The mode of origin of pulmonary acini and respiratory bronchioles in the fetal lung

Between the 50th day of embryonic life Stage 20 (O'Rahilly and Boyden, '73; Wells and Boyden, '54). and the 30th week of gestation no reconstructions of the developing parenchyma of the human lung seem to have been attempted. To bridge this long gap, only two techniques have been utilized hitherto: counting the generations of branches in the growing segments of the lung and analyzing the changing cytology with the aid of the electron microscope. At least two important objectives remain: a knowledge of when the respiratory units known as pulmonary acini appear and the order in which capillary invasion of its component parts takes place. At the 16th week of gestation, peripheral units have not attained their full complement of branches and their boundaries are not delimited. At 17 weeks, acini of varying sizes can be recognized as discrete terminal units at the periphery of the segmental trees, having attained their full, though underdeveloped, complement of fetal branches, as gauged by later counts of generations made at 30 weeks of age (Boyden, '69). The 17th is also the week in which capillaries first invade the peripheral branches of the acinus. Although significantly later stages have not yet been obtained, the evidence available (including that from unpublished work on monkey fetuses) strongly suggests that canalization (before birth), like alveolization (after birth) Boyden and Tompsett, '65; Boyden, '67. proceeds mainly in a centripetal direction.