Abstract
The insect tracheal system is a continuous tubular network that ramifies into progressively thinner branches to provide air directly to every organ and tissue throughout the body. During embryogenesis the basic architecture of the tracheal system develops in a stereotypical and genetically controlled manner. Later, in larval stages, the tracheal system becomes plastic, and adapts to particular oxygen needs of the different tissues of the body. Oxygen sensing is mediated by specific prolyl-4-hydroxylases that regulate protein stability of the alpha subunit of oxygen-responsive transcription factors from the HIF family. Tracheal cells are exquisitely sensitive to oxygen levels, modulating the expression of hypoxia-inducible proteins that mediate sprouting of tracheal branches in direction to oxygen-deprived tissues.
Highlights
Development of the tracheal systemUnlike vertebrates, which have developed two different but still coupled systems for oxygen intake and distribution – i.e., lungs or gills for oxygen intake, and a circulatory system for oxygen delivery to tissues and organs – insects rely solely on the tracheal system, an interconnected air-filled network of tubes that transports oxygen in gas phase directly to tissues and cells throughout the organism (Ghabrial et al, 2003)
The insect tracheal system is a continuous tubular network that ramifies into progressively thinner branches to provide air directly to every organ and tissue throughout the body
An atmosphere with high oxygen levels fostered the evolution of efficient oxygen delivery systems
Summary
Unlike vertebrates, which have developed two different but still coupled systems for oxygen intake and distribution – i.e., lungs or gills for oxygen intake, and a circulatory system for oxygen delivery to tissues and organs – insects rely solely on the tracheal system, an interconnected air-filled network of tubes that transports oxygen in gas phase directly to tissues and cells throughout the organism (Ghabrial et al, 2003). The development of Drosophila tracheal system begins around 4 h after oocyte fertilization, to form an interconnected network of tubes during embryogenesis This network becomes functional at the beginning of the first larval instar and ramifies dramatically during the 2nd and 3rd larval stages to accompany the increase in body size of the larva. Once tracheal cells have reached the Bnl-positive cluster, bnl expression shuts-off in that cluster, and is immediately turned-on again a few cell diameters away on the track of the forming branch This process repeats again and again throughout branch extension, promoting the continuous elongation of the tracheal branches (Sutherland et al, 1996). Augmented FGFR activation increases the ramification of terminal tracheal cells, whereas reduction of FGFR activity, once the tracheal branches have reached their final destination, leads to reduction of terminal cell sprouting (Hacohen et al, 1998)
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