AMER. ZOOL., 26:553-554 (1986) Cell Recognition During Neuronal Development in Insect Embryos' COREY S. GOODMAN, MICHAEL J. BASTIANI, JONATHAN A. RAPER, AND JOHN B. THOMAS Department of Biological Sciences, Stanford University, Stanford, California 94305 During development, neurons find and interconnect with other neurons in a remarkably precise way. The unfolding of neuronal specificity undoubtedly involves a series of highly specific recognition events between individual neurons. What cellular interactions underlie this specific neuronal recognition? What molecules underlie these specific cellular interactions? To answer these questions, we began several years ago to study cell recognition during neuronal development in the grasshopper embryo. Here we review what we have learned from cellular and immunological studies using the grasshopper embryo. Cell recognition at early stages of neuronal development is mediated largely by specific filopodial con- tacts, and leads to the stereotyped patterns of selective fasciculation. Our results sug- gest that such recognition is likely to involve the temporal and spatial expression of many different molecules, and our monoclonal antibody studies reveal cell surface anti- gens whose distribution correlates with these predictions. We end the paper by reviewing our recent studies on the same cellular interactions in the Drosophila embryo, which leads to a consideration of the future prospects for a molecular genetic solution to this problem using Drosophila. Specific cell recognition occurs through- out much of neuronal development, from cell migration, to the outgrowth of axonal processes, and the formation of specific synaptic connections. The initial question is: during which period of development, and in which species, can these events best be analyzed and manipulated at the cellular and molecular level? Insects have relatively simple nervous systems, which, in addition 1 From the Symposium on The Cell Surface in Devel- opment and Cancer presented at the Annual Meeting of the American Society of Zoologists, 27-30 Decem- ber 1983, at Philadelphia, Pennsylvania. to their complex brain (10 5 neurons), includes a chain of relatively simple seg- mental ganglia, each containing about 1000 pairs of neurons. Unfortunately, if we wait until these neurons have formed many of their specific synaptic connections late in embryonic development, even this "sim- ple" nervous system appears hopelessly complex. However, if we look early enough, when only a handful of neurons have sent out processes, then the system is indeed simple and accessible enough to allow us to ask how individual neurons distinguish their appropriate targets amongst a limited population of embryonic neurons. At these early stages of neuronal devel- opment, cell recognition occurs most dra- matically at the tips of growing axons, called growth cones, and at their finger-like extensions, called filopodia. Growth cones radiate many filopodia (approx. 0.1 ixm in diameter, up to 50 fim in length) which transiently explore their environment. Many of these filopodia contact other cell surfaces. To some of these surfaces they strongly adhere, and to others their adhe- sion is much weaker. If adhesion is weak, during the contractile cycle, the filopo- dium is retracted; if however, its adhesion is strong, then tension in that direction is increased during the contractile cycle and the leading tip of the growth cone advances towards the point of attachment (Bray, 1982; Letourneau, 1982). Our results in the grasshopper embryo suggest that cell recognition at these early stages of neuronal development is mediated largely by specific filopodial interactions. The high affinity that growth cones and their filopodia show for particular neu- ronal surfaces gives rise to the stereotyped patterns of selective fasciculation in which growth cones, confronted with a scaffold of axon fascicles, choose particular axon bundles along which to extend. 553 Downloaded from https://academic.oup.com/icb/article-abstract/26/3/553/264099 by guest on 16 April 2019