Nucleolus structural integrity during the first meiotic prophase in rat spermatocytes

Highlights: • The nucleolus is formed by its three components during the first meiotic prophase in rat spermatocytes. • The granular component is intertwined with the dense fibrillar component during leptotene and zygotene stages. • Ribonucleoprotein cluster close by the nucleolus is not the granular component. -- Abstract: A typical nucleolus structure is shaped by three components. A meshwork of fine fibers forming the fibrillar center (FC) is surrounded by densely packed fibers forming the dense fibrillar component (DFC). Meanwhile, wrapping the FC and DFC is the granular component (GC). During the mitotic prophase, the nucleolus undergoes disassembling of its components. On the contrary, throughout the first meiotic prophase that occurs in the cells of the germ line, small nucleoli are assembled into one nucleolus by the end of the prophase. These nucleoli are transcriptionally active, suggesting that they are fully functional. Electron microscopy analysis has suggested that these nucleoli display their three main components but a typical organization has not been observed. Here, by immunolabeling and electron microscopy, we show that the nucleolus has its three main components. The GC is interlaced with the DFC and is not as well defined as previously thought during leptotene and zygotene stage.