ULTRASTRUCTURAL DEVELOPMENT OF KAPOSI-SARCOMA IN RELATION TO THE DERMAL MICROVASCULATURE

Ultrastructural subtypes of endothelial cells in Kaposi's sarcoma were compared with lymphatics and the normal dermal microcirculation in different stages of lesional development. In the earliest patch stage, lymphatic channels, recognised by their dissecting growth pattern and a lack of a basal lamina and pericytes, were prominent. Venous endothelium was recognised by virtue of its multilaminated basal lamina and often showed markedly irregular luminal and abluminal cytoplasmic projections. As the histological stage progressed toward spindle cells, venous endothelium showed a tetrad of changes: dissolution of the basal lamina; fragmentation and disapperance of the pericyte sheath, decreased and often rudimentary intercellular junctions and markedly reduced numbers of Weibel-Palade bodies. These were also features of spindle cells, which were rarely seen to emerge from narrow vascular channels of indeterminate type. Spindle cells showed sparse intercellular junctions and minimal basal lamina but no Weibel-Palade bodies. These progressive venous alterations thus resulted in a mixed intermediate subtype of endothelium with the morphological traits resembling lymphatics as well as venous blood vessels. The mixed subtype included endothelial tubes surrounded by a complete basal lamina but lacking pericytes, and much more commonly, tubes with pericytes but a scanty basal lamina. Both forms had remarkably few or no Weibel-Palade bodies. In the spindle cell stage, normal vessels were largely replaced by the mixed subtype and an indeterminate type of frequently disrupted endothelial tube which lacked a basal lamina as well as a pericytic investment. Dissecting lymphatic channels could not be confidently distinguished from the latter vessels. Direct anatomical connections between morphologically recognizable lymphatics and venules, blood capillaries or endothelial tubes of mixed subtype were difficult to demonstrate. Lymphaticovenous shunts were represented by gaps in all endothelial subtypes, the gaps sometimes due to cell degeneration.