Acta histochem. 65, 233-242 (1979) Department of Human Anatomy, University of Bologna Proteoglycans and their relationship with the other components of the rabbit aorta wall observed in two different experimental conditions By ROLANDO RIVA, MAURIZIO MARCHINI, RITA 8rnOCCHI With 4 figures (Received April 27, 1979) Summary An electron microscopic study of the aorta wall of rabbit was carried out employing electron microscopic histochemical methods aimed at examining the distribution of proteoglycans and their correlation with the other structural components of the aorta wall. The ultrastructural evidence of the proteoglycans has been obtained by treating the arterial wall with alcian blue stain and supporting this staining procedure with control enzyme treatments. Moreover, in order to attain a better interpretation of the ultrastructural findings, the arterial wall has been studied under 2 different experimental conditions, the 1st one being represented by an accentuated contraction of the vase's wall, following immersion in the fixator, the other one by an extreme distension of the wall, reached prior to fixation, subjecting the tissue to its tension limit of 700 dyn/mm 2 . This type of elcetron microscopic visualization of the proteoglycans following alcian blue treatment leads on to think that at least 2 distinct macromolecular of proteoglycans exist, each one having an unusual relationship with the other components of the arterial wall. Dif- ferent site and types of interaction are to be noted for each of the 2 distinct ultrastructural proteo- glycans entities. Introduction Among the numerous morphological and biochemical research studies carried out on the arterial wall in recent time, peculiar informations have been obtained by the study of the interaction detectable at the ultrastructural level, which exists among proteoglycans and the other main arterial components: collagen fibrils, elastic fibers and muscle cells (KADAR et al. 1972; MYERS et al. 1973; WIGHT and Ross 1975; RIVA et al. 1977). In order to obtain further data concerning this subject, a histochemical ultrastruc- tural research was carried out that made it possible to visualize proteoglycan macro- molecules in greater detail. In this paper, 2 different ultrastructural aspects of these macromolecules and their assumed relationships with the other arterial components are presented. Materials and Methods A number of segments of 2 month-old rabbit thoracic aortas were placed in 0.1 MpH = 7.4 phosphate buffer and therein cut into rectangular-shaped slices, the long of which side was ar-