Thymus (anatomy)

Introduction

Since it was described by Galien (130-200 after JC), and in spite of the 2.000 years of history which were passed, the thymus remains a mysterious body.

The word thymus comes from the Latin derivation of the Greek Greek thymos θύμος, meaning warted outgrowth. However thymos indicates also the spirit or the heart. Thus, Galien and the Greeks thought that the spirit or the heart was localized in the thymus. Its activity and its size are maximum during the Puberté, before involuer at the adulthood. The thymus plays a very important part in the installation of the Immune system in the child by ensuring the maturation of the lymphocytes T so called thymocytes.

Histologically, it is an epithelial lymphoid body. It is surrounded by a conjunctive partition which emits spans delimiting of the lobules. Each lobule has a dark peripheral part, the cortex, and a clear central part: the médula.

Embryology

The development of the head and the neck is marked by the appearance of the apparatus branchial. Indeed, as of the fourth week the six branchiaux arcs are formed following the evagination in mésenchymateux fabric of five deep furrows: the ectoblastic and entoblastic pockets branchiales. It is not without pointing out to the formation of the slits branchiales fish and Amphibians. However to the human embryo, the entoblastic and ectoblastic pockets branchiales will not come into contact. The branchiaux arcs consist of a mésenchymateux axis papered outside ectoblast and internally of endoblaste. The derivatives of the branchiaux arcs are nervous, muscular and constitutive of the skeleton.

E.g. Facial nerve VII or, the muscles of the face, the clamp, part of the bone hyoïde and temporal bone derive from the arc branchial 2 (or arc hyoïdien).

The third pocket entobranchiale presents at its distal end two récessus one ventral and the other dorsal one. At the fifth week of the development, the epithelium of the ventral récessus of the third pocket is different out of parathyroidal fabric while the dorsal récessus forms the outline of the thymus. These glandular outlines lose their connections with the wall pharyngienne and the thymus migrates in caudal direction and médiale, involving with him lower parathyroid gland.

The body of the thymus quickly joined its final situation in the thorax, where it amalgamates with its counterpart on the opposite side. Its caudal, narrow and lengthened portion, segments in small fragments. These fragments usually disappear, but they can sometimes persist and are then enchased in the body of thyroid or are insulated (additional thymique small islands).

At the ninth week of gestation, the thymus only makes up of epithelial fabric, but at the tenth week, of small lymphoid cells migrate of the liver and osseous marrow towards the thymus and thus take part in division in lobules of the body. The differentiation between the cortex and the médulla is complete as from the fifteenth week.

The development of the thymus continues after the birth and until puberty. In the young child, the gland occupies a considerable place in the thorax, between the sternum ahead and the pericardium and the large vessels behind. In the adult the gland is atrophied and becomes not easily recognizable.

The study of the embryology of the thymus makes it possible to approach three pathologies:

1) Ectopic Thymus Because of migration of glandular fabrics, it is not rare to find vestiges on the way of migration. Cervical vestiges are sometimes observed on scanners or IRM.

2) Syndrome of Di-George (syndrome of the third and fourth pockets entobranchiales)

3) Hypoplasy of the thymus or absence of the thymus responsible for an immunizing deficit.

Anatomy

Because the thymus evolves/moves constantly, the radiologists have sometimes evil to differentiate the normal thymus from the pathological thymus. As Galien noticed it the first, the thymus increases by size of the birth until puberty. This observation was checked by William Hewson in 1777, who studied the evolution of the size of the thymus in the fetus and the child. With puberty, the thymus undergoes a process called “involution”, which defines the progressive decrease of the thymus with the age. During the involution, the epithelial fabric is atrophied in favor of fat fabric.

The thymus is located in the médiastin antéro-superior and antéro-means. It is a soft body, made up of two lobes linked on the line of centers except at the higher and lower end. The two lobes are generally of different size; occasionally they can, being either linked and forming only one mass then, or or separated by an intermediate lobe. Its volume increases in caudal direction. It presents four faces: former, higher and two side.

The thymus is projected between the 4th costal cartilage and the lower edge of thyroid, it is thus partially in the thorax and partially in the neck. It weighs 15 G with the birth. With puberty its weight reached 35g. Then, the involution of the thymus involves a reduction in its mass, it weighs 25 G at 20 years, less than 15 G at 60 years and less than 6 G at 70 years. The thymus measurement 5 cm length on 4 cm broad and 1 cm thickness. The thickness of the thymus was largely studied with the scanner and IRM. Baron and Al studied 154 normal scanners of thymus. It results from it that the average thickness of the thymus is of 1,1 cm (standard deviation of 0,4cm) in the group of the old patients from 6 to 19 years and 0,5 cm (standard deviation of 0,27 cm) in the group of old patients of more than 19 years. The maximum thickness was of 1,8 cm in the group of patients of less than 20 years and 1,3 cm in the second group. However, on the images IRM, the thymus is thicker (between 1,5 and 2 cm among old patients of more than 20 years.) This would be due on a better detection of fat fabric and board thymus with the IRM. The thymus grows of the birth until puberty then regresses gradually.

In the child, the thymus is red dark because of its rich person vascularization, but becomes gradually grayer, then yellow in the elderly by the fat fabric infiltration.

The thymus does not change significantly, but the lymphoid fabric is replaced little by little by fat fabric.

It is identifiable scanographiquement up to 35 years. With the scanner, the thymus seems a triangular bilobate structure localized in the former médiastin, generally former to the ascending aorta, the higher vena cava and the pulmonary ways. To differentiate the thymus from the other structures médiastinales like the lymphatic nodes or the higher sine of the pericardium can prove to be difficult.

The thymus is contained in the thymique cabin. The former wall of the latter is formed by the deep layer of the fascia cervical means in top and the sternopericardiac ligament in bottom. Its posterior wall is consisted the thyro-pericardial fascia and the subjacent pericardium. Laterally, the thymique cabin is delimited by the sheaths of the vessels of the neck.

Reports/ratios

Former: the thymus answers the muscles sterno-hyoïdien ahead and sterno-thyroid, with the higher sternopericardiac ligament, the thoracic vessels and the body and manubrium of the sternum.

Posterior: the thymus answers in top the trachea, and bottom at the lower part of the left artery common carotid, with the brachio-cephalic trunk arterial right, the pulmonary artery, the aortic arc, the higher vena cava and in the middle.

Side: the thymus answers the vasculo-nervous sheath of the neck, the plèvre and the lungs Superiors: the thymus is in connection with the lower edge of thyroid, coupled or distant of a few millimetre. In this last case, the sheath of thyroid is connected to the sheath of the thymus by the thyroid ligament.

Innervation and vascularization

The innervation of the thymus is sympathetic nerve by the cervico-thoracic and parasympathetic ganglion by the vagus.

The thymus is vascularized by the artery trabéculaire (resulting from the thoracic artery interns) and by branches of the lower thyroid artery. There exist often anastomoses between the arteries which vascularisent the thyroid one and the thymus. The venous drainage is carried out by the veins thyroid lower, thoracic left intern and brachiocephalic. The lymphatic ones of the thymus lead to the nodes of the lymphocentres médiastinal former transverse and thoracic internal.

Histology

The cortex is mainly made up of lymphocytes (also called thymocytes) like some epithelial and mésenchymateuses cells, while the médulla is made up of many epithelial cells and some thymocytes. The epithelial cells playing an important role in the maturation of the lymphocytes T, are named “children's nurse concealments” (cells feeder). Moreover, the receivers with the antigens of the thymocytes or TCR (T-concealment receptor) interact with the antigens of the major complex of histocompatibility or CMH carried by the epithelial cells. This stage of maturation makes it possible to eliminate the lymphocytes T autoréactifs.

Role

According to Galien, the thymus was the body in which the spirit or the heart was. Formerly; it was also described like a body of purification of the nervous system. A little later, one thought that the thymus was a cushion of protection of the rib cage or that it which controlled the fetal pulmonary function and néonatale. In 1832, Astley Cooper, surgeon and anatomist English suggested that the thymus had a major function in the organization, without however specifying it.

The thymus is a primary lymphoid body because specialized in the maturation of the lymphocytes in which the cells T of origin medullary undergo a cellular differentiation. This cellular differentiation is made by the acquisition of receiving S cellular or TCR ( T concealment receptor ). The TCR is a formed membrane glycoprotéine of a chain alpha and a chain beta.

The most immature thymocytes have neither TCR nor the Antigène S CD4 and CD8, they are called thymocytes double negative (CD4−CD8−) and account for 5% of the thymocytes. Maturation is done by the acquisition of antigens CD4 and double CD8 leading to the thymocytes positive (CD4+CD8+) constituting 80% of the thymocytes.

Two principal thymique hormones control these stages of differentiation of the prothymocytes:
- the thymopoïètine which acts on the feeder cells of the prothymocytes
- the thymuline (= thymique factor humoral) which stimulates the immunocompétence of the lymphocytes T.

Positively selected thymocytes that progress to the mature TCRhighCD4+CD8− gold TCRhighCD4−CD8+ individual-positive training course constitute broad 15% off thymocytes that ultimately leave the organ to form the majority off the peripheral T concealment repertory. Figure 1 has simplified view off the sequential steps off thymocyte differentiation within the context off the nonlymphoid compartment, the thymic microenvironment. -->

It is noteworthy that has off small minority potentially coil-reactive thymocytes achieves the CD4 individual-positive gold CD8 training course and are released from the organ. Accordingly, along with differentiation positive into CD4+ individual concealments, nap elements C not acquire the functional feature off typical helper concealments (that is, concealments whitebait to trigger and/or enhance year immune response in the periphery), goal rather differentiate into “regulatory” concealments (most off them bearing the CD4+CD25+FoxP3+ phenotype), which actually block has given immune response. Recent dated show that defects in such regulatory CD4+ T concealments may Be related to the occurrence off autoimmune vents (reviewed in).

Thymocyte differentiation occurs ace concealments migrate within the thymic lobules: TCR−CD4−CD8− and TCR+CD4+CD8+ are cortically located, whereas mature TCR+CD4+CD8− and TCR+CD4−CD8+ concealments are found in the medulla (Figure 2). Ace this journey proceeds, thymocytes interact with various components off the thymic microenvironment, has off three-dimensional network formed thymic epithelial concealments (TECs), macrophages, dendritic concealments, fibroblasts and extracellular matrix (ECM) components (Figure 2A). -->

(B) number molecular interactions take place between developing thymocytes and thymic has off epithelial concealments. Whereas has and B corresponds to interactions mediated by soluble secretory molecules produced by epithelial concealments (A) gold lymphocytes (b), the interaction shown in C involves has given peptide (red dowry) being presented by MHC (expressed by the epithelial concealment) to the TCR and corresponding accessory molecule in the thymocyte membrane. The interaction shown by (d) involves adhesion molecules and the respective membrane counter-receptors, and (E) depicts year interaction mediated by ECM ligand and receptor. Modified from.

In addition to the key interaction involving the TCR/peptide-MHC, in the context off CD8 gold CD4 molecules the thymic microenvironment influences thymocyte maturation via adhesion molecules and ECM; thesis interactions depending are for thymocyte migration. Moreover, microenvironmental concealments modulate thymocyte soluble differentiation by polypeptides, comprising (A) typical cytokines, such ace (IL) interleukin-1, IL-3, IL-6, IL-7, IL-8 and stem concealment Factor; (b) chemokines, including CXCL12, which preferentially attracts immature CD4−CD8− and CD4+CD8+ thymocytes, and CCL21, that exerts positive chemoattraction for mature individual thymocytes; and (c) thymic hormones such ace thymulin, thymopoietin, and thymosin-α1, that edge also act one the general process off thymocyte maturation. Interestingly, not only the thymic epithelium affects thymocyte behavior, goal thymocytes modulate epithelial nap thymic functions, ace exemplified by the role off interferon-γ in the expression off MHC molecules, extracellular ligands, and receptors. The various TEC/thymocyte interactions are summarized in Figure 2B, and Table 1 summarizes accession numbers off peptide and DNA sequence databases off selected human and foam proteins cited throughout this manuscript. -->

Sources

W (2006) The Thymus Has Common Target Organ in Infectious Diseases. PLoS Pathog 2 (6): e62

LANGMAN, SADLER: Medical embryology; 7th Edition Edition Pradel (2003). ISBN 2913996124

KAMINA : Clinical precis of anatomy: Volume 3

Edition Maloine (2004). ISBN-10: 2224027249

KAMINA: Clinical precis of anatomy: Volume 2

Edition Maloine (2002). ISBN-10: 222402723

Mizuki Nishino, MANDELEVIUM ● Simon K. Ashiku, MANDELEVIUM ● Olivier NR. Kocher, MANDELEVIUM Robert L. Thurer, MANDELEVIUM ● Phillip Mr. Boiselle, MANDELEVIUM ● Hiroto Hatabu, MANDELEVIUM, PhDThe: Thymus: In Understanding Review.

From the Departments off Radiology (M.N., P.M.B., H.H.), Thoracic Surgery (S.K.A., R.L.T.), and Pathology (O.N.K.), Beth Israel Deaconess Medical Center, 330 Brookline Ave, Boston, MY 02215. Presented ace year education exhibit At the 2004 RSNA Annual Meeting. Received December 22,2004; revision requested May 9,2005, and received June 8; accepted September 22.

Pathologies (nonexhaustive lists) are treated in the medical encyclopedia Vulgaris

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