Morgan detested it because it postulated an alternative mode of inheritance (as in planaria dividing). In that way, polarity could be generated. However, the cells responded to the inducer in a graded manner depending upon the inducer concentration. Child ( 12, 13 ), created the idea of a "gradient field" Here, a morphogenetic field existed that depended upon a system of induction (see 14 ). Gavin de Beer and Julian Huxley ( 11 ), along with C.M. Gustafson ( 10 ) represented this graphically as two intersecting gradients. ![]() Their experiments indicated that a gradient of vegetal substance was found throughout the embryo, concentrated in the vegetal-most micromeres, and there was conversely a gradient of animalizing factor in the otherdirection. The threshold gradient idea was brought about primarily by Rünnstrom ( 8 ) and Hörstadius ( 9 ) in their studies of sea urchins. His illustrations, however, show a graded character from the vegetal to the animal pole, and this makes it appear that there are threshold-related gradients in these embryos. His is more like a step-gradient, wherein the cell membrane separating two cells also separates two concentrations of the same substance. However, as Sander ( 7 ) has made explicit, Boveri's concept of a gradient is not the one that we have today. The introduction of gradients into embryology is often credited to Boveri ( 6 ), who did introduce the term " Gefälle" (gradient) into his work. There are several reasons for this, one of which was that they were ridiculed by a person who had been one of the leading investigators of such gradients, Thomas Hunt Morgan ( 5 ). Until they were resurrected in the laboratory of Christiane Nüsslein-Volhard, "gradients" had been a "dirty word" for decades ( 4 ). Here we have to digress briefly to discuss the concept of gradients in embryology. Two major sets of models were generated to explain these results. The blastopore lips from the early gastrula stages induced the most archencephalic (anterior) structures, whereas the blastopore lips of later embryos caused the differentiation of the more posterior neural elements. To further study the organizer phenomenon, Holtfreter ( 3a ) made a blastopore "sandwich," enclosing the dorsal blastopore lip between slices of undifferentiated ectoderm. Early dorsal blastopore lips gave rise to head structures late dorsal blastopore lips induced new tails. This could also be seen by transplanting the tissue at the time when it comprised the dorsal blastopore lip ( 3 ). When transplanted into the blastocoel of early gastrulae, the anteriormost mesoderm induced cement glands and eyes, posterior-most mesoderm induced tails, and the intermediate mesoderm induced the appropriate intermediate structures. Otto Mangold ( 2 ) showed that the regional position of the blastopore lip-derived dorsal mesoderm determined the anterior-posterior specification of the neural tube. ![]() Thus, some other factors are needed (Waddington's "individuators") to specify the regionality of forebrain, midbrain, hindbrain, and spinal cord. When induced by non-specific evocators, anterior neural tissue usually forms (Nieuwkoop, 1 ). A Selective History of Induction IV Regional specificity
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