The ultrastructure of the myoid cells in the thymus of 2-week old chicks was examined by electron microscopy. 2. Myoid cells are round or elongated, distributed singly or in clusters in the medulla and at the corticomedullary junction. They are interposed between reticular epithelial cells to provide support for the meshwork of the microenvironment. They are also inserted into intercellular cysts, or associated with blood vessels and endocrine-like cells. 3. The cytoplasm of the myoid cell is packed with myofibrils arranged in circular bundles around the nucleus, aligned in register to form sarcomeres or randomly distributed. The interfibrillar cytoplasm contains smooth endoplasmic reticulum, mitochondria and free ribosomes. 4. The myoid cells contribute to the formation of the structural framework of the medulla. Their association with intercellular cysts and blood vessels suggests that their contractility may affect cystic and vascular flow.
