Atlas of plant and animal histology

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The Cell

4. THE NUCLEUS

The nucleus is the structure that defines eukaryotic cells (eu: true and karyos: nucleus). It contains the DNA and all the molecular machinery to produce RNA, by a process known as transcription. Typically, cells contain only one nucleus, but some cells have more than one, such as osteoclasts, and some cells may contain many of them like the skeletal muscle fibers or the epithelial cells of some invertebrates. The shape of the nucleus is variable and usually adapted to the morphology of the cell (Figure 1). Typically, the nucleus is rounded, but other forms are also found, as in neutrophils, which contain multi-lobulated nucleus. The position of the nucleus is usually in the center of the cell, but it can also be found in peripheral locations. For example, secretory cells have their nucleus in the basal part, and skeletal muscle cells have their nuclei close to the plasma membrane. During cell differentiation, the nucleus can be moved through the cytoplasm. For instance, the two haploid pronuclei are moved in the zygote during fertilization to fuse to one another and form a diploid nucleus. Both, nuclear position and movement are driven by the cytoskeleton.

Nucleus
Figure 1. Different nuclear shapes. A. Epithelial cells from human gall bladder with rounded to elongated nuclei. B. Monocyte from blood smear showing a kidney-shaped nucleus. C. Neutrophils from blood smear containing a multi-lobed nuclei. D. Longitudinal view of a skeletal muscle fiber with many nuclei, all of them located toward the periphery (arrows).

Although the amount of DNA is almost the same in all the cell nuclei of an organism, the size of the nucleus is variable depending on the cell type (Figure 2). Furthermore, cells from species with different amount of DNA may show a similar nuclear size. These data indicate that the size of the nucleus is adapted to the size and the physiology of the cell, but does not solely depend on the amount of DNA.

Nucleus
Figure 2. The size and shape of the nucleus depends on the cell type, despite that nearly all the nuclei of an individual contain the same amount of DNA. Neurons and glial cells are shown in this picture. Nuclei of neurons contain more loosely arranged chromatin, whereas glia nuclei have highly condensed chromatin, and therefore the size is much smaller.

There are two nuclear compartments: nuclear envelope and nucleoplasm (Figure 3). The nuclear envelope is the peripheral part of the nucleus that separates the interior of the nucleus, known as nucleoplasm, from the cytoplasm, and consists of an outer membrane, an inner membrane, and a perinuclear space (between both membranes). Nucleoplasm-cytoplasm communication is regulated by the nuclear pore complexes, which are molecular scaffolds inserted in the nuclear envelope. The nuclear lamina is a layer of proteins lining the inner surface of the inner membrane of the nuclear envelope, which provides mechanical support to the nucleus.

Components of the nucleus.
Figure 3. Components of the nucleus.

In the nucleoplasm, DNA and associated proteins form the chromatin, which is referred to as heterochromatin if it is highly condensed or as euchromatin if it is loosely packaged. Decondensation of chromosomes after mitosis results in chromatin, and each chromosome decondensates in specific regions of the nucleoplasm. The nucleolus, a distinct chromatin domain, is also found in the nucleoplasm, along with other dense structures known as nuclear bodies, which are composed of chromatin and other proteins.

During mitosis (open mitosis), the nuclear envelope is disorganized and chromatin condensates to form the chromosomes. After chromosome segregation during telophase, the nuclear envelope reassembled again enclosing the chromatin and a typical new nucleus is formed

In this section of the atlas, we are not going into all the molecular processes involved in DNA transcription and replication, as well as gene regulation. In the future, a new section may be opened to deal with these topics. We will be focused in some features of the nuclear morphology and physiology, and encourage the reader to visit other sites related to these genetic topics.

Cell junctions
Nuclear envelope
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