Central Lymphoid Organs
المؤلف:
APURBA S. SASTRY , SANDHYA BHAT
المصدر:
Essentials Of Medical Microbiology 2021
الجزء والصفحة:
3rd edition , p169-170
2025-08-25
550
Bone Marrow
Almost all the cells in blood have originated from pluripotent hematopoietic stem cells of bone marrow and the process is called hematopoiesis.
- In early fetal life, hematopoiesis occurs in liver; gradually the stem cells migrate to bone marrow. By birth, the stem cells occupy most of the bone marrow space of large bones
- As the individual ages, hematopoietic activity in large bones decreases and after puberty hematopoiesis is mostly confined to axial bones such as pelvis, vertebrae, sternum, skull and ribs
- The progenitor T and B cells originate in bone marrow. Further development of B cells occurs in bone marrow itself, whereas the progenitor T cells migrate to thymus for further proliferation.
Thymus
Thymus is the site of proliferation and maturation of T cells.
Development
Thymus is developed in the embryonic life (third month) from third/fourth pharyngeal pouch. It is highly active at birth, continues to grow for many years, reaches its peak size at puberty, and then it degenerates.
Structure
Thymus has two lobes surrounded by a fibrous capsule. Septa arising from capsule divide thymus into lobules, and each lobule is differentiated into an outer cortex and an inner medulla (Fig. 1).
Fig1. Thymus (cross-section of a portion, schematic).
Cortex is densely populated and contains:
- Thymocytes: Lymphocytes of thymus are called as thymocytes. The cortical thymocytes are immature and many in number
- Cortical epithelial cells and
- Nurse cells (specialized epithelial cells with long membrane extensions that surround many thymocytes).
Medulla is sparsely populated and contains:
- Thymocytes: Medullary thymocytes are relatively more mature and fewer in number
- Medullary epithelial cells
- Interdigitating dendritic cells and
- Hassall’s corpuscles: They are concentric layers of degenerating epithelial cells.
Thymic Hormones
Several thymic hormones such as thymulin, thymopoietin and thymosin are produced from the epithelial cells of thymus. They are believed to attract the precursor T cells (progenitor T cells) from bone marrow.
Maturation of T Cells
The cell-to-cell interaction between thymocytes and thymic stromal cells (including epithelial cells, dendritic cells and macrophages) and the effect of thymic hormones help in maturation of T cells in thymus.
Central Tolerance
One very interesting fact is that only 2–5% of the developing T cells become mature and released out from thymus; remaining T cells are destroyed as they are either not capable of recognizing major histocompatibility complex (MHC) or are believed to be self-reacting in nature.
- Destruction of such self-reacting T cells prevents development of autoimmunity (immune response against self-antigens)
- Such tolerance to self-antigens mediated by thymus that occurs in embryonic life is called as central tolerance.
Defect in Thymus
Any defect in thymus leads to defect in maturation of T lymphocytes that in turn results in severe life-threatening cell-mediated immunodeficiency disorders.
- DiGeorge syndrome: It is an immunodeficiency disorder in man, characterized by congenital aplasia of thymus
- Nude mice: Mice with congenital absence of thymus are called as nude mice.
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