State of the lungs at birth.

Abstract

The lungs, during intrauterine life, pass through two principal stages of development. The first, which occupies about the first half of fetal life, is a period of increasing elongation and ramification of an original bud arising from the entodermal tube. During this time cells differentiate into proximal tall columnar bronchial epithelium, intermediate cuboidal epithelium, and terminal actively growing buds made up of very large cells with prominent basal vacuoles. These terminal buds incite the local proliferation of mesoderm so that each elongating tubular branch becomes surrounded by a sheath of connective tissue. The lobular pattern is very prominent at this time, owing to the close investiture of the entoderm by mesoderm. Throughout fetal life an increasing number of branches are produced; as they become more numerous their external surfaces become more closely approximated. The connective tissue sheaths lose their individuality and merge indistinguishably with those adjacent to them. The proportionate