[Pathology] Atlas of Reactive, Developmental, Inflammatory, and Tumorlike Conditions (part 3)
Fig. 1.20 Multilocular thymic cyst of a lymphoepithelial type shows
cystically dilated cavities surrounded by dense lymphoid tissue with
well-developed lymphoid follicles containing germinal centers. The
main differential diagnosis for these lesions involves MALT lymphoma
of the thymus, which can also show prominent cystic changes and lymphoid
follicles but which will have a monotonous population of mildly
atypical small lymphocytes infiltrating residual Hassall’s corpuscles to
form lymphoepithelial lesions
Fig. 1.21 At higher magnification, the thymic lymphoepithelial cyst
shows a cystic cavity lined by low cuboidal to stratifi ed squamous epithelium
and surrounded by a dense cuff of lymphoid tissue. In one area,
the lymphoid tissue is surrounding residual atrophic Hassall’s corpuscles (arrows)
Fig. 1.22 Thymic lymphoid follicular hyperplasia in a 45-year-old
patient with myasthenia gravis shows a slightly enlarged and thickened
thymus gland that weighed 65 g (normal for age, 25 ± 12 g). Notice the
normal-appearing, smooth outer surface and conserved normal shape of
the gland
Fig. 1.23 Histologic appearance of lymphoid follicular hyperplasia in
a patient with myasthenia gravis, showing an enlarged lymphoid follicle with
a prominent germinal center. Notice residual involuting
Hassall’s corpuscles at the periphery of the lymphoid follicle
Fig. 1.24 Gross appearance of “pure” (or “true”) thymic hyperplasia.
The thymus is enlarged to at least four times its normal size and weight.
The outer surface is smooth and shiny, and the enlargement is uniform.
This patient had no history of myasthenia gravis or other autoimmune
disorder; the lesion was found incidentally on a routine chest X-ray.
This process can also be seen as a complication of chemotherapy in
patients with Hodgkin lymphoma and germ cell tumors in adults. It can
also follow chemotherapy in cancer patients or antiretroviral therapy for
HIV infection
Fig. 1.25 Histologic appearance of the thymus in “pure” thymic
hyperplasia shows an essentially normal thymus, with preservation of
the cortical-medullary architecture and no evidence of lymphoid follicular
hyperplasia. The only abnormality is the increased size and
weight of the gland. The absence of sheeting and confluence of the
lymphocytes, the preservation of the normal architecture with abundant
Hassall’s corpuscles, and the absence of a thick fi brous capsule surrounding
the organ distinguish this condition from a well-differentiated
lymphocyte-rich thymoma
Fig. 1.26 Involuting thymus of the adult in a 54-year-old woman who
died of other causes. The reverse process of thymic hyperplasia is
thymic involution, which is a normal physiologic process that starts after
puberty and progresses into adulthood. The thymus loses volume and
becomes almost entirely replaced by fat, with only scattered microscopic islands
of residual, atrophic thymic epithelium seen on histologic
examination. It must be emphasized that the thymus does not
disappear under normal physiologic conditions; it remains as a
welldefined structure in the mediastinum
Fig. 1.27 Histologic appearance of the thymus in normal involution
shows small, residual islands of thymic epithelium surrounded by scant
lymphocytes. Notice that the thymic islands are separated by abundant
fatty tissue, which is replacing the original thymic parenchyma. Loss of
thymic parenchyma is experienced mainly at the expense of the immature
T-lymphoid cell population, which is no longer being actively
recruited to the thymus for the programming of T-memory cells
Fig. 1.28 At higher magnification, an involuting island of thymic
epithelium from an adult shows residual strands of epithelial cells admixed
with scant small lymphocytes. Notice a small gland-like structure,
which is a normal part of the process. Occasionally, these microscopic
glandular structures can display a rosette-like or trabecular architecture
resembling neuroendocrine rests
Fig. 1.29 An elongated strand of involuting thymic epithelium
(arrows) is seen arising from a small island of residual thymus (lower
right). Sometimes these elongated strands of atrophic thymic epithelium
can show complex branching and adopt a netlike configuration
like that observed in acquired multilocular thymic cysts
Continue of part 4:
[Pathology] Atlas of Reactive, Developmental, Inflammatory, and Tumorlike Conditions (part 4)
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