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newborn for extra-uterine life because the alveoli must be prepared for the transition from 3% to 21% oxygen, and for maintaining their potency by producing surfactant16. PTHrP is expressed throughout vertebrate phylogeny, beginning with its expression in the fish swim bladder, an organ that is obviously used for coping with gravity.

The expression of PTHrP by alveolar type-II cells was down regulated by 80-90% within 8 hours of rotation in a bioreactor on Earth (Figure 5-10). This lower level lasted for at least three days, suggesting that PTHrP signaling had been exploited for adaptation to 1-g conditions. Similarly, the production of PTHrP declined over the first 24 hours and maintained this level for the following 48 hours. After return to 1 g, both parameters returned to normal levels (Torday 2003).

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