Ultrastructure of epithelial cells of the chick embryo thymus

The ultrastructure of the stromal cells of the thymuses of chick embryos from the 13th to 19th day of incubation was investigated. At the 13th day, the non-lymphoid elements consist mainly of undifferentiated and epithelial cells. The dominant cystic cells are organized to form intra- and intercellular cysts and cleft-like spaces. By the 15th day, cysts are more conspicuous, and the cystic cells are marked by an increase in cytoplasmic organelle content. Isolated foci of cystic cells with no apparent connections to cysts are also observed. Myoid cells, characterized by the presence of myofibrils, are identifiable at this stage. Endocrine-like cells show synthetic and secretory activities and are distributed in cystic epithelium or among other cell types. Dendritic cells and macrophages are also observed among the other cells. Between the 15th and 19th day, there is a further increase in size and frequency of the cysts. Myoid and endocrine-like cells are also more prevalent.