Morphology: Each normal, fully differentiated, mature tissue type has a characteristic gross and microscopic appearance that varies little from individual to individual of a species. Neoplastic tissues lose these differentiated features of cellular morphology and organization to a variable extent. In general, malignant tumors appear less differentiated than benign tumors. Loss of morphologic hallmarks of tissue maturity is often accompanied by loss of functional capacity and development of aggressive behavior.

Neoplastic cells often show considerable morphologic variability compared with the normal tissue from which they are derived. Tumor cells, especially malignant tumor cells, may exhibit anaplasia (cellular atypia). Anaplastic cells are poorly differentiated cells that exhibit notable cellular and nuclear pleomorphism (variation in size and shape). In some tumors, bizarre tumor giant cells are seen (Fig. 6-6). Nuclei may exhibit extreme variability in number, size, shape, chromatin distribution, and nucleolar size and number (Fig. 6-7). Anaplastic nuclei are often hyperchromatic (darkly staining) because of increased DNA content; are disproportionately large relative to cell size, resulting in an increased nuclear : cytoplasmic ratio; and have prominent nucleoli. Mitotic figures in tumor cells may be numerous. Many of the nuclear changes seen in neoplastic cells reflect the frequent cell division, chromosomal abnormalities, and active metabolic state that characterize these cells.

Neoplastic cells often exhibit loss of characteristic cytoplasmic and nuclear features. For example, poorly differentiated mast cell tumors often lack the prominent cytoplasmic granules that are a hallmark of normal mast cells (Fig. 6-8). Special stains or immunohistochemistry may be able to highlight some characteristic morphologic feature retained in at least a subpopulation of tumor cells. As an example, characteristic granules may be revealed in some cells of feline and canine mast cell tumors by staining with toluidine blue or Giemsa. Many tumor cells have noticeably basophilic cytoplasm as a result of the presence of large numbers of ribosomes required for rapid cell growth and frequent cell division.

In tumors, normal tissue organization is usually lost to some extent. Increasing loss of normal architecture in tumors correlates with increasing independence of tumor cells from their surrounding tissue. As an example, lymphomas arising in lymph nodes often consist of solid sheets of neoplastic cells that partially or completely efface normal lymph node architecture (Fig. 6-9). In tissue that normally undergoes continual renewal, such as the skin and oral mucosa, the normal maturation sequence may be altered. Thus in squamous cell carcinomas, the orderly morphologic progression from basal cell layer to fully keratinized stratum corneum may not be seen (Fig. 6-10).

Function: Loss of differentiated function frequently accompanies loss of differentiated morphology in tumors. Thus neoplastic cells arising from alveolar lining cells of the lung generally fail to perform normal respiratory functions and tumors of primitive germ cell origin do not form normal sperm or ova. Some aspects of normal function may be retained. Thyroid adenomas may continue to produce thyroid hormones, and plasma cell tumors may secrete immunoglobulins. However, in the majority of cases, these functions are no longer regulated appropriately because the neoplastic cells have lost responsiveness to and dependence on normal regulatory pathways. Thus thyroid adenomas may produce clinical hyperthyroidism, and plasma cell tumors may cause hypergammaglobulinemia.

Behavior: Benign tumors are generally expansile and may compress adjacent tissue, whereas malignant tumors have invasive and in many instances metastatic capabilities. In malignant tumors, alterations in adhesion, motility, and protease production allow tumor cells to leave the tumor mass and penetrate surrounding tissue. Moreover, for malignant cells to invade and ultimately metastasize, they must become completely independent of local growth regulatory controls and acquire an independent blood supply. Acquisition of these features allows tumors to spread well beyond their ordinary anatomic niches.

Continuously Dividing Tissues (Labile Tissues): In continuously dividing tissues (also called labile tissues), cells proliferate throughout life, replacing those that are lost. These tissues include surface epithelia, such as stratified squamous surfaces of the skin, oral cavity, vagina, and cervix; the lining mucosa of all the excretory ducts of the glands of the body (e.g., salivary glands, pancreas, and biliary tract); the columnar epithelium of the gastrointestinal tract and uterus; the transitional epithelium of the urinary tract; and cells of the bone marrow and hematopoietic tissue. In most of these tissues, mature cells are derived from stem cells, which have an unlimited capacity to proliferate and whose progeny may differentiate into a variety of mature cell types.

Quiescent Tissues (Stable Tissues): Quiescent (or stable) tissues normally have a low level of replication; however, cells from these tissues can undergo rapid division in response to stimuli and are thus capable of reconstituting the tissue of origin. They are considered to be in the G₀ stage of the cell cycle but can be stimulated to enter G₁. This category includes the parenchymal cells of the liver, kidneys, and pancreas; mesenchymal cells, such as fibroblasts and smooth muscle; vascular endothelial cells; and resting lymphocytes and other leukocytes. The regenerative capacity of stable cells is best exemplified by the ability of the liver to regenerate after partial hepatectomy and after acute chemical injury. Fibroblasts, endothelial cells, smooth muscle cells, chondrocytes, and osteocytes are quiescent in adult mammals but proliferate in response to injury. Fibroblasts in particular may proliferate extensively.

Nondividing Tissues (Permanent Tissues): Nondividing (permanent) tissues contain cells that have left the cell cycle and cannot undergo mitotic division in postnatal life. Neurons and skeletal and cardiac muscle cells belong to this group. If neurons in the central nervous system (CNS) are destroyed, the tissue is generally replaced by the proliferation of the CNS supportive elements, the glial cells. However, recent results demonstrate that limited neurogenesis from stem cells may occur in adult brains. Although mature skeletal muscle cells do not divide, skeletal muscle does have some regenerative capacity, through the differentiation of the satellite cells that are attached to the endomysial sheaths. If the ends of severed muscle fibers are closely juxtaposed, muscle regeneration in mammals can be excellent, but this is a condition that can rarely be attained under practical conditions. Cardiac muscle has very limited, if any, regenerative capacity, and extensive injury to the heart muscle, as may occur in myocardial infarction, is followed by scar formation.

Proliferation: Cell proliferation is largely controlled by signals (soluble or contact-dependent) from the microenvironment that either stimulate or inhibit cell proliferation. An excess of stimulators or a deficiency of inhibitors leads to net growth. Although accelerated growth can be accomplished by shortening the cell cycle, the most important mechanism of growth is the conversion of resting or quiescent cells into proliferating cells by making the cells enter the cell cycle. Both the recruitment of quiescent cells into the cycle and cell-cycle progression require stimulatory signals to overcome normal physiologic blocks to cell proliferation. Cell proliferation can be stimulated under both physiologic and pathologic conditions. The proliferation of mammary epithelium under hormonal stimulation during lactation is an example of physiologic proliferation. Pathologic conditions, such as tissue injury, cell death, and mechanical alterations, also stimulate cell proliferation. Excessive physiologic stimulation may create pathologic conditions, such as enlargement of the thyroid, as a consequence of increased serum levels of thyroid-stimulating hormone.

Differentiation: Differentiation also impacts the size of a cell population and its proliferative potential. For example, myocytes and neurons are terminally differentiated cells (i.e., they are at an end stage of differentiation and are not capable of replicating). In some adult tissues, such as liver and kidney, differentiated cells are normally quiescent but are able to proliferate when necessary. In proliferative tissue, such as bone marrow and the epithelia of the skin and gut, the mature cells are terminally differentiated, short-lived, and incapable of replication, but they may be replaced by new cells arising from stem cells. Thus in such tissues there is a homeostatic equilibrium between the proliferation of stem cells, their differentiation, and the death of fully differentiated cells.

Cell Death: A variety of cell death mechanisms, including senescence, apoptosis, and autophagy, eliminate irreversibly damaged or effete cells to maintain normal tissue homeostasis. In response to DNA damage, oxidative stress, and telomere shortening, proliferating cells may undergo a permanent arrest in the G₁ phase of the cell cycle termed cellular senescence. Senescence is mediated by activation of the p53 or retinoblastoma pathways of cell cycle arrest. Senescent cells often express senescence-associated β-galactosidase.

Apoptosis is a form of “programmed cell death” that serves both as a normal physiologic process and as a response to pathologic stimuli. In proliferative tissue, such as gut epithelium, terminally differentiated cells undergo apoptosis and are thus removed from the cell population. Apoptosis may occur in response to withdrawal of survival factors from the cell environment or by binding of death factors, such as Fas ligand and tumor necrosis factor-α (TNF-α) to cell surface receptors. Hypoxia and lack of essential nutrients may end in apoptosis. DNA damage may also induce apoptosis; in this case, apoptosis is triggered by p53. Apoptosis may be stimulated by the activity of cytotoxic immune cells, including T lymphocytes and natural killer (NK) cells. Signals for apoptosis activate a variety of signaling pathways, many of which ultimately result in the release of cytochrome C from mitochondria. The final effectors of apoptosis are the caspases, intracellular proteases that selectively destroy cellular organelles and degrade genomic DNA into nucleosome-sized fragments. The morphologic hallmarks of apoptosis include margination of chromatin, condensation and fragmentation of the nucleus, and condensation of the cell with preservation of organelles. Ultimately, the cell breaks into membrane-bound apoptotic bodies that are engulfed by surrounding cells without stimulating an inflammatory response (Fig. 6-14).

Autophagy refers to degradation of a cell’s own organelles within autophagosomes. Autophagy can be a mechanism for cell survival in the face of nutrient deprivation, as it salvages important cellular components for reuse; however, extensive autophagy can also lead to a form of programmed cell death. The kinase mammalian target of rapamycin (mTOR) is the major cellular inhibitor of autophagy.

Latency: As illustrated in Fig. 6-16, the latent period for a tumor is the time before a tumor becomes clinically detectable. The smallest clinically detectable mass is about 1 cm in diameter and contains about 10⁹ cells. To form a tumor that size, a single transformed cell must undergo about 30 rounds of cell division, if all the progeny remain viable and capable of replication. Thus, by the time most tumors become clinically evident, they have probably been developing in the host for many years. However, once tumors reach a clinically detectable size, their growth may appear to be very rapid, because only 10 doubling cycles are required to convert a 1-g tumor into a 1-kg tumor. In fact, volume doubling times for tumors vary considerably, depending on the rate at which tumor cells divide, the fraction of tumor cells that are replicatively competent, and the rate at which tumor cells die. In general, benign neoplasms grow more slowly than malignant tumors, although there is considerable variation among tumors. Moreover, tumors may grow erratically, depending on their blood supply, the effect of extrinsic growth-regulating factors such as hormones, the efficacy of the host immune response, and the emergence of subpopulations of particularly aggressive tumor cells.

Proliferation: Many neoplastic cells no longer respond to extrinsic or intrinsic signals directing them into G₀ and no longer express functional p53. Thus the cells move continuously through the cell cycle. Moreover, because the tumor cells do not undergo cell-cycle arrest after DNA damage, they progressively accumulate potentially mutagenic DNA damage (Fig. 6-17). For homeostasis to be maintained, normal cells must engage in a continual dialogue with their environment. There is a constant exchange of information among cells via soluble mediators, including growth stimulatory factors, growth inhibitory factors, and hormones. These soluble mediators tightly control the growth of nonneoplastic cells. Neoplastic cells, on the other hand, often lose both their dependence on extrinsic growth stimulatory substances and their susceptibility to growth inhibitory signals from their environment. The mechanisms by which this occurs are discussed later. The end result is that tumor cells are no longer responsive to the needs of the organism as a whole and develop the capacity to drive their own replication.

The mitotic index is usually defined as the number of tumor cells in a microscopic field that contain condensed chromosomes and lack nuclear membranes (Fig. 6-18). Such cells are interpreted as being actively dividing, and the mitotic index of a tumor is considered to indicate its malignant potential. However, the mitotic index can be misleading. The fraction of tumor cells observed to be in mitosis depends not only on the number of cells undergoing mitosis but also on the length of time required to complete the process. In tumor cells, the time required for completion of the cell cycle is generally as long as or even longer than for normal cells. Mitotic figures may persist in cells unable to complete cell division and abnormal mitotic figures may be seen.

Differentiation: As discussed previously, tumor cells are generally less differentiated than normal cells. In some instances, however, some tumor cells can be forced to differentiate into more mature, near-normal cells. Leukemia cells are particularly susceptible to differentiation therapy, and retinoids are routinely employed to treat acute promyelocytic leukemia in human patients. Other differentiating agents, including vitamin D compounds and cytokines, have been less effective in this disease. Vitamin D compounds are showing some promise in differentiation therapy of human epithelial tumors, and compounds that epigenetically alter tumor cells by modifying the histones in chromatin may also enhance differentiation of tumor cells (discussed later). A common assumption underlying differentiation therapies is that more differentiated tumor cells will have a less stem cell-like phenotype and will thus have reduced proliferative potential.

Cell Death: Because the DNA replication machinery is unable to duplicate the extreme ends of DNA templates, the telomeres that form the ends of chromosomes are shortened at each cell division. Embryonic cells express telomerase, a riboprotein enzyme that allows telomeres to be replicated and even expanded; however, most adult cells do not express this protein and their telomeres shrink with each round of cell division. Very short telomeres are incompatible with continued cell division and trigger cellular senescence in normal cells. However, many neoplastic cells regain the ability to produce telomerase and thus to replicate their telomeres. Reexpression of telomerase appears to play an important role in the escape of tumor cells from senescence and their consequent immortality.

Although virtually all normal cells in the body can undergo apoptosis in response to appropriate physiologic signals, many cancer cells acquire resistance to apoptosis. This blocks a major route of tumor cell loss and enhances the overall growth rate of the tumor. Many tumor cells circumvent apoptosis by functional inactivation of the p53 gene, thus removing a key proapoptotic molecule. Additionally, tumor cells may constitutively activate survival signaling pathways, rendering the cells independent of exogenous survival factors. Finally, tumor cells may develop mechanisms for inactivating death factor signaling pathways, thus evading apoptosis in response to homeostatic signals from the cellular environment.

Autophagy plays a poorly understood and somewhat paradoxic role in tumor growth. In many tumors, authophagy is suppressed, thus presumably preventing autophagic tumor cell death. However, increased autophagy may also enhance tumor cell survival under the conditions of reduced nutrient availability that arise during therapy.