the kinds of organogenesis. Vertebrate development, including that of mammals, had become comprehensible as a branching succession of inductive interactions among neighboring members of an increasingly large number of different cell groups of the embryo. Developmental mechanisms, as understood even in the 1970s, were descriptions of the movements and interactions of cells or groups of cells. They were cellular- or tissue-level mechanisms. The all-important “inducers” were materials of unknown composition released by one cell group and received by another group. Consequently, the recipient cells took a path of development different from the one that would have been taken if they were unexposed. The progression or momentum of development also was recognized: that the individual events of interactions and responses are time-critical, and that certain subsequent aspects of development never occur if one event is prevented. Molecular mechanisms, however, were not understood at that time. Embryologists encountered the limits of the field in the 1940-1970 period, as they tried to discover the chemical nature of inducers and the responses of cells to them. The basic information and methods of biochemistry, molecular biology, cell biology, and genetics were not yet available to analyze cell-cell signaling and transcriptional regulation in embryos. In light of discouraging results, some embryologists considered that the organizer concept was faulty and that inducers were an experimental artifact (see later discussion for recent successes in understanding inductions). Although Morgan and other early geneticists had proposed that inducers and cytoplasmic localizations elicit specific gene expression and that development was in large part a problem of ever-changing patterns of gene expression (Morgan 1934), the means were not at hand to pursue those insights. Roux, Spemann, and Harrison had outlined plausible lines of inquiry into determination and morphogenesis in the early part of the twentieth century; however, the means were also not available to pursue those questions at that time. To many scientists in the 1940-1970 period, the study of development seemed messy and intractable. Researchers turned to more informative subjects such as the new molecular genetics of bacteria and phages (viruses that infect bacteria). From those inquiries came new insights in the 1950-1965 period on the nature of the gene and the code and the processes of replication, transcription, translation, enzyme induction, and enzyme repression. For example, it was only in 1961 that Monod and Jacob described gene regulation in bacteria in terms of promoters, operators, and repressor proteins (Monod and Jacob 1961). Those authors immediately saw the relevance to animal development. All of their insights made possible the invention of techniques for gene isolation and amplification, for in vitro expression of genes, for genome analysis, and, thereafter, for the new developmental biology. With so little molecular information about developmental processes, there was scarcely any understanding of the action of developmental toxicants. For example, Wilson (1973) in his book Environment and Birth Defects could only raise the following possibilities for connections between inductions and developmental defects: The free-living nematode Caenorhabditis elegans emerged as an important model system in the 1970s, as the result of pioneering work on its genetics by S. Brenner (1974). Spemann and Mangold demonstrated that the transplantation of a small amount of tissue from the dorsal side of an early embryo of Triturus (tailed amphibian) could induce host tissue to form a secondary embryonic axis, including neural tissue. Spemann concluded that the behavior of an implanted region (the dorsal lip of the blastopore) reflects its normal function in inducing the primary axis and designated it as the embryonic organizer.