Varga I, Pospisilova V, Jablonska-Mestanova V, Galfiova P, Polak S
Department of Histology and Embryology, Faculty of Medicine, Comenius University, Bratislava, Slovakia.
Bratisl Lek Listy. 2011;112(7):368-76.
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).
胸腺起源于咽区；在水生脊椎动物中，鳃就是从这个区域发育而来的。根据经典假说，人类胚胎的上皮性胸腺基质源自第三咽囊左右腹侧部分的内胚层细胞。但第三咽囊与其相应的第三咽沟外胚层紧密接触起着重要作用。第三咽囊内胚层与周围神经嵴衍生细胞外间充质之间的上皮 - 间充质相互作用对于胸腺上皮细胞的增殖和分化也是必需的。在我们的研究中，我们用照片展示了胸腺从发育第6周直至第25周的发育过程。在8至9毫米阶段，可以在第三咽囊的内胚层中看到胸腺和甲状旁腺的最初原基。我们还发现第二咽囊中存在上皮增殖，但这种“副胸腺”停止了分化。发育第7周和第8周的胸腺原基几乎只包含上皮细胞。这些细胞在外围排列成一排棱柱形细胞。间充质在胸腺上皮原基周围聚集，在发育的第9周至第12周期间，间充质形成的隔膜在上皮细胞之间折叠，并在包膜周围形成“开口”。根据我们的观察，在发育第13周时，皮质和髓质的分化变得明显，并从发育的第17周至第18周起完成。在发育第13周时检测到第一个发育中的哈氏小体。在发育的第16周和第18周之间，以及在发育的第22周和第25周之间，观察到哈氏小体数量显著增加（图14，参考文献35）。
