Structurelobules held together by delicate areolar tissue; the entire organ being enclosed in an investing capsule 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, and these differ in many essential particulars from each other.
 CortexThe 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.
 MedullaIn 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 by the thymic medullary epithelium, 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.
 VasculatureThe arteries supplying the thymus are derived from the internal mammary, and from the superior thyroid and inferior thyroids.
The veins end in the left brachiocephalic vein (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.
 FunctionIn the two thymic lobes, hematopoietic precursors from the bone-marrow, referred to as thymocytes, mature into T-cells. Once mature, T-cells emigrate from the thymus and constitute the peripheral T-cell repertoire responsible for directing many facets of the adaptive immune system. Loss of the thymus at an early age through genetic mutation (as in DiGeorge Syndrome) results in severe immunodeficiency and a high susceptibility to infection.
The stock of T-lymphocytes is built up in early life, so the function of the thymus is diminished in adults. It is largely degenerated in elderly adults and is barely identifiable, consisting mostly of fatty tissue, but it continues its endocrine function. Involution of the thymus has been linked to loss of immune function in the elderly, susceptibility to infection and to cancer.
The ability of T-cells to recognize foreign antigens is mediated by the T cell receptor. The T cell receptor undergoes genetic rearrangement during thymocyte maturation, resulting in each T-cell bearing a unique T-cell receptor, specific to a limited set of peptide:MHC combinations. The random nature of the genetic rearrangement results in a requirement of central tolerance mechanisms to remove or inactivate those T cells which bear a T cell receptor with the ability to recognise self-peptides.
 Phases of thymocyte maturationThe generation of T-cells expressing distinct T-cell receptors occurs within the thymus, and can be conceptually divided into three phases:
- A rare population of hematopoietic progenitor cells enter the thymus from the blood, and expands by cell division to generate a large population of immature thymocytes.
- Immature thymocytes each make distinct T-cell receptors by a process of gene rearrangement. This process is error-prone, and some thymocytes fail to make functional T-cell receptors, whereas other thymocytes make T-cell receptors that are autoreactive.
- Immature thymocytes undergo a process of selection, based on the specificity of their T-cell receptors. This involves selection of T-cells that are functional (positive selection), and elimination of T-cells that are autoreactive (negative selection).
MY BULLET POINTS:
- Thymus is a gland that shriks with age, it beings to atrophy at puberty.
- it produces two hormones: threse hormones are responsible for maturation and specialization fo the T-lymphocytes and the T-cells. (T-lymphocytes: responsible for cellular immunity side of body defense by rejecting foreign grafts, tumors and or virus infected cells)
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