SCANNING ELECTRON MICROSCOPIC STUDY OF CANINE LYMPHATIC VESSELS AND THEIR VALVES

Abstract

The morphology of canine thoracic duct, peripheral
collecting lymphatics and their valves was deterrnin·
ed using scanning and transmission electron micro·
scopy (SEM and TEM). Information from surface
and subsurface structures was derived simultaneously
in the SEM by using a heavy metal stain combined
with backscattered electron imaging. The peripheral
lymphatics and thoracic duct were covered by a
continuous sheet of endothelial cells, the nuclei of
which were slightly raised, ovoid in shape with coarsely
granular chromatin and long axes oriented parallel
to the direction of flow. The immediate subendothe·
lial connective tissue in the thoracic duct was primarily
smooth muscle, whereas in the peripheral lym·
phatics reticular and elastic fibers predominated.
Valves were seen in both peripheral and central
lymphatics. These were covered with endothelium
similar to the collecting lymphatic vessels, were usu·
ally bicuspid and had cusps that inserted either at a
single point or overlapped prior to a wide based insertion.
A complex network of connective tissue
and elastic fibers was seen in valves and in the vessel
walls adjacent to the valves. The similarities and
dissimilarities of the SEM appearances of lymphatics
arteries and veins are discussed.