What is thymus gland? functions and embryonic development of thymus gland along with diseases related to it

THYMUS GLAND

introduction
One of the most important component and the lymphoid organ of the human body goes by the name of thymus gland. It is a part of lymphatic system and its precursor is the third pharyngeal pouch. If we talk about the structure of the thymus gland than we can say that thymus gland is made up of two identical lobes these two lobes of the thymus gland differ slightly in size and may be united or separated. Thymic tissues may be found scattered or around the gland. A person normally can't fell the thymus gland from outside of the body as it is located behind the sternum. Thymus gland is basically made up of two different types of cells that differ in their properties and these cells are as following 
  • thymic stromal cells 
  • hematopoietic cells (that are derived from bone marrow cells that are known as hematopoietic stem cells )
thymus gland is the most largest in size and most active during the neonatal and pre-adolescent periods. By the time of early teens thymus gland activity as well as its size starts to decrease and the tissues of thymus gland gets replaced by adipose tissues by the time when a person will be 75 years old the thymus gland would be a little more than a fatty tissue despite of all these changes in the thymus gland with respect to time it still caries out its most important function through out the life of a person and that important role is the production of the T-lymphocytes that are  very crucial component of the immune system of the human body as primary function of the thymus gland is not production but maturation of the T-lymphocytes these T-lymphocytes are not produced in the thymus gland but its sequence is like that it first are produced in the bone marrow cells than it partially matures in the thymus gland after this step T-lymphocytes leaves the thymus gland and continues to mature in the peripheral circulation. T-lymphocytes are very crucial to the adaptive immune system in which the body adapts and prepares it self for the foreign invasion. Thymus gland is very important as it provides totally functional and self tolerant T-lymphocytes it is also responsible for the for the induction of the central tolerance by which there is elimination of any developing T-lymphocytes that are reactive to themselves. The hormone of the thymus gland goes by the name of thymosin they are name so because they were originally isolated from the thymus gland thymosin have diverse biological activity. Thymosin is a polypeptide hormone that stimulates the development of precursor  T-lymphocytes in the thymus to the mature T-lymphocytes. Once the T-lymphocytes are fully matured in the thymus they migrate to the lymph nodes throughout the body where they aid the immune system in fighting many kind of diseases. Thymus gland plays a dual role in the body such as thymus gland function as an endocrine gland as well as a lymphatic gland. There are two main regions of the each lobe of the thymus gland ion play there own distinct roles in the body the name of the two sections of the thymus gland are as following
  • thymic cortex is the peripheral part 
  • thymic medulla is the central part 
upon reaching the medulla the surviving T-lymphocytes continues to mature and are presented with body own antigens. T-lymphocytes that binds with the body own antigens test positive for the autoimmunity and they are eliminated by the process of negative selection after every thing around 2% of the immature T-lymphocytes reaches maturity. Developing T-lymphocytes in the thymus are known by the name of thymocytes and they are of hematopoietic origin. Thymus gland revives immature  T-lymphocytes from the bone mature and thymus makes these immature  T-lymphocytes into mature and fully functional  T-lymphocytes. Firstly the  T-lymphocytes resides within the cortex of the thymus gland where they come across various antigens the immature  T-lymphocytes that responds to the antigens corresponds to foreign cells that survives matures and than they migrate to the medulla section of the thymus gland while those  T-lymphocytes that cant make up to this process dies and are cleared from the thymus gland. Thymus gland will not function throughout a full lifetime but it has a very big responsibility to protect the body against autoimmunity. The arteries that supply blood to the thymus gland are the branches of the main arteries that are as following

  • internal thoracic artery
  • inferior thyroid artery
  • and sometimes superior thyroid artery
whereas the veins that carry deoxygenated blood from the thymus gland are as following

  • left brachiocephalic vein
  • internal thoracic vein
  • inferior thyroid veins 
and these all veins sometimes end up giving deoxygenated blood of the thymus gland into the superior vena cava  whereas the nerves that supply to the thymus comes from the following

  • vagus nerve
  • cervical sympathetic chain
thymus gland is  pinkish grey in color it is very soft to touch and it have two lobes at the time of birth it is about 4-6 cm long 2.5-5 cm wide and appropriately 1 cm thick it increases to its maximum till the time of puberty at which its weight is about 40 grams after which its decline starts and the process by which the size of the thymus and its activity decreases is known as involution.

embryonic development of thymus gland
The epithelium of the thymus gland developers first that appears as two outgrowth one on either side of the third pharyngeal pouch that is the precursor of the thymus gland. Thymus gland can sometimes also evolve from the fourth pharyngeal pouch. In such a way that sometimes additional portions of the thymus gland tissues are developed from the fourth pharyngeal pouch. The epithelium forms very finely divided lobules as a result of which it developers into a sponge like structure. For the development as well as proper and optimal activity of the thymus gland iodine is very essential and needed as iodine is necessary for the function of the thyroid gland. During the initial period of the development of the body thymus gland is most required because of extensive developmental activities at that point of time till to the time of puberty it increases to about 20-50 grams. After this period of major development that is mainly till the puberty a time comes when there would be decline in the growth of thymus as well as its activity and such a condition is known by  the name of involution. By the end of the first year after the birth thymus grows tremendously and attain a relatively maximum size. But during the time involution fat cell that ultimately forms the adipose tissues invades the thymus from the walls between the lobules first and than invade the cortex and medulla section of the thymus gland. This process of accumulation of fat in the thymus continues as the age of a person progresses and at a very old age there would be such a condition when the detection of the thymus gland would become very difficultly and and it would not be more than a fat tissue and at a very old age it weights about 5-15 grams that why older person's immunity is way much weaker than young people. The main reason of the change of the thymus with the time may be that thymus gland is mainly required after the birth till the puberty but it does not means it is not required for the rest of the life because it has a role of production of T-lymphocytes fro the whole life of a person. There is nothing in the bodies of human or animals that is unnecessary or not needed but the thing is that during the coarse of life there are changes in body requirements and organs adapt to that changes production rates of organs changes throughout but it does not means that we don not need that organ any more 

functions of the thymus gland
Human bodies have three barriers of defense or three lines of defenses the third all the last line of defense of the body is system that we all known and that is the immune system . One of the most important component of the immune system or the immunity are the lymphocytes such as the T and B lymphocytes. If we talk about the B-lymphocytes than we can say that they are produced as well as matures in the bone marrow but case is not the same with the T- lymphocytes. T-lymphocytes are involved in the cell mediated responses and immunity. The thing of T-lymphocytes that is different from the B-lymphocytes is that T-lymphocytes are not produces and matures at the same place such that it is produced in the bone marrow from where the immature T-lymphocytes migrates to the thymus gland gland for the maturation and being fully functions that means without the thymus gland immature T-lymphocytes can become functional by only this fact we can deduce the importance of the thymus gland in the body. At the time of maturation of the T-lymphocytes at the thymus gland they are known as thymocytes. At first there they react with the antigens but it is made sure that they do not react with the antigens of the body and such T-lymphocytes that shows positive autoimmunity are destroyed at the level of thymus and once they are matured and fully prepared they move out from the thymus gland to the body to perform their function. Most of the T- lymphocytes have a unique receptor that is according to a specific substance that are called as antigens most of the T-lymphocytes binds to the major histocompatibility complex on cells of the body. The mature lymphocytes fr the sake of proper functioning needs to bind with major histocompatibility complex molecule and must not react with the antigens that are part of the humans tissues. There are two types of type of selections that are involved with maturation and proper functioning of lymphocytes and the type of selections are as following 
  • positive selection 
  • negative selection
this all selection occurs in the thymus gland and if the T-lymphocytes this whole process of selection they than leaves the thymus gland and perform their function. Through the negative selection those T- lymphocytes are eliminated that attacks its own body cells or proteins. Negative selection of the T-lymphocytes occurs in the medulla section of the thymus whereas the positive selection occurs in the cortex of the thymus gland. T-lymphocytes undergo a process of maturation which involves ensuring that cells the right antigens and bind with the major histocompatibility complex molecule after leaving the thymus further maturation of the T-lymphocytes occur in the peripheral circulation.

diseases of thymus gland
Thymus gland is like any other gland or organ of the body that work for the body and if any sort of abnormality occurs in the thymus it surely does effect the body. Thymus disorders may be in the form of any sort of production disturbance structure related problems there can also be tumor formation in te thymus or thymus gland can also be effected by various autoimmune disorders or there can be any type of cyst formation in the thymus . Some of the symptoms related to the thymus gland disorders are as following 
  • chest pain
  • trouble swallowing
  • shortness of breath
  • loss of appetite
  • weight loss
these are just some of the symptoms that appears as a result of thymus disorder. Some of the disorders relate to the thymus gland are as following
  • cervical cyst formation
  • surgical removal
  • lymphomas
  • thymoma
  • myasthenia gravis
  • thymoma associated multiorgan autoimmunity
  • autoimmune polyendocrine syndrome
  • immunodeficiency

myasthenia gravis
Masthenia gravis is an autoimmune disease that is cased by antibodies that blocks the acetylcholine receptors it is also associated with the hyperplasia. In myasthenia gravis immune system produces antibodies that block or destroy many of the muscles receptor sites for acetylcholine that is neurotransmitter outside the CNS and because of fewer sites for receptors available muscle receives fewer nerves signals resulting in weakness. Myasthenia gravis is characterized by weakness and rapid fatigue of any of the muscle under voluntary control. As the receptor sites would be decreased than there would also be breakdown in the normal communication between the nerves and muscles as there would be blockage of acetylcholine receptors by antibodies mage by the thymus gland. There's no cure for this disease but treatment can help relieve signs and symptoms. Muscle weakness caused by it worsens as the effected muscle is used. Some of the common symptoms of the myasthenia gravis are as following
  • eye problems (ptosis and diplopia)
  • impair speaking
  • chewing problems
  • changed facial expressions
  • difficulty in swallowing
  • weakness in arms neck and legs
thymoma associated multiorgan autoimmunity
thymoma associated multiorgan autoimmunity is an autoimmune disorder like the Graft versus host disease it usually occurs in patients with thymoma. In such patients rather than a donor being a source of pathogenic T-lymphocytes the patient's own malignant thymus produces self directed T- lymphocytes that causes a autoimmunity in the body and kills own body cells. It is a paraneoplastic syndrome that also involves thyroid liver and also skin. People having this disease can face symptoms such as
  • chronic diarrhea
  • abnormal liver enzyme 
  • histopathology of skin or liver
generally its treatment includes a refractory disease course but some patients may also respond to photothearpy

immunodeficiency

Immunodeficiency is an autoimmune disorder as it names also tells us. As we all know that the T-lymphocytes are matures in the thymus gland it there would be any problem in the thymus gland or the thymus gland would fail to produce a certain level of T-lymphocytes as a result of which there would be a deficiency of T-lymphocytes in the body and this deficiency of T-lymphocytes in the body is known as the primary immunodeficiency syndrome. It may be caused as a result of some sort of genetic mutation or it can be some sort of syndrome such as Digeorge' syndrome that is caused as a result of deletion of 22nd chromosome as a result of which there is a failure of production of the third and the fourth pharyngeal pouch. As we know that and have discussion above that third pharyngeal pouch is the precursor of the thymus and if there would be a failure of development of the third and the fourth pharyngeal pouch and then there will be a failure in the production of the thymus gland. it can also cause other associated problems such as failure of development of the parathyroid gland. Such a condition is generally treated by thymus transplantation.

autoimmune polyendocrine syndrome
It is also an autoimmune disorder and it also goes by the name of polyendocrinopathy candidiasis ectodermal dystrophy. It is a rare genetic syndrome that cause defects in autoimmune regulators such as AIRE gene that stimulates the expression of self antigen with medulla as a result of which self antigens(negative selection) are not expressed as a result of which T-lymphocytes are not prepared to tolerate tissues of the body and they may treat them as foreign tissues and stimulate autoimmune responses and try to destroy them and result in autoimmunity it can also effect multiple endocrine tissues such as 

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