Cell-Cell Interaction During Drosophila Embryogenesis
71
Recently, molecules with cell migration guidance properties have
been identified in invertebrates and vertebrates. Genetic studies in C.
elegans have led to the identification of a gene (unc-6) which is required
for mesoblast migration and pioneer axon extension in dorsal and ventral directions on the body wall (Hedgecock et al. 1990). unc-6 encodes
a secreted protein with an N-terminus homologous to laminin subunits
and is now generally referred to as a netrin. Multiple netrin cues are
important for proper regulation of the dorsal guidance receptor UNC-5,
a cell surface receptor of the immunoglobulin superfamily which mediates some cellular responses to the UNC-6 guidance cues (Hamelin et al.
1993). More recently, netrins have been identified in vertebrates and in
Drosophila where they are involved in axon guidance (Hedgecock and
Norris 1997). The netrins identified thus far in Drosophila were not
involved in directed cell migration during tracheal development (unpublished results). Orthologs for several other vertebrate guidance molecules or their receptors (Wehrle-Haller and Weston 1997) have not been
found yet in Drosophila, therefore it has not been possible to test their
role in tracheal development.
5.2.1 Control of Cell Migration During Tracheal Development
in Drosophila
Due to our interest in intercellular signaling, we started to study trachea
formation during embryonic development in Drosophila melanogaster a
few years ago, with special emphasis on the genetic control of directed
cell migration. The stereotyped tubular network of the larval trachea
develops from ten individual small bulges containing approximately 90
cells, the tracheal placodes, which form in lateral positions in the trunk
segments on either side of the embryo (Manning and Krasnow 1993;
Samakov1is et al. 1996). Upon invagination of the tracheal cells (generating a tracheal sac), the complex branching pattern of the tracheal
system is established via cell migration, extension, and fusion in the
absence of further cell division. During this process, a defined number
of cells migrate from each placode as anterior branches and as ventral
and dorsal branches (e.g., five to seven cells migrate dorsally and eventually line up to form the dorsal branch which targets the dorsal vessel
and the dorsal muscles). Thus, the establishment of the tracheal network
71
Recently, molecules with cell migration guidance properties have
been identified in invertebrates and vertebrates. Genetic studies in C.
elegans have led to the identification of a gene (unc-6) which is required
for mesoblast migration and pioneer axon extension in dorsal and ventral directions on the body wall (Hedgecock et al. 1990). unc-6 encodes
a secreted protein with an N-terminus homologous to laminin subunits
and is now generally referred to as a netrin. Multiple netrin cues are
important for proper regulation of the dorsal guidance receptor UNC-5,
a cell surface receptor of the immunoglobulin superfamily which mediates some cellular responses to the UNC-6 guidance cues (Hamelin et al.
1993). More recently, netrins have been identified in vertebrates and in
Drosophila where they are involved in axon guidance (Hedgecock and
Norris 1997). The netrins identified thus far in Drosophila were not
involved in directed cell migration during tracheal development (unpublished results). Orthologs for several other vertebrate guidance molecules or their receptors (Wehrle-Haller and Weston 1997) have not been
found yet in Drosophila, therefore it has not been possible to test their
role in tracheal development.
5.2.1 Control of Cell Migration During Tracheal Development
in Drosophila
Due to our interest in intercellular signaling, we started to study trachea
formation during embryonic development in Drosophila melanogaster a
few years ago, with special emphasis on the genetic control of directed
cell migration. The stereotyped tubular network of the larval trachea
develops from ten individual small bulges containing approximately 90
cells, the tracheal placodes, which form in lateral positions in the trunk
segments on either side of the embryo (Manning and Krasnow 1993;
Samakov1is et al. 1996). Upon invagination of the tracheal cells (generating a tracheal sac), the complex branching pattern of the tracheal
system is established via cell migration, extension, and fusion in the
absence of further cell division. During this process, a defined number
of cells migrate from each placode as anterior branches and as ventral
and dorsal branches (e.g., five to seven cells migrate dorsally and eventually line up to form the dorsal branch which targets the dorsal vessel
and the dorsal muscles). Thus, the establishment of the tracheal network