Thymus Gland

Background

The thymus gland is located in the anterior mediastinum. It increases in size until puberty to approximately 20-50 gms and then undergoes progressive atrophy to 5-15 gm in older patients. It is pyramidal shaped with two fused lobes. It is divided into a cortex and medulla. There are characteristic structures known as Hassall corpuscles which are epithelial cells with keratinized cores.

The thymus is the site where bone marrow progenitor cells migrate and differentiate into T cells. Prothymocytes in the superficial cortex (CD2) give rise to cortical thymocytes (CD1a, CD2, CD3) and CD4 and CD8 (T helper and suppressor cells). Medullary thymocytes are fewer and larger and express CD4 or CD8. There are also scattered epithelial cells, macrophages, dendritic cells, and myoid cells.

Thymic neuroendocrine carcinoma (Thymic Carcinoid)

Thymoma

OUTLINE

Histopathological Features and Variants

Commonly Used Terms

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HISTOLOGICAL TYPES CHARACTERIZATION

B CELLS

B cells in epithelial and perivascular compartments of human adult thymus

Kristina G. Flores, PhD
Jie Li, BS
Laura P. Hale, MD, PhD

Hum Pathol 2001;32:926-934 Abstract quote

The thymus is the site of T-cell differentiation. However, the relatively recent observation that B cells are also present in the human thymus has prompted studies to determine the origin and function of these B cells.

Our studies show that phenotypically distinguishable B cell populations are located within both the thymic medulla and the thymic perivascular space and that cellular trafficking occurs between these compartments, including B cells trafficking from the periphery. The numbers of thymic B cells increase with age, correlating with increases in lymphocyte-rich regions of thymic perivascular space that are prominent between ages 10 and 50 years. B cells within both thymic epithelial and perivascular compartments contain mutated immunoglobulin VH sequences characteristic of post–germinal center B cells, suggesting that the B cells that most often give rise to thymic B-cell lymphomas may originate from either the thymic medulla or perivascular space.

VARIANTS

SKIN

Dermal thymus. A light microscopic and immunohistochemical study.

Barr RJ, Santa Cruz DJ, Pearl RM.

Department of Dermatology, University of California Irvine Medical Center 92668.

Arch Dermatol 1989 Dec;125(12):1681-4 Abstract quote

Two rare cases of distinctive thymic remnants occurring in the skin are described.

The lesions were present at birth, and involved the side of the neck. One child had a complete cleft lip and palate. The other had multiple congenital anomalies consistent with a rare syndrome entitled branchio-oculo-facial syndrome.

Microscopically, lobular foci were present in the dermis and consistent with both prethymic and thymic remnants. Immunohistochemical studies using antibodies to T cells and B cells showed a distribution similar to that seen in normal thymus.

Dermal thymus appears to be a distinct entity and may be associated with other faciobranchial defects.

RHABDOMYOMATOUS MULTILOCULAR THYMIC CYST

Rhabdomyomatous Multilocular Thymic Cyst

Runjan Chetty, MBChB, FRCPath, FRCPC, DPhil,1 and Anu Reddi, MBChB, FCS
Am J Clin Pathol 2003;119:816-821 Abstract quote

The thymus is the seat of a diverse array of pathologic conditions given its embryologic roots. Multilocular thymic cysts, although well described, are uncommon, and one associated with rhabdomyomatous elements has not been described previously.
A 15-year-old boy complained of sudden-onset chest pain of a month's duration, but was otherwise well. Chest radiographs localized the mass to the anterior mediastinum, arising from the thymus. A computed tomography scan demonstrated the lesion to be a multilocular fluid-containing cyst.

A large, 15-cm cyst contiguous with the thymus was removed. Histologic evaluation confirmed a multilocular cyst lined mainly by mucinous epithelium with focal areas of ciliated and squamous lining. A prominent finding was skeletal muscle elements in the form of spider cells, strap-like cells, and foci reminiscent of fetal-type muscle with cross-striations. At the periphery of the cyst, thymic tissue with branching ducts and Hassall corpuscles were noted. No evidence of skin and/or its appendage structures, cartilage, or other differentiated tissue was seen despite generous sampling of the specimen.

The muscle elements, most likely, were derived from thymic myoid cells, while the multilocular cyst arose from remnants of the thymomedullary system.

Henry JB. Clinical Diagnosis and Management by Laboratory Methods. Twentieth Edition. WB Saunders. 2001.
Rosai J. Ackerman's Surgical Pathology. Eight Edition. Mosby 1996.
Sternberg S. Diagnostic Surgical Pathology. Third Edition. Lipincott Williams and Wilkins 1999.
Robbins Pathologic Basis of Disease. Sixth Edition. WB Saunders 1999.
DeMay RM. The Art and Science of Cytopathology. Volume 1 and 2. ASCP Press. 1996.
Weedon D. Weedon's Skin Pathology Second Edition. Churchill Livingstone. 2002
Fitzpatrick's Dermatology in General Medicine. 5th Edition. McGraw-Hill. 1999.
Weiss SW and Goldblum JR. Enzinger and Weiss's Soft Tissue Tumors. Fourth Edition. Mosby 2001.

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Last Updated 5/25/2003

Histopathological Features and Variants
Commonly Used Terms
Internet Links

HISTOLOGICAL TYPES	CHARACTERIZATION
B CELLS
B cells in epithelial and perivascular compartments of human adult thymus Kristina G. Flores, PhD Jie Li, BS Laura P. Hale, MD, PhD	Hum Pathol 2001;32:926-934 Abstract quote The thymus is the site of T-cell differentiation. However, the relatively recent observation that B cells are also present in the human thymus has prompted studies to determine the origin and function of these B cells. Our studies show that phenotypically distinguishable B cell populations are located within both the thymic medulla and the thymic perivascular space and that cellular trafficking occurs between these compartments, including B cells trafficking from the periphery. The numbers of thymic B cells increase with age, correlating with increases in lymphocyte-rich regions of thymic perivascular space that are prominent between ages 10 and 50 years. B cells within both thymic epithelial and perivascular compartments contain mutated immunoglobulin VH sequences characteristic of post–germinal center B cells, suggesting that the B cells that most often give rise to thymic B-cell lymphomas may originate from either the thymic medulla or perivascular space.
VARIANTS
SKIN
Dermal thymus. A light microscopic and immunohistochemical study. Barr RJ, Santa Cruz DJ, Pearl RM. Department of Dermatology, University of California Irvine Medical Center 92668.	Arch Dermatol 1989 Dec;125(12):1681-4 Abstract quote Two rare cases of distinctive thymic remnants occurring in the skin are described. The lesions were present at birth, and involved the side of the neck. One child had a complete cleft lip and palate. The other had multiple congenital anomalies consistent with a rare syndrome entitled branchio-oculo-facial syndrome. Microscopically, lobular foci were present in the dermis and consistent with both prethymic and thymic remnants. Immunohistochemical studies using antibodies to T cells and B cells showed a distribution similar to that seen in normal thymus. Dermal thymus appears to be a distinct entity and may be associated with other faciobranchial defects.
RHABDOMYOMATOUS MULTILOCULAR THYMIC CYST
Rhabdomyomatous Multilocular Thymic Cyst Runjan Chetty, MBChB, FRCPath, FRCPC, DPhil,1 and Anu Reddi, MBChB, FCS	Am J Clin Pathol 2003;119:816-821 Abstract quote The thymus is the seat of a diverse array of pathologic conditions given its embryologic roots. Multilocular thymic cysts, although well described, are uncommon, and one associated with rhabdomyomatous elements has not been described previously. A 15-year-old boy complained of sudden-onset chest pain of a month's duration, but was otherwise well. Chest radiographs localized the mass to the anterior mediastinum, arising from the thymus. A computed tomography scan demonstrated the lesion to be a multilocular fluid-containing cyst. A large, 15-cm cyst contiguous with the thymus was removed. Histologic evaluation confirmed a multilocular cyst lined mainly by mucinous epithelium with focal areas of ciliated and squamous lining. A prominent finding was skeletal muscle elements in the form of spider cells, strap-like cells, and foci reminiscent of fetal-type muscle with cross-striations. At the periphery of the cyst, thymic tissue with branching ducts and Hassall corpuscles were noted. No evidence of skin and/or its appendage structures, cartilage, or other differentiated tissue was seen despite generous sampling of the specimen. The muscle elements, most likely, were derived from thymic myoid cells, while the multilocular cyst arose from remnants of the thymomedullary system.