A Practical Approach, Second Edition=Ronald D. Ho.pdf

POSTNATAL DEVELOPMENTAL MILESTONES 1023EmbryonicperiodStage ofmicrovascularmaturationAlveolarstageCanaucularstagePseuooglandularstageSaccularstageNormal growthperiodLUNG DEVELOPMENTLUNG GROWTHFertilization10 20 30 3 6 9 1 2 3 4 5 6 7Weeks Months YearsAGEBirthFigure 1 Timing of stages of human lung development. (Reproduced from Zeltner and Burri, 1987(1)).Inter-species Comparison of Lung Growth and DevelopmentDevelopmental stages identified for the human lung are also found in other mammalian speciesreported in the literature. In this section, the growth and development of the pulmonary system incommon laboratory species will be discussed and compared with that of humans.The timing of each developmental stage and the degree of lung development at birth varieswidely between species (3). As an example, at birth the opossum lung is very primitive; rat andmouse lungs have no alveoli; kittens, calf and humans have relatively few alveoli; and the lamblungs are quite well developed. Postnatal development of the lung has also been studied in otherspecies: guinea pig, hamster, dog, monkey and humans. A comparative table of the timing of eachstage for several species is presented below (7).Species Glandular Canalicular Saccular AlveolarMouse 14–16 16.5–17.4 17.4– PD 5–Rat 13–18 19–20 21–PD PD 7–21Rabbit 19–24 24–27 27– —Sheep –95 95–120 120– —Human 42–112 112–196 196–252 252–childhoodRat Lung Growth and DevelopmentRats are perhaps the most extensively studied laboratory species for lung development (2). Manyhypotheses regarding human lung development are based on research conducted in rats. Postnatalchanges in the rat lung occur in 3 phases: short expansion; proliferation; and equilibration. Thefirst phase lasts from birth to day 4 of postnatal age. During this period, the lung transforms fromthe saccular stage into the alveolar stage. During the proliferating phase, alveolar numbers increasesmore rapidly than body weight. So does the lung volume and lung weight. During the last phase,© 2006 by Taylor & Francis Group, LLC