Ultrastructural observations on cytodifferentiation of thymic cystic cells of chick embryo.

Thymuses from chick embryos from the 13th-19th day of incubation were examined by light and electron microscopy. At the 13th day of incubation, the primitive medulla consists of undifferentiated, cystic, reticular and lymphoid cells interspersed with developing cysts. Undifferentiated cells contain indented nuclei, poorly developed endoplasmic reticulum and numerous free ribosomes. Synthesis of cytoskeletal material and assembly of microvilli denote their differentiation to cystic cells. Cystic cells possess rough- and smooth-surfaced endoplasmic reticulum, a Golgi complex, mitochondria and microvilli. They are organized individually or in clusters to form intra- and intercellular cysts respectively. By the 15th day of incubation, cystic structures are more apparent. Their lumina contain a low density flocculent or moderately dense granular material. At the 19th day, there is a further increase in number and complexity of the cysts. The results demonstrate the existence of undifferentiated cells and their differentiation to cystic cells. Morphological evidence suggests secretory and/or absorptive activities of the cystic cells in the prehatching period.