title: The ontogeny of T cell recirculation during foetal life.
abstract: Studies on the ontogeny of the immune system in the sheep foetus, which remains immunologically naive until after birth, indicate that a large scale recirculation of T cells is just as much a feature of the foetal immune system as it is in animals after birth. An extensive recirculation of T cells and dendritic cells through peripheral tissues-including the gastrointestinal tract and skin-develops early in foetal life, although a population of gut-homing memory T cells does not develop until postnatal life. Current evidence suggests that two populations of thymic emigrants with distinct tissue-homing specificities to spleen and lymph node contribute to the development of the foetal peripheral T cell pool. CD8(+) thymic emigrants mostly target spleen while CD4(+) thymic emigrants home predominantly to lymph nodes. The lifespan of thymic emigrants is uncertain, although cells entering lymph are long-lived and form the basis of a diverse pre-immune repertoire of recirculating T cells. The life history and growth rates of non-recirculating T cells in spleen and lymph nodes during foetal life are at present unknown.
mesh_terms: D000818:Animals; D002465:Cell Movement / Q000276:immunology*; D005260:Female; D005333:Fetus / Q000276:immunology; D011247:Pregnancy; D012756:Sheep; D013601:T-Lymphocytes / Q000502:physiology*; D013950:Thymus Gland / Q000166:cytology / Q000196:embryology* / Q000276:immunology*