A freeze-etch study of clustered nuclear pores

Freeze-etch studies of cell nuclei from cockroach blood cells and leaf-hopper Malpighian tubule cells reveal clustered nuclear pores over the membrane surface. Pores within clusters are arranged in incomplete and complete hexagonal arrays. In some replicas, areas different in texture from the general nucleoplasm, and evidently corresponding to regions of heterochromatin, could be seen in the nucleoplasm. Observations on the relative particle density on the inner and outer membrane fracture faces as seen from both cytoplasmic and nucleoplasmic aspects support the concept of a lipoprotein membrane asymmetry. Specifically, the inner fracture face of the outer membrane and the outer fracture face of the inner membrane bear fewer particles/unit area than the opposing fracture faces of the nuclear membrane complex.