DR. ALBERTINI'S PHOTO GALLERY

BRIEFING: Spheres within spheres within spheres! That is just what we are demonstrating this month as our image gallery takes aim at the nucleus of the oocyte (or egg as we call it once it has matured). As in all cells, the nucleus contains the DNA where "genes" are organized into linear sequences forming a template for the production of RNA molecules. The DNA is complexed with proteins and supercoiled into something called chromatin that, in oocytes, takes on an unusual disposition, referred to as chromosomes (just a high school refresher!). But the nucleus of oocytes itself is highly specialized and has historically been known as the germinal vesicle (or GV, for short).

Besides the chromosomes, seen as filamentous or fibrous structures (see FIGURES 2-3), the GVs of most animal oocytes (yes, they look very similar whether from a mouse, a monkey, a cow, or a human) contain a single dense spherical structure. This nucleolus is a factory for the synthesis of ribosomal RNAs that constitute the protein-synthesizing machinery of the cytoplasm. Oocytes, being the largest cells in the body, also contain an abundance of ribosomes, which are used during the development of the embryo following fertilization.

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FIGURE 1 shows a section of a small follicle. The oocyte is outlined in pink (surrounded by the zona pellucida). The central nucleus (or GV) has a large blue sphere - the nucleolus. Two small, undeveloped follicles are seen at the image border.

FIGURE 2 illustrates in more detail the central nucleus and nucleolus (GV), including its red chromosomes, some of which encircle the nucleolus (black hole).

FIGURE 3 shows a more high-powered view of a GV at the edge of the oocyte and chromosomes, and the nucleolus are seen as an interconnected network.

FIGURE 4 is a 3-dimensional reconstruction of the entire GV after chromatin (DNA) labeling. The beaded appearance along the length of each chromosome reflects inactivated areas of the genome unable to participate in the process of RNA production (transcription).