POSTNATAL LUNG GROWTH - EVIDENCE THAT THE GAS-EXCHANGE REGION GROWS FASTEST AT THE PERIPHERY

The site(s) at which the gas-exchange region enlarges after alveoli ar e formed from the saccules of the immature lung is unclear; however, t his information might be important to studies of the regulation of lun g growth. Although aware of important assumptions on which it rests, w e undertook this study to test the idea that the lung's gas-exchange r egion enlarges more rapidly in the immediate subpleural region than mo re centrally' To label the interstitium of the gas-exchange region, ra ts were provided silver-containing water from age 23 to 135 days (112 days). Some were killed at age 135 days, others 51 days after silver e xposure ended (age 186 days). We considered silver grains that formed in the interstitium as a marker of lung present or formed during silve r exposure; tissue added after exposure would diminish the numerical d ensity of grains and the fastest growing sites would have the lowest g rain density. Rats killed on the 112th day of silver exposure had a si lver grain density in their immediate subpleural gas-exchange matrix t hat was 28% lower than in their more central gas-exchange matrix. Rats killed 51 days after silver exposure ended had a grain density in the immediate subpleural region that was 65% lower than in the central ma trix of the gas-exchange region. The grain density in the rats killed 51 days after silver exposure ended was 33% lower in the central matri x and 67% lower in the peripheral matrix than in the respective region s of rats killed on day 112 of exposure. These observations are consis tent with the notion that the gas-exchange region enlarges centrally a nd subpleurally, but the latter is the fastest site of postnatal growt h.