Thymosins and thymic hormones
Also known as: thymosin α1, thymulin, thymopoietin, thymic hormone
A family of peptides (thymosin α1, thymulin, thymopoietin) produced by thymic epithelial cells that promote the differentiation and maturation of T lymphocytes.
Overview
The thymus is both a primary lymphoid organ and an endocrine gland. Its epithelial reticular cells secrete peptides that, together with direct cell–cell contact and cytokines (IL-7), support the maturation of T-cell precursors arriving from the bone marrow: thymosin α1 (enhances T-cell differentiation and cytokine production), thymulin (a zinc-dependent nonapeptide that induces T-cell markers) and thymopoietin (influences early thymocyte development and neuromuscular transmission). Thymosin β4, named in the same family, is in fact a ubiquitous intracellular actin-sequestering protein.
Thymic hormone output is greatest in childhood. From puberty the thymus undergoes involution, with its parenchyma progressively replaced by adipose tissue, and thymic hormone levels fall — contributing to reduced naïve T-cell output in old age.
- Location
Source: epithelial reticular cells of the thymus (anterior superior mediastinum). Targets: developing T lymphocytes (thymocytes) and peripheral T cells.
Function
- Promote T-cell differentiation and maturation.
- Modulate immune responses.
- Physiology
Class: peptides. Mechanisms are partly paracrine within the thymus and incompletely defined.
- Regulation
Declines with age and thymic involution; reduced by glucocorticoids and zinc deficiency (thymulin).
Clinical significance
Thymic aplasia and therapeutic thymosin
DiGeorge syndrome (3rd/4th pouch failure) causes thymic hypoplasia and T-cell deficiency. Thymosin α1 (thymalfasin) is used in some countries as an immunomodulator in chronic hepatitis B/C and some cancers.
Compiled from standard anatomy and physiology references — see the Endocrine System sources. For education only; not medical advice.