FIGURE 1-15 Structure of the nucleus. (a) Schematic diagram of the structure of a typical cell nucleus and the connection of the outer nuclear membrane with the rough endoplasmic reticulum. The small black dots attached to the membrane of the rough endoplasmic reticulum are ribosomes that are synthesizing membrane and secreted proteins. (b) Electron micrograph of a pancreatic acinar cell from the bat Myotis lucifugus. The nucleolus is a subcompartment of the nucleus and is not surrounded by a membrane; most ribosomal RNA is produced in the nucleolus. Darkly staining areas in the nucleus outside the nucleolus are regions of heterochromatin.

[Part (b) Don W. Fawcett/Science Source.]