Thymus
The thymus is a bilobed primary lymphoid organ located in the cranial mediastinum (thoracic portion) with extensions along the ventral trachea in the neck (cervical portion). It is essential for the development and education of T-lymphocytes: immature thymocytes from the bone marrow undergo positive and negative selection in the thymic cortex and medulla to produce self-tolerant, MHC-restricted T-cells. The thymus is largest and most active in the fetus and neonate, reaching maximum size in the first months of life before undergoing gradual involution. By adulthood, it is largely replaced by adipose tissue, though small islands of functional thymic tissue persist.
The thymus is a primary lymphoid organ where T-lymphocyte maturation and selection occurs. It is largest in foals and young horses, undergoing progressive fatty involution with age (largely replaced by fat by 2-3 years, though remnants persist). In foals, the cervical portion extends along the trachea and may be palpable. Thymomas in adult horses arise from residual thymic tissue in the cranial mediastinum and are typically diagnosed by thoracic ultrasound, radiography, and cytology of pleural fluid.
Neoplasm of thymic epithelial cells. The most common cranial mediastinal mass in adult horses. May be associated with paraneoplastic syndromes.
Lymphoid neoplasia involving the thymus, typically in young horses. Distinguished from thymoma by the monomorphic lymphoid cell population on cytology.
Lethal autosomal recessive disorder in Arabian foals causing complete absence of functional T and B lymphocytes due to a DNA-PKcs gene mutation. The thymus is hypoplastic and lacks lymphocytes.
Neoplasm of thymic epithelial cells. The most common cranial mediastinal mass in adult horses. May be associated with paraneoplastic syndromes. Signs include: Pleural effusion; ventral edema of head and forelimbs (cranial vena cava compression); dyspnea; jugular distension; weight loss
Lymphoid neoplasia involving the thymus, typically in young horses. Distinguished from thymoma by the monomorphic lymphoid cell population on cytology. Signs include: Rapidly enlarging cranial mediastinal mass; pleural effusion; respiratory distress; cranial vena cava syndrome; may respond partially to corticosteroids initially
Lethal autosomal recessive disorder in Arabian foals causing complete absence of functional T and B lymphocytes due to a DNA-PKcs gene mutation. The thymus is hypoplastic and lacks lymphocytes. Signs include: Recurrent infections starting at 2-5 months (when maternal antibodies wane); pneumonia (often adenoviral or Pneumocystis); diarrhea; failure to thrive; death by 5 months; no response to antimicrobial treatment
The thymus is a primary lymphoid organ where T-lymphocyte maturation and selection occurs. It is largest in foals and young horses, undergoing progressive fatty involution with age (largely replaced by fat by 2-3 years, though remnants persist). In foals, the cervical portion extends along the trachea and may be palpable. Thymomas in adult horses arise from residual thymic tissue in the cranial mediastinum and are typically diagnosed by thoracic ultrasound, radiography, and cytology of pleural fluid.
See the Thymus on our interactive 3D horse model. Open 3D Model