Thymus help

The Thymus is part of the Lymphatic system, and lies in the anterior superior mediastinum; between the sternum and large blood vessels above the heart. Structurally and therefore functionally speaking, the Thymus is different from the other lymphatic organs. It does not have lymphatic vessels filtering through it, and instead is the site of thymic lymphocytes (T-cells) maturation; isolated from the rest of the body.

It is a capsulated structure in which an enveloping layer of loose connective tissue called Areolar tissue holds two lobes closely together. Loose connective tissue is strong enough to form this attachment, yet maintains flexibility. Areolar tissue itself is constructed by fibroblasts and collagenous fibers arranged in loosely branched cells and empty space giving a mesh-like appearance.

After puberty, adipose tissue and areolar connective tissue replace the original thymic stroma (framework) and lymphocytes. Endothelial cells are joined by tight junctions, forming a barrier keeping T-cell preventing antigens from entering the thymus.

Thymic epithelium cells are developed early, attracting thymocyte precursors and macrophages. These are located in the medulla, arranged in concentric layers of flat cells. These cells degenerate in a process called involution, eventually to be filled with keratin; these clusters are called Thymic Corpuscles.

Epithelial Reticular Cells (ERC) are specialized epithelial cells connected by Desmosomes, structurally forming a mesh-work known as Syncytium. Functionally, ERC sends information to pre-T cells called Positive Selection; also assuring the T-cells develop solely in the cortex. The ERC also secrete Thymosin, a hormone regulating differentiation of the T-cells allowing for maturation.