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Ontogeny of gut associated immune competence in the chick

To accommodate the rapid transition to external nutrients, the chick's gastrointestinal tract undergoes dramatic changes within the first few days of life. These include a rapid increase in mass, villi number and length, enterocyte number, crypt depth and proliferating cells. Concomitant with the development of digestive structures and functions a rapid development of the gut associated lymphoid tissue (GALT) occurs. This lymphoid system works within and in concert with digestive tract parenchyma, however, there is little information describing the normal development and immunological function of the avian GALT in the immediate post-hatch period. The purpose of this review is to summarize current knowledge on the structure and function of the avian GALT during the early post-hatch period. At hatch, the gut is poorly populated by both innate immune leukocytes and lymphocytes. The basal numbers of lymphocytes are the result of early waves migrating from the thymus and bursa of Fabricius. Further waves of lymphocyte migration occur after 4 days of life and continue intermittently with time. In concert with this pattern of lymphocyte population adaptive immunity develops. Hence, the gut of the hatchling is unprotected by adaptive immunity during the first few days of life. Protection during this critical period might be the result of maternal antibody activity or that of the innate immune system. This system appears to be functional at this time, though much work is needed to establish this possibility. Upon maturity of the immune system, most of the immunological activity within the chick GALT is concentrated in the hindgut, and specifically so in the caeca and bursa of Fabricius. Once immune responses have become established the relevant cells disseminate systemically and to other areas of the small intestines. Finally, observations on the beneficial effects of early feeding on development of gut and GALT are discussed with reference to management of hatchlings.