The thymus: picture review of human thymus prenatal development.

The thymus is derived from pharyngeal region; a region from which, in case of aquatic vertebrates, the gills develop. According to the classical hypothesis, the epithelial thymus stroma of human embryos is derived from the endodermal cells of the left and right ventral parts of the third pharyngeal pouches. But a close contact of the third pharyngeal pouch with its corresponding third pharyngeal clefts ectoderm plays an important role. Also an epithelial-mesenchymal interaction between the third pharyngeal pouch endoderm and surrounding neural crest derived-cells ectomesenchyme is necessary for the proliferation and differentiation of thymic epithelial cells. In our work we photographically presented the development of thymus from the 6th up to the 25th week of development. The first primordia of the thymus and parathyroid glands within the endoderm of third pharyngeal pouches can be seen in 8 to 9 mm stages. We found also an epithelial proliferation in the second pharyngeal pouches, but this "thymus secundus" stopped their differentiation. The thymus primordia at the 7th and 8th week of development contain almost exclusively epithelial cells. These cells are arranged at the periphery as a row of prismatic cells. The mesenchyme accumulates around the epithelial thymic primordium, and during 9th to 12th weeks of development, septa from mesenchyme fold between the epithelial cells and create the "openings" in the capsular surround. According to our observations, in the 13th week of development the differentiation of cortex and medulla becomes obvious and is completed from the 17th up to 18th week of development onward. The first developing Hassall's corpuscle was detected in the 13th week of development. The striking increase in the number of the Hassall's bodies was observed between the 16th and the 18th week of development, as well as between the 22nd and the 25th week of development (Fig. 14, Ref. 35).