THE ULTRASTRUCTURE OF PULMONARY LYMPHATIC CAPILLARIES OF NEWBORN RABBITS AND OF HUMAN INFANTS

Abstract

For a long time morphological studies of the pulmonary lymphatics have erroneously been considered as the simple search for a more precise knowledge of a problem which seemed only a delicate and intriguing anatomical puzzle. This usually involved the disputed presence (1-10) or absence (11-21) of true alveolar lymphatics, the localization and orientation of lymphatic valves and consequently the problem of the direction of lymphatic flow, and the differentiation of tissue clefts or small blood vessels from true lymphatics. Recent physiological (22-24), embryological (25), pathological (26-28, 82) and clinical (23, 24) investigations have revealed however that these problems have a basic and challenging importance in the understanding not only of the structure and function of the normal and diseased lung, but even in the regulation of body fluids. Such investigations might well shed some light on many other related problems of the pulmonary lymphatic microcirculation, on which opinions differ or are nonexistent, such as the distinction between (pulmonary) lymph and interstitial fluid, and the mechanisms governing their formation and subsequent removal, the way or ways in which fluids and cells enter lymphatics, and the forces which propell them into the larger lymph ducts. 

To answer some of these questions, we previously made serial histological (13, 29), morphometric (28) and radiologic studies (23, 30) as well as investigations on corrosion casts after the injection of the pulmonary lymphatics (29-31). The results obtained led to electron microscopical studies (30, 32, 33). Apart from other advantages, a careful electron microscopical investigation avoids some of the difficulties usually encountered in light optical microscopy, (where the distinction between the smallest blood vessels and true lymphatic vessels is often impossible) or in injection or corrosion specimens in which the formation of possible artefacts due to injection of tissue clefts is a potential source of error. Moreover, an electron microscopical investigation seemed warranted as no previous studies are known to us on this subject, except for the studies of Kato (34, 35), Lauweryns et al. (30, 32, 33) and two low-power micrographs by Schulz (36). Neonatal infant and rabbit lungs were more specifically studied, as the pulmonary lymphatics are especially prominent in such lungs (13).