RESUMO
To investigate the human lung development and the distribution of elastic and reticular fibers during the fetal period proper, lung tissues taken from the periphery of the right lower lobes of Korean fetuses (n=49) of both sex were studied. The fetuses were the prodocts of spontaneous or therapeutic abortions and were found to have no associated lesions or anomalies at autopsy. The fetal age were estimated from crown-rump length or foot length. Paraffin sections, cut at 5-7 µm, were stained with routine hematoxylin and eosin for general structure, acid orcein and a1dehyde fuchsin for elastic fiber, and with Gomori's silver technique for reticular fiber, respectively. The lung development during fetal period proper, could be subdivided into three continuous periods according to the relation between airspaces, surrounding mesenchymal tissue, their structural changes and distribution, i.e., an early stage of the formation of conductive airways (pseudoglandular period, before 16th week of gestation), a middle stage of the development of lung parenchyma and new blood vessels (canalicular period, between 16th and 28th week of gestation), and a late stage of transition of respiratory portion to vascular organ (terminal sac stage, after 28th week of gestation). In places, secondary septa of sac or saccule formed by capillaries, capillary connective tissue, elastic and reticular fuel could be identified by the 33rd week of gestation. Elastic fibers could be noted in pleura, subepithelial areas of bronchioles and the wall of blood vessels in the late stage of pseudoglandular period. By the 28th week of gestation, elastic fibers were seen in the wall of small blood vessels or capillaries in the septal wall among the airspaces. And these fibers were observed in the tip of the secondary septa by the 33rd week of gestation but were not still completely developed in the walls of primary or secondary septa. Reticular fibers were already developed and widely distributed in fetal lung by the 10th week of gestation. These fibers were concentrated particular around the subepithelial area of bronchicoles, the airspaces and the blood vessel wall in the canalicular period. By the late stage of terminal sac period, reticular fibers formed a network along the small blood vessels in the septum of airspaces. These results indicate that primitive alveoli might be formed by the late stage of fetal period proper. The fibrous framework could partially formed by collagenous and reticular fibers during the pseudoglandular period, by addition of elastic fiber to the preformed network, and incompletely still finally by the three kinds of connective tissue fiber.