Although crystalline, viral, and other inclusions are sometimes found in the nucleoplasm, until recently, morphologic techniques showed it to be amorphous. It must be assumed, however, that many proteins and other metabolites reside in or pass through the nucleus in relation to the synthetic and metabolic activity of the chromatin and nucleolus. New structures have recently been identified within the nucleoplasm, including intranuclear lamin-based arrays, the protein filaments emanating inward from the nuclear pore complexes, and the active gene-tethered RNA transcription and processing machinery itself.
Somatic cells in the adult organism may be classified according to their mitotic activity.
The level of mitotic activity in a cell can be assessed by the number of mitotic metaphases visible in a single high-magnification light microscopic field or by autoradiographic studies of the incorporation of tritiated thymidine into the newly synthesized DNA before mitosis. Using these methods, cell populations may be classified as static, stable, or renewing.
FIGURE 3.8 • Cryoelectron tomography of the nuclear pore complex. These surface renderings of electron tomograms obtained from the frozen-hydrated Dictyostelium nuclei show detailed structure of the nuclear pore complex (NPC). 320,000.
a. Cytoplasmic face of the NPC shows eight protein fibrils arranged around the central channel. They protrude from the cytoplasmic ring subunits and point toward the center of the structure. Note a presence of the central plug or transporter within the central pore, which represents either ribosomes or other protein transporters captured during their passage through the NPC. b. Nuclear face of the NPC shows the nucleoplasmic ring subunits connected by nuclear filaments with the basket indicated in brown color. (Adapted from Beck M, Fster F, Ecke M, Plitzko JM, Melchior F, Gerisch G, Baumeister W, Medalia, O. Nuclear pore complex structure and dynamics revealed by cryoelectron tomography. Science 2004;306:1387–1390.)
FIGURE 3.9 • Sagittal section of the nuclear pore complex. Cryoelectron tomographic view of a sagittal section of the nuclear pore complex shown in Figure 3.8 is compared with a schematic drawing of the complex. Note that the central plug/transporter has been removed from the central pore. 320,000. Each pore contains eight protein subunits arranged in an octagonal central framework at the periphery of the pore. These subunits form a nuclear pore complex that is inserted between two cytoplasmic and nucleoplasmic rings. Eight short protein fibrils protrude from the cytoplasmic rings into the cytoplasm. The nuclear ring anchors a basket assembled from eight thin filaments joined distally into terminal ring which diameter can be adjusted to meet nuclear pore transport requirements. The cylindrical central framework encircles the central pore, which acts as a close-fitting diaphragm. (Adapted from Beck M, Frster F, Ecke M, Plitzko JM, Melchior F, Gerisch G, Baumeister W, Medalia, O. Nuclear pore complex structure and dynamics revealed by cryoelectron tomography. Science 2004;306:1387–1390.)  Static cell populations consist of cells that no longer divide (postmitotic cells), such as cells of the central nervous system and skeletal or cardiac muscle cells. Under certain circumstances some of these cells (i.e., cardiac myocytes) may enter mitotic division.
 Stable cell populations consist of cells that divide episodically and slowly to maintain normal tissue or organ structure. These cells may be stimulated by injury to become more mitotically active. Periosteal and perichondrial cells, smooth muscle cells, endothelial cells of blood vessels, and fibroblasts of the connective tissue may be included in this category.