Transmission electron micrograph of a portion of the nucleus and adjacent cytoplasm in a monkey Sertoli cell from the terminal segment of the seminiferous tubule. Near the junction with the tubuli recti and rete testis, where there are very few germ cells, the Sertoli cytoplasm in some species is crowded with filaments. At the arrows, we can see numerous microtubules among the 10 nm filaments.