title: Age-related changes in the cellular composition of the thymus in children.
abstract: BACKGROUND
  T-cell development in the thymus is an extensively studied subject, mainly in mice. Nevertheless, the normal composition and cell numbers of the noninvoluted human thymus are largely unknown.
  
  
  OBJECTIVE
  We aimed to gain insight into age-related changes in different thymic subpopulations and to provide reference values for the distribution of thymocyte subsets. The composition of the normal thymus may serve as a reference for thymi in pathological conditions and may aid diagnoses of immunodeficiency diseases.
  
  
  METHODS
  Thymic lobes of 70 children (58 immunologically normal and 12 diseased), ranging in age from 8 days to 8 years old, were studied by 4-color flow-cytometric analysis. Detailed staining and gating strategies allowed us to dissect small subsets, including immature CD4(-) CD8(-) populations and thymic B, natural killer, and T-cell receptor gammadelta + cells.
  
  
  RESULTS
  We demonstrate that distribution of thymocyte subsets changes with age and correlates with age-related fluctuations of T-lymphocyte counts in peripheral blood. Thymi of children 3 to 6 months old appear to be the most active: they have high numbers of total thymocytes, the highest percentage of double-positive cells, and large numbers of CD34 + progenitors in their thymi. Furthermore, we show that the human thymus is a site for B-cell development, because all B-cell progenitor stages that can be found in the bone marrow are also present in the thymus.
  
  
  CONCLUSIONS
  We conclude that T-cell development in children is a dynamic process, answering the demands of a maturing and expanding immune system.
authors.affiliation: Department of Immunology, Erasmus MC, University Medical Center Rotterdam, Dr Molewaterplein 50, 3015 GE Rotterdam, The Netherlands.
authors.forename: Floor
authors.initials: F
authors.lastname: Weerkamp
authors.forename: Edwin F E
authors.initials: EF
authors.lastname: de Haas
authors.forename: Brigitta A E
authors.initials: BA
authors.lastname: Naber
authors.forename: W Marieke
authors.initials: WM
authors.lastname: Comans-Bitter
authors.forename: Ad J J C
authors.initials: AJ
authors.lastname: Bogers
authors.forename: Jacques J M
authors.initials: JJ
authors.lastname: van Dongen
authors.forename: Frank J T
authors.initials: FJ
authors.lastname: Staal
mesh_terms: D000375:Aging; D002648:Child; D002675:Child, Preschool; D005434:Flow Cytometry; D006801:Humans; D007107:Immune System / Q000254:growth & development*; D007150:Immunohistochemistry; D016130:Immunophenotyping; D007223:Infant; D007231:Infant, Newborn; D008214:Lymphocytes / Q000166:cytology*; D013950:Thymus Gland / Q000166:cytology* / Q000254:growth & development*