Transmission electron micrograph of the cytoplasm of a Sertoli cell from the testis of a common ground squirrel (Citellus lateralis), in culture. In cells in their normal environment in vivo 6 to 7 nm actin filaments occurring invidually or in small groups interlace to form a complex meshwork in the cytoplasm. When the same cell types are explanted and maintained in culture, the filaments tend to aggregate into coarse bundles. In the accompanying micrograph of a Sertoli cell in vitro there are many individual filaments and several large aggregations of parallel filaments.