The thymus is a lobulated organ, as you will see best in examples of fetal autopsy thymus. Each lobule is made up of an outer cortex (the darker, more densely lymphocytic area) and an inner medulla (pale due to lower cellularity). Embedded invisibly within these areas is an extensive epithelial network, which is only noticeable on H&E as the squamous nests within the medulla called Hassall’s corpuscles (Figure 23.1) but can be nicely highlighted by staining for cytokeratins. The epithelial cells within the cortex and medulla differ in their antigen profile and are presumed to give rise to different varieties of thymoma.

The lymphocytes within the thymus are a mix of B and T cells . The T cells begin life as lymphoblasts at the subcapsular perimeter of the thymus (Figure 23.2); like other lymphoblasts they are positive for the blast marker TdT by immunostain. As they mature and become exposed to antigens, they migrate inward through the cortex and to the medulla, finally leaving the thymus from the medullary vessels as mature T cells. Before maturity these immature T cells are called thymocytes . The B cells are not undergoing maturation but may organize into germinal centers as they do everywhere else in the body. The presence of lymphoid follicles and germinal centers is not, by itself, pathologic, but the autoimmune disease myasthenia gravis is associated with follicular hyperplasia within the thymus.

The thymus gradually involutes throughout life, which is to say the lymphoepithelial structure is gradually replaced by fat. In an adult, all that may remain is a mass of fat in the mediastinum with microscopically visible nests of lymphocytes and Hassall’s corpuscles. It is the recognition of these corpuscles that enables you to definitively identify the tissue as the thymus. Most thymomas occur in adults, so evidently the involution is not protective against neoplastic transformation. The thymus may also be found anywhere along the anterior mediastinum, as far up as the thyroid, so always think of it when you see tiny squamous whorls within lymphoid tissue—interpreting ectopic thymus as metastatic carcinoma would be particularly egregious.

Thymic hyperplasia is more of a clinical diagnosis than a pathologic one, as the thymus appears histologically normal, but simply weighs more than it should for age. True thymic hyperplasia is not associated with prominent lymphoid follicles and may be unrelated to autoimmune disease. Thymic lymphoid hyperplasia (also called thymic follicular hyperplasia or thymic germinal center hyperplasia ), on the other hand, is characterized by prominent germinal centers and is often associated with myasthenia gravis. The weight or size of the thymus may be normal.

The term thymoma refers to tumors of thymic epithelium. Many types of thymoma also have a lymphocytic component, but the lymphocytes are polyclonal bystanders; thymomas are not lymphomas. Lymphomas do occur in the thymus but most are the familiar varieties covered in other chapters (Hodgkin disease, T cell lymphoblastic lymphoma, anaplastic large cell lymphoma, etc.) and will not be discussed here. Of the thymomas, there are three basic categories (Table 23.1): thymomas presumably arising from medullary epithelium (called type A, usually indolent), thymomas presumably arising from cortical epithelium (type B), and a mixed type called Type AB. What used to be called a “Type C” thymoma is now designated thymic carcinoma. The carcinomas are often squamous but may take the morphology of almost any other carcinomas found elsewhere in the body. The thymomas, however, are unique to the thymus.