Stereomicroscopic Funnel-like Architecture of Pulmonary Lymphatic Valves*

Abstract

Although lymphatic valves are basically important in channeling lymph and establishing
the lymphatic microcirculation, not much attention has been paid to their structure
in the past. They are still classically, and as will be shown, erroneously considered to
occur as two semilunar cusps, attached to opposite sides of the vessel wall, and hanging
into the vessel lumen like two closely apposed swallow's nests. This widely accepted
textbook description (1, 2, 4-7, 15-17, 19, 20) has been largely extrapolated from observations
on venous valves situated in the large veins (i. e. the saphenous veins) of the
body. Also the appearance of routine histologic tissue sections falsely suggests and
mimicks a bicuspid architecture, as our preliminary graphic anatomic reconstructions
of 26 pulmonary lymphatic valves from serial (6 μ.) histologic tissue sections have
revealed (3, 11, 14). Reconstructed valves seemed to have the shape of a cone or a
funnel, oriented obliquely in relation to the axis of the vessel and attached over its
entire length to the wall of the vessel. The lumen appeared to be situated near the
deepest point of the funnel.
These anatomic graphic reconstructions were however only an indirect, probably a
more or less biased approach to the real architecture of the pulmonary lymphatic valves.
Indeed, due to their overall dimensions and their specific "intra vascular" localization,
it was impossible to locate accurately the reference points which are an essential condition
in anatomical reconstructions. Even if major deformations could be avoided due to the
inherent complexity of the lymphatic vessel network and of its valves, and because of the
rather limited number of the 6 micron sections used for the actual reconstructions ( 18),
slight lateral deformations were unavoidable. Moreover (3), still another cause of
artefactual lateral deformation, lenghtening and stretching of the graphic models,
occurred because we reconstructed in perspective under a very large angle rather than
by orthogonal projection.
Hence our graphic anatomic reconstructions did not constitute an exact replica of
reality and were undoubtedly subject to various artefacts to some extent. Therefore
the present study was undertaken in order to correct these earlier results. As only a
direct observation of the lymphatic valvular architecture seemed appropriate, we studied
lymphatic valves stereomicroscopically on single and serial 150-250 micron thick
histologic sections through the entire lung lobes.