Thymus
| Thymus | |
|---|---|
 Thymus | |
 The thymus of a full-term fetus, exposed in situ. | |
| Details | |
| Precursor | third branchial pouch |
| Artery | derived from internal mammary artery, superior thyroid artery, and inferior thyroid artery |
| Nerve | vagus |
| Lymph | tracheobronchial , parasternal |
| Identifiers | |
| MeSH | D013950 |
| TA98 | A13.1.02.001 |
| TA2 | 5152 |
| FMA | 9607 |
| Anatomical terminology | |
In human anatomy, the thymus is an organ located in the upper anterior portion of the chest cavity just behind the sternum. Hormones produced by this organ stimulate the production of certain infection-fighting cells. It is of central importance in the maturation of T cells.
Development
Embryology
The two main components of the thymus, the lymphoid thymocytes and the thymic epithelial cells, have distinct developmental origins. The thymic epithelium is the first to develop, and appears in the form of two flask-shape endodermal diverticula, which arise, one on either side, from the third branchial pouch (pharyngeal pouch), and extend lateralward and backward into the surrounding mesoderm and neural crest-derived mesenchyme in front of the ventral aorta.
Here they meet and become joined to one another by connective tissue, but there is never any fusion of the thymus tissue proper. The pharyngeal opening of each diverticulum is soon obliterated, but the neck of the flask persists for some time as a cellular cord. By further proliferation of the cells lining the flask, buds of cells are formed, which become surrounded and isolated by the invading mesoderm. Additional portions of thymus tissue are sometimes developed from the fourth branchial pouches. [1]
During the late stages of the development of the thymic epithelium, hematopoietic lymphoid cells from bone-marrow precursors immigrate into the thymus and are aggregated to form lymphoid follicles.
The thymus continues to grow between birth and puberty and then begins to atrophy, a process directed by the high levels of circulating sex hormones. Proportional to thymic size, thymic activity (T cell output) is most active before puberty. Upon atrophy, the size and activity are dramatically reduced, and the organ is primarily replaced with fat (a phenomenon known as "organ involution"). The atrophy is due to the increased circulating level of sex hormones, and chemical or physical castration of an adult results in the thymus increasing in size and activity. [2] Patients with the autoimmune disease Myasthenia gravis commonly (70%) are found to have thymic hyperplasia or malignancy.[3] The reason or order of these cirumstances has yet to be determined by medical scientists.
| Age | Mass |
| birth | about 15 grams; |
| puberty | about 35 grams |
| twenty-five years | 25 grams |
| sixty years | less than 15 grams |
| seventy years | less than ½ grams |
Structure
Each lateral lobe is composed of numerous lobules held together by delicate areolar tissue; the entire organ being enclosed in an investing capsule[4] of a similar but denser structure. The primary lobules vary in size from that of a pin's head to that of a small pea, and are made up of a number of small nodules or follicles.
The follicles are irregular in shape and are more or less fused together, especially toward the interior of the organ. Each follicle is from 1 to 2 mm in diameter and consists of a medullary and a cortical portion[5], and these differ in many essential particulars from each other.
Cortex
The cortical portion is mainly composed of lymphoid cells, supported by a network of finely-branched epithelial reticular cells, which is continuous with a similar network in the medullary portion. This network forms an adventitia to the blood vessels.
The cortex is the location of the earliest events in thymocyte development, where T cell receptor gene rearrangement and positive selection takes place.
Medulla
In the medullary portion, the reticulum is coarser than in the cortex, the lymphoid cells are relatively fewer in number, and there are found peculiar nest-like bodies, the concentric corpuscles of Hassall. These concentric corpuscles are composed of a central mass, consisting of one or more granular cells, and of a capsule formed of epithelioid cells. They are the remains of the epithelial tubes, which grow out from the third branchial pouches of the embryo to form the thymus. Each follicle is surrounded by a vascular plexus, from which vessels pass into the interior, and radiate from the periphery toward the center, forming a second zone just within the margin of the medullary portion. In the center of the medullary portion there are very few vessels, and they are of minute size.
The medulla is the location of the latter events in thymocyte development. Thymocytes that reach the medulla have already successfully undergone T cell receptor gene rearrangement and positive selection, and have been exposed to a limited degree of negative selection. The medulla is specialised to allow thymocytes to undergo additional rounds of negative selection to remove auto-reactive T cells from the mature repertoire. The gene AIRE is expressed in the medulla, and drives the transcription of organ-specific genes such as insulin to allow maturing thymocytes to be exposed to a more complex set of self-antigens than is present in the cortex.
Vasculature
The arteries supplying the thymus are derived from the internal mammary, and from the superior thyroid and inferior thyroids.
The veins end in the left innominate vein, and in the thyroid veins.
The nerves are exceedingly minute; they are derived from the vagi and sympathetic nervous system. Branches from the descendens hypoglossi and phrenic reach the investing capsule, but do not penetrate into the substance of the organ.
Cancer
Two primary forms of tumours originate in the thymus.
Tumours originating from the thymic epithelial cells are called thymomas, and are found in about 10-15% of patients with myasthenia gravis.[6] Symptoms are sometimes confused with bronchitis or a strong cough because the tumor presses on the recurrent laryngeal nerve. All thymomas are potentially cancerous, but they can vary a great deal. Some grow very slowly. Others grow rapidly and can spread to surrounding tissues. Treatment of thymomas often requires surgery to remove the entire thymus. Tumours originating from the thymocytes are called thymic lymphomas.[6]
Other animals and second thymus
The thymus is also present in most vertebrates, with similar structure and function as the human thymus. Some animals have multiple secondary (smaller) thymi in the neck; this phenomenon has been reported for mice [7] and also occurs in 5 out of 6 human fetuses.[8] As in humans, the Guinea pig's thymus naturally atrophies as the animal reaches adulthood, but in the hairless "Skinny pig" breed (which arose from a spontaneous laboratory mutation) it often possesses no thymic tissue whatsoever, and the organ cavity is replaced with cystic spaces.
When animal thymic tissue is sold in a butcher shop or at a meat counter, thymus is known as sweetbread.
References
- ^ Swiss embryology (from UL, UB, and UF) qblood/lymphat03
- ^ Sutherland JS. Activation of thymic regeneration in mice and humans following androgen blockade. J Immunol 2005 15;175(4):2741-53
- ^ Kumar, Parveen, Michael Clark (2002). Clinical Medicine 5th edn. Saunders. p. 1222. ISBN 0-702-02606-9.
{{cite book}}: CS1 maint: multiple names: authors list (link) - ^ Histology image: 07403loa – Histology Learning System at Boston University
- ^ Histology image: 07401loa – Histology Learning System at Boston University
- ^ a b Huete-Garin, A. (2005). "Chapter 6: "Mediastinum", Thymic Neoplasm". In J.K.T. Lee, S.S. Sagel, R.J. Stanley and J.P. Heiken (ed.). Computed Body Tomography with MRI Correlation. Philadelphia: Lippincott Williams & Wilkins. pp. pp. 311-324. ISBN 0781745268.
{{cite book}}:|pages=has extra text (help); Unknown parameter|coauthors=ignored (|author=suggested) (help)CS1 maint: multiple names: editors list (link) - ^ Terszowski G et al. (2006) Evidence for a Functional Second Thymus in Mice. Science. 2 March 2006. PMID 16513945
- ^ Surprise organ discovered in mice, Nature News, 2 March 2006
Millington GWM, Buckingham JC. (1992) Thymic peptides and neuroendocrine immune communication. J Endocrinol 133: 163-168