📝 STANDARD DESCRIPTION:
Type: Primary lymphoid (immune) organ + endocrine.
Position:
- Two parts in young calf: Cervical thymus (in the neck along trachea) + Thoracic thymus (in cranial mediastinum, in front of heart).
- Connected by an isthmus.
Function: Site of T-lymphocyte (T-cell) maturation.
Age changes:
- LARGEST around birth & in young animal.
- Begins to shrink (involute) after puberty.
- In adult, replaced almost entirely by fat — only small remnants remain.
📚 DETAILED DESCRIPTION:
THE TRAINING SCHOOL FOR T-CELLS — THYMUS
The thymus is a soft, pinkish-grey organ that is huge in the newborn but slowly disappears with age. Its job is to make and 'train' T-lymphocytes that defend the body against germs.
POSITION (especially well-developed in calf):
• Cervical thymus: A long lobe lying along the trachea in the neck (sometimes reaching as far up as the larynx).
• Thoracic thymus: A larger lobe in the cranial mediastinum, in front of the heart, between the two lungs.
• A narrow isthmus connects the two parts at the thoracic inlet.
This 'two-piece' arrangement is typical of ruminants and pigs. In horses and dogs, the thymus is mostly thoracic.
STRUCTURE: Made of many small lobules. Each lobule has:
• An outer cortex — packed with rapidly dividing immature T-cells.
• An inner medulla — fewer cells, contains Hassall's corpuscles (whorls of degenerating epithelial cells, characteristic of thymus).
FUNCTION:
• Site where T-lymphocytes are 'educated' — they learn to recognize the body's own tissues vs. foreign material. T-cells that would attack self are killed; survivors are released into the blood.
• Also produces hormones (thymosin, thymopoietin) that further support immune cell development.
AGE INVOLUTION:
• At birth and in young calf — VERY LARGE (relative to body size).
• Reaches maximum at puberty.
• Then steadily shrinks; replaced by fat.
• In old animal — only a tiny fatty remnant.
CLINICAL/MEAT NOTE: The cervical thymus of calves is sold as 'sweetbread' in the meat trade.
// COMPARATIVE ANALYSIS
| HORSE |
Mainly thoracic; cervical part minimal. Involutes after puberty. |
| DOG |
Almost entirely thoracic; small cervical extension. |
| PIG |
Like ox — both cervical and thoracic parts well developed. |
| FOWL |
7 pairs of separate thymic lobes arranged in a row down each side of the neck (cervical only). No thoracic thymus. |
CLINICAL RELEVANCE
Thymic Lymphoma (calves & young dogs): Massive enlargement of the thymus → cranial mediastinal mass → presses on trachea & cranial vena cava → coughing, breathing difficulty, brisket oedema.
