[THYMUS HISTOLOGY]. Thymus is an indispensable organ for the maturation of T-cells in fetal and early post-natal life. It is a lymphoid organ composed of two lobes connected to each other by an isthmus. Entire thymus is covered by a thin capsule. Two anatomically and functionally distinct regions can be identified, the outer cortex (top yellow arrow) and the inner medulla (yellow arrowhead). Fibrous septae arising from the capsule (bottom black arrows) penetrate as far deep as the corticomedullary junction creating numerous thymic lobules.
[THYMUS HISTOLOGY]. The cortical region of each lobule is separated from the adjacent one by fibrous capsule (arrows) but the medullary region is shared by all lobules. The cortex appears darker because of the much higher number of precursor T-cells than medulla. Early in embryo, thymus is populated by precursor T-cells produced in yolk sac and liver but with further development precursor T-cells are derived from bone marrow. Blood and lymphatic vessels traverse through fibrous septa as well as nerve supply to the organ.
[THYMUS HISTOLOGY]. With ageing in postnatal life thymus is progressively replaced by adipose tissue (oblique line dividing lobules). Thymic mass is reduced by involution in adults but remnants of thymic tissue can be identified in the anterior mediastinum. Note the fibrous septum (top arrow), cortex (central arrow), and medulla (bottom arrow).
[THYMUS HISTOLOGY]. A cytokeratin immunostaining highlights the lobulated architecture of the organ. The outer region is mostly composed of cortex whereas the inner region as outlined by arbitrary lines is the medulla.
[THYMUS HISTOLOGY]. Beneath the capsule a continuous layer of thymic epithelial cells is present (right arrow), which essentially forms a blood-thymic barrier around blood vessels entering and leaving the capsule. Just underneath this epithelial layer is a thin subcapsular cortical region composed of precursor T-cells, called thymocytes, that are derived from bone marrow (or from yolk sac and fetal liver in embryonic stages). These are the most undifferentiated precursor T-cells (thymocytes) which progressively mature and differentiate into distinct T-cell types. The big central arrow indicates the cortex rich in thymocytes whereas the lighter region indicated by the arrowhead is the medulla not as densely populated by thymocytes as the cortex.
[THYMUS HISTOLOGY]. TdT+CD7+ Precursor T-cells from bone marrow undergo progressive differentiation as they enter into the thymus. The process involves acquisition of certain cell surface antigens including CD1a, CD2, CD3, CD5, and both CD4/CD8. In addition to thymocytes (black arrowheads), thymic epithelial cells comprise another major cellular component (yellow arrows). The epithelial cells are round to oval cells that not only provide scaffolding to the tissue but also provide a microenvironment where maturation of T-cells occurs. Thymic epithelial cells contain long processes creating an interconnected network and a framework of supporting cells. The smaller darker thymocytes undergo maturation directed by self-MHC recognition. Thymocytes that fail to recognize self-MHC and those that react to self-MHC undergo apoptotic death. Immunohistochemically, most cortical thymocytes express both CD4 and CD8.
[THYMUS HISTOLOGY]. A cytokeratin immunostaining highlights scattered thymic epithelial cells, which provide a microenvironment for the development of thymocytes.
[THYMUS HISTOLOGY]. Thymic epithelial cells form a continuous external layer just underneath the capsule that follows the lobulation and ensheath the blood vessels entering and leaving the organ creating a functional blood-thymic barrier (top curved arrow). This epithelial layer not only provides a structural support and barrier it also provides a functional microenvironment. Note small thymocytes (arrowheads) and slightly larger histiocytes (arrows) present in the subcortex.
[THYMUS HISTOLOGY]. Immunohistochemcal staining for CD3 shows positive cytoplasmic staining in cortical thymocytes. The precursor T-cells, called T-lymphoblasts, undergo progressive maturation and differentiation into mature T-cells. Thymocytes move down from subcapsular region into the medulla. Cortical thymocytes also express CD1, CD7, CD5 and both CD4 and CD8 together, whereas medullary thymocytes express either CD4 or CD8 but not both.
[THYMUS HISTOLOGY]. The thymic medulla contains fewer thymocytes than the cortex (arrowheads). The thymocytes here are either CD4+ or CD8+ and do not express CD1. The thymic epithelial cells are larger cells with pale cytoplasm and round to oval nuclei (arrows).
[THYMUS HISTOLOGY]. A cytokeratin immunostaining of the medulla shows scattered epithelial cells (arrows) among smaller thymocytes (arrowheads) which are negative for cytokeratin.
[THYMUS HISTOLOGY]. Hassall’s corpuscles are a characteristic feature of thymic medulla and composed of a concentric arrangement of flattened thymic epithelial cells in a whorl-like formation. They are present at birth and increase in number with ageing. Their exact function is unknown but they are probably involved in phagocytosis cellular debris formed as a result of apoptosis. The center of Hassall’s corpuscles is keratinized.
[THYMUS HISTOLOGY]. A cytokeratin immunostaining of the medulla shows a large Hassall’s corpuscle and a few epithelial cells intensely positive for pan cytokeratin. Note the epithelial cells converging onto the Hassall’s corpuscle.
[THYMUS HISTOLOGY]. Thymus does contain scattered B-cells but B-lymphoid follicles are normally not seen. However, thymic hyperplasia is characterized by proliferation of B-lymphoid follicles with reactive germinal centers (arrow). A thymic Hassall’s corpuscle is shown (arrowhead).
[THYMUS HISTOLOGY]. The CD21 shows follicular dendritic cell network in a reactive lymphoid follicle.
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