~Thymus.&.Tonsils.Comprehensive.Histology.Jun.2022

Thymus- embryology, physiology, histology and pathologic correlations

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2. Outlines Introduction Embryology Anatomy and function Histology Age related changes Pathologic correlation: thymoma Tonsils 2

3. Introduction 3 Bilobed Encapsulated lymphoid organ Mediastinum Overlying the great vessels of the heart Thyme leaves

4. Embryology of the thymus 4 Origin: Endoderm, 3 rd and 4 th pharyngeal pouches 6 th week endodermal lining creates a saccule Thymic premordia 10 th Week prothymocytes populate thymus Antero-superior mediastinum Tonsils : 2 nd pharyngeal pouch ERCs

5. Developmental abnormalities Ectopic tissues Parathyroid tissue Sebaceous, salivary gland Ectopic thymus: Failure of the organ to migrate Submandibular, paratracheal areas Parathyroid or thyroid glands, lung, pleura Di- George syndrome: thymic aplasia T- cell deficiency 5

6. Anatomy Antero-superior mediastinum 20g at birth, 35-50g puberty Soft, roughly triangular, bilobed mass Base and borders A thyme leaf Capsulated Each lobe of the thymus is divided into several lobules Outer cortex & inner medulla 6

7. 7 Blood supply & innervation Arterial supply Internal mammary , superior & inferior thyroid arteries Venous drainage L eft brachiocephalic, internal thoracic & inferior thyroid veins Lymph vessels A rise in the interstitium of the lobular septa Merge to form large lymph vessels Innervation CN X and cervical sympathetic nerves

8. Functions Cell-mediated immunity Supply the circulating lymphocyte Immunological self-tolerance Secretion: thymulin , thymopoietin and thymosins Regulation of inflammation 8 Thymus

9. Role of the Thymus in T-Cell maturation and  selection 9 TCR (T-cell receptor) I Negative selection: in the medulla I Positive selection: in the cortex

10. FIG. Thymus gland (panoramic view). Stain: hematoxylin and eosin. Low magnification 10 An overlying capsule Sends trabeculae Form lobules: functional units Not completely separated Histology

11. FIG. Thymus gland (panoramic view). Stain: hematoxylin and eosin. Low magnification 11 Two compartments: Thymic epi thelial compartment Peri vascular compartment Histology

12. FIG: Normal lobular architecture of the thymus demonstrating clear separation between the cortex and medulla. Trabeculae, lobules seen 12 Cortex : outer darkly basophilic Medulla: inner lightly stained Staining differences Density of lymphoblasts Histology

13. (b) The infant thymus (a) is a lobulated organ invested by a loose collagenous capsule C from which interlobular septa S containing blood vessels radiate into the substance of the organ. The thymic tissue is divided into two distinct zones, a deeply basophilic outer cortex Cx and an inner eosinophilic medulla M; distinction between the two is most marked in early childhood, as in this specimen. (b) Examination of the thymus at low magnification reveals the lobules (L) composed of a dark-staining basophilic cortex (C) and a lighter-staining & relatively eosinophilic medulla (M) . The cortex contains numerous densely packed lymphocytes, whereas the medulla contains fewer lymphocytes. Hassall’s corpuscles (Arrows) 13

14. 14 Histology: Epithelial Cells AKA, epithelial reticular cells (ERCs) Cortical and medullary Plump, round- oval nuclei Prominent nucleoli Cytoplasm with numerous prolongations Cytoreticulum I Blood thymus barrier I T-cell maturation ROLES:

15. (a) LM of part of the cortex of a child’s thymus at high magnification. Many closely packed lymphocytes ( Ly ) with small, round, densely stained nuclei predominate. ERC s have large, round euchromatic nuclei, with prominent nucleoli. Their cytoplasm is eosinophilic. Processes of these cells ( arrows ) appear to invest capillaries ( Cap ), many of which are seen in transverse section. This pattern in the cortex constitutes the blood-thymus barrier, which limits access of blood-borne antigens to immature lymphocytes. 820×. H&E. 15 (b) The epithelial reticular cells throughout the cortex are APCs & extend long processes bound together by desmosomes to make the framework, the cytoreticulum, for the lymphocytes, having a cytoreticulum consisting of APC cellular processes. rather than the more common network of simple reticulin fibers, allows regulated specificity of lymphocyte binding via the changing antigens on MHC proteins. Some cortical epithelial cells also secrete cytokines that promote T-cell maturation Histology: Epithelial Cells

16. B. The medulla usually possesses varying numbers of circular bodies called Hassall’s, or thymic, corpuscles (HC) . Stain readily with eosin and can be distinguished easily with low magnification, as in top figure and lower left (arrows) . The center of a corpuscle, particularly a large one, may show evidence of keratinization and appear somewhat amorphous 16 Histology: Hassall’s corpuscles Concentric layers of flattened epithelial cells Central hyaline core often keratinizes Unique to medulla! Helps to distinguish Diameter 20 - 150μm Cytokeratins Thymic stromal thymopoietin

17. B. The medulla usually possesses varying numbers of circular bodies called Hassall’s, or thymic, corpuscles (HC) . The corpuscles are large concentric layers of flattened type VI epithelioreticular cells (Ep) . They stain readily with eosin and can be distinguished easily with low magnification, as in top figure and lower left (arrows) . The center of a corpuscle, particularly a large one, may show evidence of keratinization and appear somewhat amorphous B. A. 17

18. FIG. Hassall corpuscles showing ( A ) cystic dilatation with accumulation of cellular debris, ( B ) dystrophic calcification, and ( C ) accumulation of foamy macrophages 18 Changes in thymic corpuscles

19. FIG. Normal thymic cortex. There are numerous cortical thymocytes, Most of which have small nuclei with densely packed chromatin 19 Histology: thymocytes Predominant in cortex Size : variable Nucleus: Round - oval nucleus Nucleoli: 1-2 prominent nucleoli Cytoplasm: relatively abundant, strongly basophilic

20. B - cells Can be found aggregated as lymphoid follicles or scattered as individual cells Macrophages Mainly located in the cortex, show phagocytic activity Dendritic cells are mainly located in the medulla Eosinophils, Mast cells - few Neuroendocrine, myoid and plasma cells - rare! 20 Histology: other cells

21. IMMUNO HISTOCHEMISTRY Thymic Epithelial Cells Thymocytes: CD1, CD14, CD38, and TdT 21

22. 22 Age related and other changes Thymic Involution Physiologic starts at puberty Early stages: ↓Nꝋ of cortical thymocytes, sparing of the epithelial cells Advanced stages: Both depleted Cystic, adipose tissue Serpiginous

23. A: A slender, thin, serpiginous strand of atrophic thymic epithelium admixed with scant lymphocytes is seen coursing through the fat and originating from a small island of involuting thymus ( top right ). B: Immunohistochemical stain for p63 at higher magnification shows strong nuclear positivity in the thymic epithelial cells C: Microscopic island of involuting thymic epithelium is present in the fat which is composed of oval to spindle epithelial cells with eosinophilic cytoplasm and a sprinkling of small lymphocytes. D: Another microscopic island of involuting thymic epithelium is shown composed of atrophic and tightly packed spindle cells admixed with scattered small lymphocytes. 23 Age related and other changes

24. 24 Age related and other changes A: Thymic remnant showing elongated configuration with prominent spindling of the cells B: Involuting thymus with epithelial rosettes

25. A: Thymic remnant composed predominantly of small lymphocytes simulating a lymphoid nodule. Note the single row of flattened epithelial cells at the periphery. B: Thymic remnant composed of cortical and medullary portion, the latter containing a small Hassall corpuscle ( bottom half ) 25 Age related and other changes

26. FIG. Strands of residual thymic epithelium arranged in small nests resembling neuroendocrine growths 26 Age related and other changes

27. FIG (C) Residual thymic island with predominance of lymphocytes, small solid epithelial cell clusters at the periphery, and calcified Hassall corpuscle. ( D) Anastomosing strands of epithelial cells admixed with small lymphocytes embedded within a collagenized stroma 27 Age related and other changes

28. FIG. (A) Focus of involuting thymic epithelium showing cystic dilatation. The cells lining the cyst range from flat- to cuboidal- to columnar-ciliated epithelium. ( B) Another focus of involuting thymic epithelium shows cystically dilated spaces lined by a layer of cuboidal epithelial cells simulating glandular structures. ( C) Large, cystically dilated spaces are seen flanked by islands of involuting thymic epithelium. The island on the left shows the typical appearance of involuted thymus in adults; the island on the right simulates a lymph node. ( D) Immunostaining for p63 in the solid nodule resembling a lymph node highlights numerous scattered involuting thymic epithelial cells with strong nuclear positivity 28 Cystic degeneration

29. Restricted to neoplasms of thymic epithelial cells Nearly all present in adult life Largely solid , yellowish gray, and separated into lobules by septa 29 Pathologic correlation: thymoma

30. 30 Micro scopically: Neoplastic epithelial cells Epithelial cells : Round–polygonal (“plump”), stellate , or spindle- oval The Nuclei are vesicular and smooth contour Nucleolus +- conspicuous

31. 31 Pathologic correlation: thymoma

32. Tubal tonsil Invaginations of surface epithelium ( Ep ) form tonsillar crypts ( arrows ). Intensely basophilic areas under the epithelium contain densely aggregated lymphoid tissue ( LT ). 7.5× H&E Tonsils 32 Discrete aggregates of lymphoid nodules Part of MALT Waldeyer's ring Palatine tonsils Lingual tonsil Pharyngeal tonsil Tubal tonsil

33. FIG. 12.24 Photomicrograph of a palatine tonsil a. This low-magnification photomicrograph shows an H&E–stained palatine tonsil. The stratified squamous epithelium that forms the surface of the tonsil dips into the underlying connective tissue in numerous places, forming tonsillar crypts. 25. b. This higher-magnification photomicrograph of the rectangular area in a shows the stratified squamous epithelium ( SSE ) lining the tonsillar crypt. In the portion of the photomicrograph below the lumen of the crypt, SSE is well defined and separated by a connective tissue layer ( CT ) from the lymphatic nodule ( LN ). In the upper portion of the photomicrograph, the SSE is just barely recognized because of the heavy infiltration of lymphocytes; the epithelial cells are present, however, although they are difficult to identify. In effect, the lymphatic nodule has literally grown into the epithelium, distorting it and resulting in the disappearance of the more typical, well-defined epithelial–connective tissue boundary. 450. Palatine tonsil 33 Tonsil, Almond-shaped 1-2.5 cm Nonkeratinized stratified squamous Crypts Partial capsule

34. A. LM of the palatine tonsil at higher magnification. Under the surface epithelium ( Ep ) lie many lymphoid nodules ( LN ), some of which contain germinal centers ( white arrows ). Parts of the epithelium ( circle ) are unrecognizable and appear to be eroded because of extensive lymphocyte infiltration. Tonsillar crypts ( arrows ) vary in depth and appear as blind-ended invaginations of epithelium. A mucous gland ( MG ) sits close to the base of the tonsil, which is partly encapsulated by dense connective tissue ( CT ). 30×. H&E B. Epithelium ( E ) surrounding tonsillar crypts ( C ) often becomes infiltrated with lymphocytes and other leukocytes and can become difficult to recognize histologically. Adjacent connective tissue at the top of the photo also contains numerous lymphocytes. X200. H&e. 34

35. FIG. The posterior surface of the tongue has a relatively smooth stratified squamous epithelium ( E) overlying lymphoid tissue L containing lymphoid follicles ( F) Like the palatine tonsils epithelial crypts Cr penetrate the lingual tonsil Lingual tonsil 35 Pharyngeal and tubal tonsils Pseudostratified Respiratory lining epithelium Similar to palatine tonsils

36. Pharyngeal tonsil AKA adenoid , or tonsil of Luschka A single pyramidal -shaped Midline of the nasopharyngeal wall Does not have typical crypts! Adenoid hypertrophy 36

37. Refer ences Histology for Pathologists 5 th Ed. Junquiera’s Basic Histology Text and Atlas 13 th Ed. Netter's Essential Histology Netter’s Atlas of Human anatomy Ross Histology Text and Atlas 6 th Ed. Wheater's Functional Histology- A Text and Colour Atlas 6 th Ed. Color Atlas And Text Of Histology 7 th Ed. diFiore's Atlas of Histology with Functional Correlations 11 th Ed. Rosai & Ackerman’s Surgical Pathology 11 th Ed. Researchgate. net 37

38. The end! 38