Chordotonal Organs of Insects

More documents

Recommendations

Info

L.H. Field & T. Matheson Advances in Insect Physiology 27 (1998) Page 168cases some larval neurons persist through the late pupal stage (Lakes and Pollack 1990).The formation of adult chordotonal organs is known only for the blowfly Phormia regina,where post-larval morphogenesis of leg chordotonal organs has been elucidated using HRPimmunochemistry (Lakes and Pollack, 1990). At the prepupal stage of the larva, each imaginalleg disc contains 8 neurons (7 in Drosophila) derived from the embryo. One is a scolopidialneuron located on the dorsal surface of the disc, but this cell probable degenerates early inpupal development. As the leg disc enlarges into the limb bud during pupation, the first fewadult sensory neurons arise, and by one day after pupation most of the neurons are present. Bythree days (half the pupal period) the adult pattern is formed.The major chordotonal organ in the dipteran prepupal leg bud is the FeCO, which comprises atleast 50 FeCO neurons in the distal end of the presumptive femur. Two hours after pupation,these neurons sprout axons in two fascicles, which grow to the limb base apparently without anestablished pioneer neuron nor with guidepost cells. By 7.5h a tarsal chordotonal organ appearsand by 26h a tibial chordotonal organ is evident in the distal tibial segment of the bud.Meanwhile, the FeCO dendrites have all oriented distally towards the FeCO apodeme and beginto divide into proximal and distal clusters. The proximal cluster migrates to its final positionnear the trochanteral border. At 51h it appears as two recognisable scoloparia, one dense andone dispersed (Debaisieux, 1938). The proximal FeCO has thus decreased its total nerve lengthby 750 µm, apparently through axonal shortening. Phormia is the only known case in which theFeCO includes two widely separated organs. In adult Drosophila the FeCO lacks the distalcluster and comprises only two scoloparia from the proximal cluster.10.5 MORPHOGENESIS OF NERVE PATHWAYSThe developmental mechanisms by which chordotonal organ axons find their way to the CNSinvolves two processes: first, growing axons must navigate across surfaces in the absence ofprior neuronal tissue cues, and second, axons contact and fasciculate with established pathways,probably using each other as contact guides towards targets (Bate, 1978; Jan and Jan, 1993).Insights into inherent principles of directed growth and targeting during development are basedupon studies of grasshopper leg nerve morphogenesis. Two major concepts have emerged:pioneer neurones, and the guidepost hypothesis.10.5.1 Pioneer neurones and the guidepost hypothesisPathway formation is initiated by one or a few early pioneer neurons (Bate, 1976) whichdifferentiate from the embryonic epithelium and send the first axons toward the CNS. Themechanisms by which they establish the two main
L.H. Field & T. Matheson Advances in Insect Physiology 27 (1998) Page 169leg nerve pathways from the periphery are proposed by the “guidepost hypothesis” (Keshishianand Bentley, 1983b).1. Filipodial exploration. Extensive filipodia from pioneer cell bodies, and later axons, samplethe epithelial environment and may locate guidepost cells.2. Cell to cell recognition. Pioneer axons grow along a chain of immature neurons, which havedistinctive surface features that allow pioneers to make selective junctions with them.3. Axonal contact guidance. New axons grow alongside existing axons.4. Positional cell determination. Immature neurons differentiate from the epithelium instrategic locations and at times that allow them to determine pioneer routes by acting asguideposts. The resultant pathways become the adult nerve routes.The critical feature of the guidepost hypothesis is point (4), in which stereotyped nerve routesare created by a chain of guidepost neurons or “follower neurons”, serving to guide thepioneers centrally. The followers also serve as target points where newly arising chordotonalorgan axons join pioneer tracts, thereby creating nerve branch points (Keshishian and Bentley,1983a,b).The following two sections show how the two main branches of the leg nerve (N5B1, N5B2),both of which contain chordotonal organ afferents, develop in the embryo.10.5.1.1 N5B1 development In grasshopper legs the first pioneers are a pair of tibial epithelialcells, collectively labeled Ti1, which differentiate from the distal limb bud epithelium at about30% of development and soon become displaced to the level of the tibia (Fig. 32b). They sendaxons bearing 75-100 µm long filipodia on their growth cones over the epithelial surfacetowards the limb base and into the CNS. The Ti1 neurons grow along a chain of non-axonicpartially differentiated follower cells: the CT1 pair near the coxo-trochanteral margin and F1and F2 in the proximal and distal femur. The follower neurons subsequently send axonscentrally along the Ti1 axons to establish a fasciculated pathway by the time chordotonal organaxogenesis commences. Thus the first embryonic leg nerve (N5B1 in the adult) is establishedand maintained, including stereotyped branch points determined by follower neurons (Fig.32b). The follower neurons F1 and F2 later become Type II multipolar stretch receptors(Keshishian and Bentley, 1983a).The first chordotonal organ axons to arise are those of the FeCO. At 38% development theygrow toward and directly contact the F1 cell body (demonstrated by TEM) before growingproximally along the Ti1 axons. Later the subgenual organ (SGO) axons sprout and projectonto the Ti1 cell bodies (Fig. 32b) and grow along the Ti1 pathway. Occasional dye couplinghas been observed between chordotonal organ axons and pioneer cells in embryos earlier than50%
Page 2 and 3:
L.H. Field & T. Matheson Advances i
Page 4 and 5:
Page 6 and 7:
Page 8 and 9:
Page 10 and 11:
Page 12:
Page 15 and 16:
Page 17 and 18:
Page 19 and 20:
Page 21 and 22:
Page 23 and 24:
Page 25 and 26:
Page 27 and 28:
Page 29 and 30:
Page 31 and 32:
Page 33 and 34:
Page 35 and 36:
Page 37 and 38:
Page 39 and 40:
Page 41 and 42:
Page 43 and 44:
Page 45 and 46:
Page 47 and 48:
Page 49 and 50:
Page 51 and 52:
Page 53 and 54:
Page 55 and 56:
Page 57 and 58:
Page 59 and 60:
Page 61 and 62:
Page 63 and 64:
Page 65 and 66:
Page 67 and 68:
Page 69 and 70:
Page 71 and 72:
Page 73 and 74:
Page 75 and 76:
Page 77 and 78:
Page 79 and 80:
Page 81 and 82:
Page 83 and 84:
Page 85 and 86:
Page 87 and 88:
Page 89 and 90:
Page 91 and 92:
Page 93 and 94:
Page 95 and 96:
Page 97 and 98:
Page 99 and 100:
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
Page 107 and 108:
Page 109 and 110:
Page 111 and 112:
Page 113 and 114:
Page 115 and 116:
Page 117 and 118: L.H. Field & T. Matheson Advances i
Page 167: L.H. Field & T. Matheson Advances i
Page 219 and 220:
Page 221 and 222:
Page 223 and 224:
Page 225 and 226:
Page 227 and 228:
Page 229 and 230:
show all

Chordotonal Organs of Insects

Create successful ePaper yourself

Delete template?

Save as template?