Microsoft Word - NEO178BF

Stefan A. Tschanz, MD, Institute of Anatomy, Bühlstrasse 26, CH-3000 Beme 9 (Switzerland) In human lung development we distinguish a preand a postnatal period. After the formation of the lung diverticulum from the foregut at gestation day 26, the further development can be subdivided into 5 stages [1]. In the pseiidoglandular stage (prenatal weeks 5-17) the lung bud grows and divides dichoto-mously several times into a loose meshwork of mesenchyme giving the organ a gland-like appearance. During the next stage {canalicular stage, weeks 17-24) the future lung parenchyma is ‘canalized’ by numerous capillaries. Epithelial differentiation is started, so that type-I pneumocytes contribute to a thin air-blood barrier and type-II pneumocytes begin to secrete surfactant into the liquid-filled air spaces. In the following saccular stage (24 weeks to birth) the future gas-exchange region expands, the interstitial tissue in between the air spaces decreases giving the lung a more aerated appearance. The stage alveolization starts a few weeks before birth and lasts beyond it. The air spaces of the saccular stage are now being subdivided by new septa, thus forming the alveoli. Postnatally the alveolar formation continues until about the 2nd year, but the ‘bulk alveolar formation, is likely to be completed until about the 6th month of age [2]. During alveolization the saccular walls and also the new septa consists of a broad central sheet of connective tissue with a capillary layer on each side. In our oponion this double capillary layer is a prerequisite for alveolar formation as the newly formed septa arise by upfolding of one of the two layers. By this process, the prospective parenchymal air spaces turn into alveolar ducts and sacs lined by the future alveoli. In the last stage of lung development the bilayered ‘primitive, septa turn into the slender mature interalveolar walls of the adult lung containing only a single capillary layer. This period of microvascular maturation begins shortly after birth in parallel to the alveolization. It is characterized by a pronounced decrease in interstitial tissue mass leading to the approximation of the two capillary layers which come into close contact and merge [3]. The Influence of Glucocorticoids Glucocorticoids (GCs) are well known as a major therapeutic agent in preterm neonates with respiratory problems. They were initially found prenatally to accelerate the maturation