THE THYMUS first appears as a proliferation of epithelial cells lining the third and fourth branchial pouches at about the sixth or seventh week of embryonic life. Lymphoid tissue first appears in the thymus at about the eighth week of gestation, presumably as a result of a thymic microenvironmental influence on pluripotential stem cells that have migrated there from the yolk sac, fetal liver, or spleen. In the thymus, these cells acquire surface differentiation antigens and function (Fig 8-1). Thymic cells forming spontaneous rosettes with sheep erythrocytes (E) have been noted as early as eight weeks. Lymphocytes with membrane markers characteristic of T cells constitute 65% to 100% of thymus cells by 18 weeks. Reactivity to phytohemagglutinin first appears in the thymus at ten weeks, in the spleen at 13 weeks, and in the peripheral blood at 14.5 weeks. Mixed lymphocyte reactivity is first detected in the thymus at 12.5
Cooper MD, Buckley RH. Developmental Immunology and the Immunodeficiency Diseases. JAMA. 1982;248(20):2658–2669. doi:10.1001/jama.1982.03330200082017
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