ROLE OF THE PERIVASCULAR EPITHELIUM IN THE HISTOGENESIS OF HASSALLS CORPUSCLES - A MORPHOLOGIC AND IMMUNOHISTOLOGICAL STUDY

Thirteen human thymuses and one thymoma were morphologically and immun ohistologically investigated to define the histogenesis of Hassall's c orpuscles (HCs). The following monoclonal antibodies: antisquamous cyt okeratin on paraffin sections and TE-4 and TE-8 on frozen sections, we re used to show the distribution of the epithelial components; PAL-E o n frozen and anti-CD31 and anti-CD34 on paraffin sections detected the endothelial cell distribution. In the thymoma, epithelial onion-like structures, looking like true HCs, were found to originate from the pe rivascular epithelium lining dilatated spaces and some of them partial ly obliterated the space where the blood capillary showed thickened wa ll and endothelial regressive changes. Antisquamous cytokeratin staine d: (1) in the thymus: subcapsular, medullary, and HC epithelial cells; (2) in the thymoma: epithelial cells lining the perivascular spaces a nd forming HCs. TE-4 stained: (1) in the thymus: the subcapsular and m edullary epithelium; (2) in the thymoma: the epithelium lining the per ivascular spaces and epithelial cells forming HCs. TE-8 stained: (1) i n the thymus: HCs only; (2) in the thymoma: HCs and perivascular epith elial cells. PAL-E, CD31, and CD34, which specifically react with endo thelial cells, stained remnants of capillary structures in the core of some HCs. The results indicate that: (1) corpuscular structures in th ymoma originate from perivascular epithelium; (2) thymus medullary epi thelial cells stained by cytokeratin and TE-4 correspond to perivascul ar epithelial cells whose staining is well documented in thymoma; (3) the subcapsular-perivascular epithelium and HCs represent different st eps of differentiation of a single ectodermal cell lineage; (4) the PA L-E-, CD31-, and CD34-positive reaction in the core of some HCs sugges ts that the perivascular epithelium would be stimulated to transform i nto HCs as a consequence of endothelial changes with fragmentation of the capillary included in the perivascular space.