Structural and immunomorphological characteristics of the human thymus during embryonic development

Abstract

The thymus of 100 human fetuses was studied between the 4th and 34th weeks of intrauterine development by means of histological, histochemical, immunomorphological, and electron-microscopic methods. Development of the organ from the standpoint of development of the functional system is described. The anlage of the thymus can be detected at the 5th week of fetal development; it reflects the properties of the epithelium of a foregut organ. By the 7th–8th week differentiation of the reticuloendothelium and population of the organ with lymphocytes are beginning to take place and antigenic specificity is found on the surface of the lymphocytes. The zone of growth of the reticuloendothelium of the thymus, the significance of Hassall's corpuscles, the appearance of two subpopulations of T lymphocytes, and their quantitative changes are described. In the period from the 11th until the 34th week of fetal development the number of T lymphocytes forming rosettes with sheep's red blood cells virtually does not change (70–90%), whereas the number of T lymphocytes forming rosettes with autogenous red cells increases during this period from 23 to 70%.