3)Lymphoid organs a.Central lymphoid organs: thymus, /bone marrow developed earlier blood-borne stem cell comes from yolk sac e microenvironment + proliferation promoting send lc to pld and lt two weeks before borne3) Lymphoid organs a. Central lymphoid organs: thymus, bone marrow developed earlier blood-borne stem cell comes from yolk sac microenvironment → proliferation promoting send LC to PLD and LT two weeks before borne