Primary lymphoid organs Bursa of Fabricius of birds, bone marrow and thymus gland of mammals constitute the primary lymphoid organs involved in the production and early selection of lymphocytes. These lymphocytes become dedicated to a particular antigenic specificity . Only when the lymphocytes mature in the primary lymphoidal organs, they become immunocompetent cells . In mammals, B cell maturation occurs in the bone marrow and T cells maturation occurs in the thymus.
XII Std Biology-Zoology Chapter- XII Std Biology-Zoology Chapter- Human Health and Diseases is differentiated into two compartments, the outer compartment or outer cortex, is densely packed with immature T cells called thymocytes, whereas the inner compartment or medulla is sparsely populated with mature thymocytes. One of its main secretions is the hormone thymosin . It stimulates the T cell to become mature and immunocompetent . By the early teens, the thymus begins to atrophy and is replaced by adipose tissue (Fig.
. ). Thus thymus is most active during the neonatal and pre-adolescent periods . Fig.
. Primary lymphoid organ - Thymus A) Location B) Structure 7K\URLG JODQG 7UDFKHD 6HSWDH /REXOH /XQJV +HDUW 7K\PXV $ % 5LJKWOREH /HIWOREH Bone marrow Bone marrow is a lymphoid tissue found within the spongy portion of the bone. Bone marrow contains stem cells known as haematopoietic cells. These cells have the potential to multiply through cell division and either remain as stem cells or differentiate and mature into different kinds of blood cells.