The Cell: Basic Structure and Function, and Molecular Basis of Neoplasia

Overview

Cells are the basic structural and functional units of all living organisms. Estimates about the total cell count of a human body vary widely, a number as large as 10 ¹⁴ has been proposed. Although the basic components of all cells are very similar, the differentiation of cells results in a wide variation of cellular morphology and function.

The smallest human cells, by diameter, are spermatozoa with a size of ~3 µm, followed by the anucleate erythrocytes (8 µm). The largest cells are female oocytes that can be as large as 35–40 µm and are visible to the naked eye. Motor neurons are extremely long cells, with their axons reaching from the spine to the distal extremities (up to 1.4 m in length).

Most cells can only be functional in large united structures, such as organs or suborganic structures. Other cells, mainly of hemato- or lymphopoietic origin, are mobile and autonomous, although, in many cases, their functionality is dependent on interaction with other cells.

Cytopathology studies diseases on the cellular level. While in histopathology, cells are assessed in the spatial context, in virtually all cytological applications, they are removed from their spatial context and have to be assessed isolated or as cell sheets.

Apart from the loss of the spatial information, cells can have considerably altered morphology when taken out of the united structures. Cell–cell contacts are important features that build the shape of a cell. Many structural elements within a cell are connected to proteins that are attached to other cells or the basal membrane. This has to be considered when the same cell types are compared in histological versus cytological assessments.

Another important difference between histology and cytology is that, in histology, tissue sections are assessed 2-dimensionally, while in most cytology applications complete cells are analyzed that still have some 3-dimensional features, although they might appear flat in the microscope.

In this section, an overview of the most important cellular structures and functions that are relevant for cytopathology are presented ( Fig. 1-1 ). We have assembled the most important information on cellular structures by describing the regular function in brief, the relevance for cytopathology and the morphology in normal and abnormal cells. For more detailed information about cellular structures and functions, a cell biology textbook is recommended.

Nucleus

The nucleus contains the genomic DNA, histones, and several proteins that are responsible for DNA replication, repair, and transcription of genetic information ( Fig. 1-2A ).

The assessment of a cell's nucleus is one of the most important tasks in cytopathology. The size of the normal nucleus is highly variable, depending on the underlying cell type. In many malignant cells, nuclei are considerably enlarged. Apart from nuclear size, the chromatin density, the nuclear membrane, and the presence of nucleoli are important features of nuclear morphology and will be described in detail.

The nucleus contains about 25% dry substance, of which 18% is DNA, plus a similar amount of histone proteins. The rest of the dry substance contains the non-histone proteins, nucleoli, and the nuclear membrane.

Hematoxylin

In many cytological applications, the chromatin is stained with hematoxylin. Hematoxylin is a basic dye extracted from the heartwood of the tree Haematoxylum campechianum . By itself, hematoxylin is a very weak stain. Different mordants, such as potassium alum, are used to generate the typical dark blue or purple staining. Hematoxylin strongly binds to acidic components of a cell, most importantly to the phosphate groups of nuclear DNA; the stained structures are therefore called “basophilic” ( Fig. 1-3A ).

Based on the nuclear stain, a wide variation of chromatin alterations can be observed, alterations of both structure and staining intensity. Structural aberrations include chromatin margination, i.e., aggregation of chromatin to the nuclear membrane, which is a sign of cell degeneration. Other chromatin alterations are coarsening and clumping that is usually accompanied by chromatin thinning in other regions.

Hyperchromasia, i.e., increased staining intensity, can result from increased chromatin loads or from a decreased nuclear volume, which inversely applies to hypochromasia. In addition, chemical modifications of the chromatin (e.g., during specific stains or cell treatments) can increase the stain uptake simulating hyperchromasia.

Nucleoli

Nucleoli are small basophilic spherical bodies located in the nucleus. Usually, they can be found in the central nuclear region but may also be close to the nuclear membrane. A nucleolus is built by a nucleolus organizing region (NOR) of a specific chromosome. These regions contain the genes for ribosomal RNA subunits that build the protein synthesis machinery. Since, in a diploid human cell, in total 10 chromosomes containing NORs exist, in principle 10 nucleoli per nucleus could be present. Usually, only one or two nucleoli are found, since NORs from several chromosomes build a common nucleolus. Nucleoli have two distinctive regions, the pars fibrosa that contains the proteins required for transcription and the pars granulosa that contains the ribosomal precursors. During mitosis, nucleoli disappear and are reconstituted in the daughter cells. Shortly after cell division, a larger number of nucleoli can be observed that fuse gradually.

Depending on the cell type, the presence of nucleoli is physiological or can indicate malignant processes: liver cells that regularly produce a lot of protein can frequently exhibit nucleoli. In reactive or regenerative cells, nucleoli can become more prominent. In hepatocellular carcinoma, usually more than 50% of the cells show multiple prominent nucleoli. Intestinal epithelial cells regularly show single nucleoli. In ageing and starving cells, a shrinking of nucleoli can be observed. In cancer cells, nucleoli can vary substantially with regard to size and shape.

In many malignant cells, multiple nucleoli can be observed that appear disjointed, odd-shaped, and spiculated. Proteins associated with nucleolar organizer regions can be visualized by a simple argyrophilic staining method. The structures highlighted by this method are called “argyrophilic nucleolar organizer regions” (AgNORs). Different distributions of AgNORs have been described between normal, dysplastic, and malignant tissues. In several cancer sites, AgNOR aberrations were found to have independent prognostic significance with respect to patient survival. Increased NOR counts have been explained by increased metabolism with a high demand of ribosomes, but also by aneuploidy leading to increasing numbers of NOR regions in cancer cells.

Nuclear Envelope and Nuclear Shape

The nuclear envelope (NE) consists of two lipid membranes. The inner membrane is associated with the telomeres and anchors the chromosomes, while the outer membrane is part of the endoplasmic reticulum. The space between the two lipid layers is called the “perinuclear cisterna.” The nuclear envelope constitutes the nucleus and separates the genomic material from the cytosol. During cell division, the nucleus disappears; the nuclear envelope is broken down to vesicles and is reassembled during telophase. The nuclear envelope builds a strong barrier between nucleus and cytosol; a number of nuclear pore complexes regulate the traffic between both compartments. There can be passive diffusion or active transport; in general, proteins synthesized in the cytoplasm require a specific nuclear signal in order to have access to the nucleus.

Inside the nuclear envelope is a network of chromatin fibrils and a nuclear lamina built from laminins. The nuclear envelope can be visible in light microscopy. The regular nuclear shape is that of a smooth sphere or spheroid, based on the orderly arrangement of the chromosomes and the nuclear lamina. Many factors can affect the shape of the nucleus: stress, transcriptional, as well as synthetic, activities can disturb the arrangement of interphase chromosomes, DNA amplifications can lead to uneven distribution of the nuclear material and to nuclear enlargements. At the same time, aberrations of the nuclear envelope can lead to alterations of the nuclear skeleton, resulting in altered chromosomal distributions. It has been assumed that changes of the nuclear envelope occur mainly after mitosis, when the nuclear envelope is reassembled. Alterations of the nuclear envelope have been directly linked to oncogene activity. Six hours after transfection with the RET oncogene, increasing cell counts with NE alterations were observed in human thyroid cells, indicating that nuclear alterations may occur even independent of postmitotic re-assembly. NE alterations and the respective nuclear shape are an important diagnostic feature of many malignancies, especially papillary thyroid cancers and different types of leukemias.