Microscopic features of all four different epidemiological-clinical types of KS do not differ. Early lesions of the skin or the mucosa are uncharacteristic and present with subtle vascular proliferation {2216}. In the patch stage, vascular spaces are increased in number, of irregular shape, and may dissect collagen fibres in the superficial corium. They often run parallel to the epithelium. The vascular proliferation is often perivascular and periadnexal. Endothelial cells lining the spaces are flattened or more oval, with little atypia. Preexisting blood vessels may protrude into the lumen of new vessels. Admixed are sparse lymphocytes and plasma cells; frequently, extravasated erythrocytes and deposits of hemosiderin surround the vas cular structures. Slits lined by attenuated endothelial cells between collagen bundles are also seen. In some cases, there is a proliferation of spindle or oval endothelial cells around pre-existing blood vessels in the dermis or submu-cosa. Slit-like spaces, lymphocyte and plasma cell infiltration and extravasated erythrocytes are also observed. In plaque stage, all characteristics of patch stage are exaggerated. There is more extensive angio-proliferation with vascular spaces showing jagged outlines. Inflammatory infiltrate is denser and extravascular red cells and siderophages are numerous. Hyaline globules (likely representing destroyed red blood cells) are frequently found.

Nodular stage is characterized by well-defined nodules of intersecting fascicles of spindle cells with only mild atypia and numerous slit-like spaces containing red cells. Peripherally, there are ectatic blood vessels. Many spindle cells show mitoses. Hyaline globules are present inside and outside the spindle cells. Some patients, usually with endemic nodular type KS, develop lesions that closely resemble lymphangioma.

The main differential diagnosis includes Kaposiform haemangioendothelioma {775}.


The lining cells of clearly developed vascular structures are usually positive for vascular markers, while the spindle cells consistently show positive reaction for CD34 and commonly for CD31 but are factor VIII negative. All cases, irrespective of epidemiologic subgroup, are HHV-8 positive. The new marker FLI1, a nuclear transcription factor, appears to be expressed in almost 100% of different vascular tumours, including KS {780}.

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