Mitosis in thyroid epithelial cells in vivo. V. Characteristic apical protrusions are associated with mitosis in unstimulated glands

The ultrastructure of mitosis in thyroid follicular epithelial cells of untreated 7-day-old rats was investigated. While most of the nuclear and cytoplasmic events in these animals were similar to those previously observed in goitrogen-stimulated adult animals, one new feature of mitosis in the young animals was the consistent finding of a characteristic, mitosis-associated apical protrusion (MAAP), extending into the follicular lumen. MAAPs were generally dome-shaped, with a 3- to 5- μm base. Their cytoplasmic content was essentially indistinguishable from that of the remainder of the cell. MAAPs had a high concentration of microvilli. These microvilli were frequently club-shaped with distal expansions which were more electron-lucent than normal microvilli. Club-shaped microvilli were also often observed on late prophase and prometaphase cells, preceding the actual development of protrusions, which were observed only from metaphase through telophase. A maximum of one MAAP was found per cell, but MAAPs were observed in 87% of those sections of mitotic cells from metaphase to telophase in which at least 2 μm of apical surface was visible.