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Methyl-methacrylate embedding makes it possible to obtain semi-thin sections rich in detail and without tissue shrinkage. The procedure requires considerable time and labour, and great care must be taken to prevent heat damage to the tissue during the exothermic polymerization process. The original method given by Burkhardt (1966) and later modifications are described and discussed, with special attention to the practical problems encountered and their solutions.

The anatomy and pathology of the splenic red pulp was studied in three-dimensional reconstructions of methylmethacrylate embedded blocks of tissue obtained after splenectomy, as well as by morphometrical analysis of a large number of specimens. The sinuses of the spleen form a plexus of anastomosing vessels with remarkable buds. Capillaries end as sheathed capillaries in the cord tissue, the 'filtering' area, but a large proportion of the red pulp cords appear to be 'non-filtering'. These might form part of the lymphatic compartment, which is separate from the white pulp and its extension along the capillaries. This area has not yet been described in man. The change in the volume and structure of the various components of the red pulp were studied in 60 controls and in cases of traumatic rupture, idiopathic thrombocytopenic purpura, aplastic anaemia, autoimmune haemolytic anaemia, congenital spherocytosis, splenic congestion, and Hodgkin's disease. Significant differences were found in the volume of filtering and non-filtering areas, the size of the sinus compartment, and the degree of vascularization; these differences were only partially expected, for instance in disorders with excessive erythrocyte sequestration. A decrease of the 'non-filtering' area in Hodgkin's disease might indicate an unknown aspect of this disease. In agreement with our previous paper on the amount of white pulp, spleens removed because of traumatic rupture and those incidentally removed during abdominal surgery may not be combined as a single control group, because of significant and probably functional differences in the composition also of the red pulp.