<prompt> All are true about thymus, EXCEPT:Derived from third and fouh pharyngeal pouchLocated in the anterior mediastinumIt is composed of epithelial and stromal cellsThe medulla of thymus contains 85 of the lymphoid cells  <response>: THYMUS: Derived from the third and fouh pharyngeal pouches Located in the anterior mediastinum. Composed of epithelial and stromal cells derived from the pharyngeal pouch and lymphoid precursors derived from mesodermal cells. It is the site to which bone marrow precursors migrate to complete their differentiation. It has coex and the medulla. The coex contains 85 of the lymphoid cells, and the medulla contains 15. Thymic abnormalities are very rare. If the thymus does not develop properly, severe immunodeficiency is seen e.g., DiGeorge syndrome. If a lymphoid cell within the thymus becomes neoplastic, it is a lymphoma. The majority of lymphoid tumors that develop in the thymus are precursor T cell lymphoblastic lymphoma Rare B cell neoplasm in the thymus is mediastinal thymic B cell lymphoma Ref: Harrison, E-18, Chapter e-20 