neoplasia
TRANSCRIPT
Neoplasia
Neoplasia means “new growth,” and a newly grown cell mass is called a neoplasm.
Definition of neoplasm by eminent British oncologist Willis:
“A neoplasm is an abnormal mass of tissue, the growth of which exceeds and is uncoordinated with that of the normal tissues and persists in the same excessive manner after cessation of the stimuli which evoked the change.”
Neoplasia
Cancer is not a single disease but many disorders that share a profound growth dysregulation.
Some cancers, such as Hodgkin lymphoma, are curable, whereas others, such as pancreatic adenocarcinoma, have a high mortality.
Cancer cells are immortal and have limitless proliferative capacity
A tumor is said to be benign when its microscopic and gross characteristics are considered relatively innocent, implying that it cannot spread to other sites
Malignant neoplasm is a lesion that can invade and destroy adjacent structures and spread to distant sites (metastasize) to cause death. Not all cancers pursue such deadly course.
Malignant tumors are collectively referred to as cancers
Characteristics of Benign and Malignant Neoplasms
In general, benign and malignant tumors can be distinguished on the basis of the following characteristics:
1. Differentiation and anaplasia
2. Rate of growth
3. Local invasion
4. Metastasis.
Characteristics Discussion on characteristics
Differentiation and anaplasia
Differentiation refers to the extent to which neoplastic parenchymal cells resemble the corresponding normal parenchymal cells, both morphologically and functionally; lack of differentiation is called anaplasia. In general, benign tumors are well differentiated.Malignant neoplasms are characterized by a wide range of parenchymal cell differentiation, from surprisingly well differentiated to completely undifferentiated. The morphologic diagnosis of malignancy in well-differentiated tumors may sometimes be quite difficult.Malignant neoplasms that are composed of poorly differentiated cells are said to be anaplastic. Lack of differentiation, or anaplasia, is considered a hallmark of malignancy.
Rate of growth The rate of growth of a tumor is determined by three main factors: the doubling time of tumor cells, the fraction of tumor cells that are in the replicative pool, and the rate at which cells die. Because cell cycle controls are deranged in most tumors, tumor cells can be triggered to cycle without the usual restraints. The dividing cells, however, do not necessarily complete the cell cycle more rapidly than do normal cells. The progressive growth of tumors and the rate at which they grow are determined by an excess of cell production over cell loss. In some tumors the imbalance is large. Example: Some leukemias
Characteristics Discussion on characteristics
Local invasion Nearly all benign tumors grow as cohesive masses that remain localized to their site of origin and do not have the capacity to infiltrate, invade, or metastasize to distant sites, as do malignant tumors. Because they grow and expand slowly, they usually develop a rim of compressed connective tissue, sometimes called a fibrous capsule, which separates them from the host tissue. This capsule is derived largely from the extracellular matrix of the native tissue due to atrophy of normal parenchymal cells under the pressure of an expanding tumor. Such encapsulation does not prevent tumor growth.The growth of cancers is accompanied by progressive infiltration, invasion, and destruction of the surrounding tissue. In general, malignant tumors are poorly demarcated from the surrounding normal tissue
metastasis Metastases are tumor implants discontinuous with the primary tumor. Metastasis unequivocally marks a tumor as malignant because benign neoplasms do not metastasize. The invasiveness of cancers permits them to penetrate into blood vessels, lymphatics, and body cavities, providing the opportunity for spread.In general, the more aggressive, the more rapidly growing, and the larger the primary neoplasm, the greater the likelihood that it will metastasize or already has metastasized. Approximately 30% of newly diagnosed individuals with solid tumors present with metastases .Metastatic spread strongly reduces the possibility of cure
INVASION AND METASTASIS
Invasion and metastasis are biologic hallmarks of malignant tumors. They are the major cause of cancer-related morbidity and mortality. Studies in mice and humans reveal that although millions of cells are released into the circulation each day from a primary tumor, only a few metastases are produced.
Each step in the process is subject to a multitude of controls; hence, at any point in the sequence the breakaway cell may not survive.
Carcinogenic Agents and Their Cellular Interactions
The following agents can cause carcinogenesis:
1. Chemical2. Radiation3. Microbial
Carcinogenic Agents and Their Cellular Interactions(contd.)
1. Carcinogenesis caused by chemical agents:
Some chemical carcinogens are called initiators and they cause initiation of carcinogenesis in cells. Initiation causes permanent DNA damage (mutations). Initiation alone, is not sufficient for tumor formation. A different type of chemical carcinogens called Promoters, can induce tumors in initiated cells, but they are nontumorigenic by themselves. promoters leads to proliferation and clonal expansion of initiated (mutated) cells. Driven to proliferate, the initiated clone of cells suffers additional mutations, developing eventually into a malignant tumor.
Figure: Carcinogenesis caused by chemical agents
2. Carcinogenesis caused by radiation:
Different radiations can cause carcinogenesis. UV light is responsible for causing skin cancers, and ionizing radiation exposure from medical or occupational exposure, nuclear plant accidents, and atomic bomb detonations has produced a variety of cancers.
The carcinogenicity of UVB light is attributed to its formation of pyrimidine dimers in DNA. This type of DNA damage is repaired by the nucleotide excision repair pathway. When UV radiation is excessesive, the repair mechanism fails and error-prone alternative DNA-repair mechanisms become active that result in survival of the cell at the cost of genomic mutations. These mutations may lead to cancer.
Carcinogenic Agents and Their Cellular Interactions(contd.)
3. Carcinogenesis caused by viruses:
Virus/bacteria Cancer: mechanism of cancer generation
Oncogenic RNA Viruses (ex. Human T-Cell Leukemia Virus Type 1)
T-cell leukemia: A component of viral genome called Tax stimulates proliferation of T cells with molecular and chromosomal abnormalities.
Oncogenic DNA Viruses (ex. Hepatitis C Virus)
Liver cancer: Components of the HCV genome, such as the HCV core protein, may have a direct effect on tumorigenesis, possibly by activating a variety of growth-promoting signal transduction pathways
Bacteria (Helicobacter pylori) gastric adenocarcinoma: increased epithelial cell proliferation in a background of chronic inflammation. There is an initial development of chronic gastritis, followed by gastric atrophy, intestinal metaplasia of the lining cells, dysplasia, and cancer.
Carcinogenic Agents and Their Cellular Interactions(contd.)
Oncogenes and cancerFour classes of normal regulatory genes—the growth-promoting proto-oncogenes, the growth-inhibiting tumor suppressor genes, genes that regulate programmed cell death (apoptosis), and genes involved in DNA repair—are the principal targets of genetic damage.
Mutant alleles of proto-oncogenes are considered dominant, because they transform cells despite the presence of a normal counterpart.
In contrast, typically, both normal alleles of the tumor suppressor genes must be damaged before transformation can occur.
Genes that regulate apoptosis may behave as proto-oncogenes or tumor suppressor genes.
Mutations of DNA repair genes do not directly transform cells by affecting proliferation or apoptosis. Instead, DNA-repair genes affect cell proliferation or survival indirectly by influencing the ability of the organism to repair nonlethal damage in other genes, including proto-oncogenes, tumor suppressor genes, and genes that regulate apoptosis. A disability in the DNA-repair genes can predispose cells to widespread mutations in the genome and thus to neoplastic transformation. Cells with mutations in DNA repair genes are said to have developed a mutator phenotype.
A new class of regulatory molecules, called microRNAs (miRNAs), has recently been discovered. Even though they do not encode proteins, different families of miRNAs have been shown to act as either oncogenes or tumor suppressors. They do so by affecting the translation of other genes