THE POSTNATAL GROWTH OF THE LUNG IN THE DOG

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Abstract
This study is based on a detailed, if preliminary, analysis of the lobus intermedius of the dog lung. By means of dissections, graphic reconstructions from serial sections, plastic injections and 3-dimensional drawings obtained by piling acetate plates upon which sections have been traced, it has been observed that there is a substantial reduction postnatally in the number of non-respiratory branches of the axial bronchus and of non-respiratory peripheral bronchioles[long dash]those with which the fetus was born. After birth, the latter are rapidlyconverted from passages lined by cuboidal epithelium into respiratory units. Following inflation of the bronchioles by inspired air the cuboidal epithelium becomes drawn out into metameric clumps of cells between which alveoli, and even alveolar sacs, subsequently arise. For example, in terminal sprays of the thin right margin of the lobe a segment of terminal bronchiole 2 mm long increases its length by one-half in the first 10 1/2 days after birth and it (and as many as ten of its branches) may be transformed into respiratory passages. No evidence was found to support the Broman-Bremer theory that after birth the cuboidal epithelium of terminal bronchioles invades the alveolar sacs formed in fetal stages while new respiratory units develop by compensatory budding of terminal alveoli. New terminal alveoli, indeed, are arising, but the method seems to be that of ingrowth of new septa.

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