Development of the trachea
The tracheal system develops from 10 segmental specializations of the lateral ectoderm, the so-called tracheal placodes, which are first apparent in stage 10. Cells of the tracheal placodes divide and invaginate to form the tracheal pits of about 80 cells. From this point on, subsequent processes of tracheal morphogenesis are based on cell migration and cell shape changes, without any further division.

In every hemisegment, the tracheal cells follow highly stereotypic migration pathways. Five primary branches form when cells of the tracheal sacks migrate into different directions. Later in development, secondary branches, which are formed by single cells, sprout out from the primary branches.

The tracheal branches of each hemisegment fuse to branches of the neighboring segment and to those of the contra lateral side, forming a widely ramified, interconnected mesh. The opening of each tracheal pit to the outside disappears in stage 12, during germ band shortening, when the pits close over. At the end of development, the tracheal system is open to the outside only at the posterior spiracles, which develop from special cells in the abdominal segment a8. These structures fuse to the first and eighth tracheal pit, respectively.
The Drosophila homologue of the FGF receptor breathless (btl) and its ligand branchless (bnl) play a central role in the formation and migration of the primary branches in tracheal development. bnl is expressed in defined ectodermal and mesodermal cells, acting as a chemo attractant for tracheal cells, which express btl.

In addition, further branch-specific determinants are required in the tracheal cells to ensure correct migration behavior. knirps and knirps related specify the cells of the presumptive dorsal and ventral branches. This is crucial for their correct response to the bnl signal. knirps and knirps related are induced in tracheal cells in response to decapentaplegic signaling. Similarly, spalt, which is induced by wingless signaling, is required in the tracheal cells that will form the dorsal trunc. A single hunchback-expressing mesodermal cell is required to connect the anterior dorsal trunc of one segment to the posterior dorsal trunc of its neighboring tracheal metamere.



Media list
Tracheal system (0.2 MB)
Tracheal development..........additional information

Genes discussed
Gene
Gene product - Domains
Function
Links
branchless (bnl)
fibroblast growth factor homolog
ligand for Breathless and Heartless - Drosophila's only known FGF
breathless (btl)
transmembrane receptor, FGF-receptor homolog
guides migraton of trachea and CNS midline cells
decapentaplegic (dpp)
ligand - TGF-ß homolog
Dpp establishes the dorsal / ventral pattern and defines compartment boundaries
hunchback (hb)

transcription factor - zinc finger
gap gene, later required for proper temporal generation of NB sublineages
Interactive Fly
knirps (kni)
transcription factor - steroid receptor - zinc finger
gap gene, later organizes the development of the second wing vein
knirps related (also named knirps-like (knrl))
ligand-dependent nuclear receptor -    transcription factor
regulation of mitosis
spalt (salm)
transcription factor - zinc finger - PRDII-BF1 homolog
a target of Dpp signaling - Spalt and Spalt-related regulate vein-specific expression of knirps and the iroquois complex
wingless (wg)
ligand - wnt family
segment polarity gene, intercellular signaling protein