Although the Barr body was originally found in sectioned tissue, it was subsequently shown that any relatively large number of cells prepared as a smear (e.g., scrapings of the oral mucous membrane from the inside of the cheeks or neutrophils from a blood smear) can be used to search for the Barr body. In cells of the oral mucous membrane, the Barr body is located adjacent to the nuclear envelope. In neutrophils, the Barr body forms a drumstick-shaped appendage on one of the nuclear lobes (Fig. 3.3). In both sections and smears, many cells must be examined to find those whose orientation is suitable for the display of the Barr body.
FIGURE 3.3 • Photomicrograph of a neutrophil from a female patient’s blood smear. The second X chromosome of the female patient is repressed in the interphase nucleus and can be demonstrated in the neutrophil as a drumstick-appearing appendage (arrow) on a nuclear lobe. 250.
The nucleolus is the site of ribosomal RNA (rRNA) synthesis and initial ribosomal assembly.
The nucleolus is a nonmembranous region of the nucleus that surrounds transcriptionally active rRNA genes. It is the primary site of ribosomal production and assembly. The nucleolus varies in size but is particularly well developed in cells active in protein synthesis. Some cells contain more than one nucleolus (Fig. 3.4). The nucleolus has three morphologically distinct regions:  Fibrillar centers contain DNA loops of five different chromosomes (13, 14, 15, 21, and 22) that contain rRNA genes, RNA polymerase I, and transcription factors.
 Fibrillar material (pars fbrosa) contain ribosomal genes that are actively undergoing transcription and large amounts of rRNA.
 Granular material (pars granulosa) represents the site of initial ribosomal assembly and contains densely packed preribosomal particles.
The network formed by the granular and the fibrillar materials is called the nucleolonema. rRNA is present in both granular and fibrillar material and is organized, respectively, as both granules and extremely fine filaments packed tightly together. Genes for the ribosomal subunits are localized in
FIGURE 3.4 • Electron micrograph of the nucleolus. This nucleolus from a nerve cell shows fibrillar centers (FC) surrounded by the fibrillar (F) and granular (G) materials. Such a network of both materials is referred to as the nucleolonema. The rRNA, DNA-containing genes for the rRNA, and specific proteins are localized in the interstices of the nucleolonema. 15,000.
the interstices of this network and are transcribed by RNA polymerase I. After further processing and modification of rRNA by small nucleolar RNAs (snoRNAs), the subunits of rRNA are assembled using ribosomal proteins imported from the cytoplasm. The partially assembled ribosomal subunits (preribosomes) are exported from the nucleus via nuclear pores for full assembly into mature ribosomes in the cytoplasm.
The nucleolus is involved in regulation of the cell cycle.