Veterinary Immunology and Serology

THYMUS

• Thymus is the first lymphoid organ to develop in mammals.
• The thymus is located in the anterior mediastinum and develop from invaginations of the ectoderm.
• In horse, cattle, sheep, pigs, and chickens, it extends up to the neck as far as to the thyroid gland.
• In human it develops from 3^rd bronchial clefts (paired structure).
• The size of thymus varies considerably. It increases progressively during fetal and neonatal life and attains maximum size during puberty. After puberty it atrophies but remnants do persists in old age.

Structure

• It is a bilobed lympho- epithelial organ.
• Each lobe consists of lobules of loosely packed epithelial cells and  covered by a connective tissue capsule.
• The outer part of each lobule is cortex and inner part is called medulla.
• Cortex
  • It is composed of numerous lymphocytes (called thymocytes) of various stages of development , many epithelial cells (called reticular cells, which are stellate shaped with abundant cytoplasm and contact with other cells by desmoses) and a few macrophages.
  • Yolk sac, Bone marrow or foetal liver derived immature T (thymus dependent) cell lineage enter the thymic cortex through the blood vessels.
  • The capillaries that supply thymic cortex are surrounded by an abnormally thick basement membrane and a continuous layer of epithelial cells, which prevent antigen from entering thymic cortex.
  • Enroute the thymocytes mature and express receptor for antigens and surface markers.
  • Maturation begins in the cortex and as the immature thymocytes migrate towards medulla they come in contact with epithelial cells, macrophages and dendritic cells  where they undergo positive selection (imposes self MHC-restriction on T cells) and negative selection (results into central tolerance).
  • Because of the positive and negative selection of thymocytes, 90-95% thymocytes die in the thymus by process of apoptosis and only5-10% of thymocytes mature as mainly either CD4 ⁺ or CD8 ⁺ T-cells.
  • Thymic epithelial cells secrete hormones like thymopoietins, thymosins, thymulin, and thymostimulins.
  • Under the influence of these hormones the cells mature. Thus the medulla contains mostly mature T cells.
• Medulla
  • It contains few epithelial cells (reticular cells) and at places Hassall’s corpuscle (which are composed of tightly packed whorls of epithelial cells that may be remnants of degenerating cells).
  • Medulla also contain small thymocytes but majority are matured with surface markers (CD4⁺, CD8⁺) and receptors expressed over it.
  • Only mature T cells from medulla exit the thymus through efferent vessels into blood circulation and peripheral lymphoid tissues.
• Function
  • Differentiation of immature thymocytes to immunologically competent T lymphocytes
  • Development of secondary lymphoid organs like spleen, lymph nodes and other lymphoid tissues. As thymic dependent lymphocytes colonize in these organs