![]() ![]() 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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