Thymus
The thymus is a bi‐lobed organ in the cranial mediastinum of the thoracic cavity. It is composed of a network of epithelial cells filled with developing T cells (“thymocytes”) and fewer other hemopoietic cells. The epithelial cells are derived from the endoderm of the third pharyngeal pouch. The formation of the thymus starts around ED 11, and the initial population of the thymus by hemopoietic cells begins at ED 12. Under the influence of the transcription factor FOXN1, thymic epithelial precursor cells differentiate into cortical and medullary epithelial cells [10]. The cortex of the thymus is densely populated with immature T cells which obscure the presence of the cortical epithelial network on H&E sections. Macrophages containing apoptotic fragments of immature T cells (“tingible body macrophages”) are scattered throughout the cortex. The medulla is less densely populated with more mature T cells intermixed with macrophages, dendritic cells, B cells, and eosinophils. The medullary epithelial cells are plump with more cytoplasm compared with the dendritic morphology of the cortical epithelial cells. Medullary epithelial cells undergo a maturation process and undergo cornification upon terminal differentiation. Hassall's (thymic) corpuscles are clusters of cornified epithelial cells that are typical of the thymic medulla. They are not as well organized and less prominent in commonly used strains of mice such as C57BL/6 and BALB/c as in humans and many other mammals. However, certain mouse strains including NZW/LacJ and NZBW F1 mice have many Hassall's corpuscles [11] (Figure 7.2).
Figure 7.2 Thymus. (a, b). Medulla of thymus of a female C57BL/6J mouse (a) and a female NZW/LacJ mouse with prominent Hassall's corpuscles (b). (c) Cyst in thymus of a 24‐week old CBA/2J mouse. (d) Cystic remnant in FOXN1‐deficient NU/J mouse. Inset: higher magnification of ciliated epithelial cells.
Cervical thymus tissue may be found in up to 90% of mice depending on the genetic background. These are likely derived from remnants of thymic embryonal tissues that are left behind during the migration of primordial thymus from the pharyngeal region to the cranial mediastinum. The morphology ranges from small clusters of lymphocytes to well‐differentiated tissue with a distinct medulla and cortex typical of the thymus.
The thymus plays a critical role in T cell development. Every day, approximately 100 precursor T cells enter the thymus from the bone marrow. Differentiation, positive and negative selection, and maturation of T cells occur as the precursors migrate from the outer cortex to the medulla. Approximately 1 million mature T cells leave the thymus daily via blood vessels at the corticomedullary junction.
Examination of the thymus: For routine examination, H&E slides prepared from cross sections of the thymus fixed in formalin will suffice. For more detailed investigation of the stromal and epithelial cell composition, immunohistochemistry with antibodies against specific cell populations is necessary. To examine the effect of changes on T lymphocyte development, flow cytometry with multiple fluorochrome‐conjugated antibodies is required.
Aging‐associated changes: The thymus reaches its maximum size of 25–45 mg at about 5–6 weeks of age. When mice reach sexual maturity, the thymus undergoes involution with a gradual loss of lymphocytes. The loss of parenchymal tissue is partially compensated by an increase of stromal cells. There are marked differences between mouse strains in the weight of the thymus and degree of involution [12].
Cysts: Cysts are typically found in the medulla and at the corticomedullary junction of the thymus. They are lined by ciliated or squamous cells and may contain proteinaceous fluid, lymphocytes, and dendritic cells (Figure 7.2). The prevalence of these cysts is age and strain‐dependent.
Thymic hypoplasia and aplasia: The thymus and parathyroid glands are both derived from the endoderm of the third pharyngeal pouch. Neural crest‐derived mesenchyme contributes to the migration and early formation of the thymus, but the thymus in young adult mice contains few if any neural crest‐derived cells. Functional deletion of genes involved in the formation and patterning of the pharyngeal arches cause thymic aplasia or hypoplasia and frequently affect other tissues that are derived from these embryonal structures (Table 7.1). The most common defect in thymus development in humans is DiGeorge syndrome, caused by a deletion in chromosome 22q11.2, and characterized by cardiac and facial abnormalities and hypothyroidism. This chromosomal segment contains the Tbx1 gene and deletion of this gene in mice recapitulates the features of DiGeorge syndrome. A more selective defect in thymus development is induced by deletion of Foxn1, which is the genetic defect in the nude mouse. As mentioned earlier, FOXN1 is critical for the differentiation and maturation of both cortical and medullary epithelial cells, and spontaneous or engineered deletion induces thymic aplasia with sometimes only a cystic remnant present in the cranial mediastinum (Figure 7.2). FOXN1 also plays a role in the formation of the hair shaft as this transcription factor regulates various keratin gene. Nude mice have hair follicles, but lack visible hair because of abnormal and fragile hair shafts (see Chapter 10 on skin, hair, and nail for details).Mice with defects in the production of T lymphocytes have small thymuses comprised predominantly of epithelial and stromal cells with few lymphocytes. This includes mice with impaired formation of T cell receptors including mutations in the Prkdc gene that underlies the scid mutation [22], and in the Rag1 or Rag2 genes [23, 24], and mice with mutations in the Il2rg gene that encodes for the IL2 receptor common gamma chain (CD132) [25, 26].
Thymic hyperplasia: Focal hyperplasia of thymic epithelial cells has been reported in older B6C3F1 mice [27]. Focal lymphoid hyperplasia and development of follicles sometimes with germinal centers in the thymus medulla have been reported in NZB and NZBWF1 mice that are prone to systemic autoimmune disease similar to systemic lupus erythematosus [11, 28].
Secondary Lymphoid Organs
Lymph Nodes
Among invertebrate and vertebrate animals, lymph nodes are found only in mammals. Afferent lymph vessels collect lymph fluid from organs and tissues, which allows LNs to monitor peripheral tissues for infections and tissue damage and to act as a barrier to further dissemination of potential pathogens. Specialized mesenchymal stromal cells and endothelial cells secrete chemotactic factors that control the migration of lymphocytes that enter LNs via high endothelial venules and afferent lymph vessels. In this manner, LNs play an essential role in maximizing the chances for antigen to find the rare B and T lymphocytes that express the receptors for that specific antigen. The number of lymph nodes in mice raised in a clean environment is fairly constant with about 22 LNs found in specific locations [29].
Lymph node anlagen are formed as a result of interactions between lymphoid tissue organizer (LTo) cells and lymphoid tissue inducer (LTi) cells during ED14 to ED17 [30, 31]. Secretion of CXCL13 by LTo cells recruits LTi cells released from the fetal liver. Accumulation and activation of LTi cells results in further recruitment of LTi cells in a lymphotoxin and RANK‐dependent manner followed by the recruitment of B and T lymphocytes and differentiation of stromal cells in subtypes that populate the different compartments of the lymph node. LTi cells are a type of innate lymphoid cells, whereas the LTo cells include mesenchymal stromal cells and lymphatic endothelial cells. The relative
