Cryptic responses to tissue manipulations in avian embryos

Experimental embryology performed on avian embryos combines tissue manipula tions and cell-labeling methods with increasing opportunities and demands f or critical assays of the results. These approaches continue to reveal unex pected complexities in the normal patterns of cell movement and tissue orig ins, documentation of which is critical to unraveling the intricacies of ce ll and tissue interactions during embryogenesis. Viktor Hamburger's many pi oneering contributions helped launch and promote the philosophical as well as technical elements of avian experimental embryology. Furthermore, his sc holarship and profoundly positive presence influenced not just those of us fortunate to have trained with him, but several generations of developmenta l biologists. The first part of this article presents examples of the oppor tunities and rewards that have occurred due to his influences. Surgical man ipulation of avian embryonic tissues always introduces a greater number of variables than the experimenter can control for or, often, readily identify . We present the results of dorsal and ventral lesions of hindbrain segment s, which include defects in structures within, beside, and also at a consid erable distance from the site of lesion. Extramedullary loops of longitudin al tract axons exit and re-enter the neural tube, and intra-medullary proli feration of blood vessels is expanded. Peripherally, the coalescence of neu ral crest- and placode-derived neuroblasts is disrupted. As expected, motor neurons and their projections close to the sites of lesion are compromised . However, an unexpected finding is that the normal projections of cranial nerves located distant to the lesion site were also disrupted. Following br ainstem lesions in the region of rhombomeres 3, 4 or 5, trigeminal or oculo motor axons penetrated the lateral rectus muscle. Surprisingly, the ability of VIth nerve axons to reach the lateral rectus muscle was not destroyed i n most cases, even though the terrain through which they needed to pass was disrupted. These axons typically followed a more Ventral course than norma l, and usually, the axons emerging from individual roots failed to fascicul ate into a common VIth nerve, which suggests that each rootlet contains pat hfinder-competent axons. The lesson from these lesions is that surgical int ervention in avian embryos may have substantial effects upon tissues within , adjacent to, and distant to those that are being manipulated. (C) 2001 IS DN. Published by Elsevier Science Ltd. All rights reserved.