Nonneoplastic and Nonhyperplastic Thymus in Myasthenia Gravis: An Immunohistochemical Study with Double Immunoenzymatic Labeling of Basement Membrane and Cellular Components

Abstract

Histologically normal thymus (type A) in patients with myasthenia gravis (MG) was immunohistochemically compared with hyperplastic MG thymus (type B) and normal non-MG thymus. In formalin-fixed, paraffin-embedded sections of ten type A, ten type B, and eight non-MG cases, the thymic epithelium and other cellular components were stained in conjunction with the basement membrane by a double immunoenzymatic method. This technique demonstrated a moderate architectural disturbance in type A thymus, with distended perivascular space (PVS), elongated medullary epithelium, and disrupted basement membrane. These changes were more prominent in type B thymus but were minimal to lacking in non-MG thymus. Compared with those in non-MG thymus, the myoid cells in MG thymuses of both types tended to cluster around the Hassall's corpuscles, with a slight decrease in number in type B but not in type A. B-lymphocytes were present in type B, type A, and non-MG thymuses in that order of abundance; the cells were confined to the medullary parenchyma in the non-MG group but were numerous both in the PVS and medulla in the MG groups. T-lymphocytes were increased in the expanded PVS of type A and B MG thymuses. The number of interdigitating reticulum cells was similar in the three groups, but the cellular distribution was more dispersed in MG thymuses of both types. These findings, although previously described in type B thymus, have not been well recognized in type A thymus. They support the view that a common abnormality (presumably chronic thymitis), differing in degree only, underlies MG thymuses regardless of the presence of follicular hyperplasia.

Comparison of the histological and immunohistochemical features of the thymus in young- and elderly-onset myasthenia gravis without thymoma

“…Also in MG patients without thymoma the thymus frequently seems atrophic in both microscopic and gross appearance, particularly in elderly patients, despite the presence of architectural changes such as distended perivascular space and disrupted basement membrane (9)(10)(11). In the present study severely atrophic thymus with abundant fat infiltration was seen in nine of the elderly MG patients (45%), and this positive rate was similar to that previously including CD23 and L29 (16)(17)(18).…”

The spectrum of micronodular thymic epithelial tumours with lymphoid B‐cell hyperplasia

“…The significance of the hyperplastic B‐lymphoid stroma still remains unclear. Lymphoid hyperplasia with germinal centres has been observed in thymus and thymoma with myasthenia gravis and other immune‐mediated diseases, 25 suggesting the important role of the thymus in the production of autoantibodies. In the present series, one case had myasthenia gravis and two had hypogammaglobulinaemia.…”

Tumours and tumour‐like conditions of the thymus other than thymoma; a practical approach

“…There are no firm criteria for diagnosing follicular thymic hyperplasia. Although finding lymph follicles with germinal centres is typical in myasthenia, this is not always the case, as the thymus gland in myasthenic patients may be devoid of lymphoid follicles and may even show an atrophic appearance altogether with only subtle changes 149–151 . Five grades of follicular thymic hyperplasia were distinguished by the myasthenia study group in Japan (grade 0, involuted thymus; grade I, accumulation of lymphocytes in the distended medulla; grade II, one follicle in one section; grade III, two to four follicles in one section; and grade IV, more than five follicles in one section or more than one follicle in each lobule) 152 .…”