p53 Gene A normal suppressor gene that controls cell division and stimulation of repair enzymes to rebuild and restore damaged regions of the DNA. Damage or inactivation of the p53 gene is considered a contributing cause of most cancers. Peripheral Nervous System (PNS) All nervous tissue outside the central nervous system. Peripheral Neuropathy Abnormal and detrimental changes to nervous tissue outside the brain or spinal cord. Permanent Cells Body cells that never divide and do not have the ability for replication even when stressed or when some cells die.  Examples are neurons and muscle cells. Peroxisomes Very small, membrane-bound organelles which contain a large variety of enzymes that perform a diverse set of metabolic functions. Physiological Adaptation The ability of the body to adapt to changes or stresses so that the change is beneficial.  Increase in muscle mass with exercise is an example of physiological adaptation. PNS The peripheral nervous system.  All nervous tissue outside the CNS. Point Mutation A change in the DNA sequence in a gene. Pons A section of the brain that functions as a relay center and assists in somatic and visceral motor control. Poorly-differentiated The change in a cell so that it has lost much of the normal appearance. Progression Stage The third recognized step in the carcinogenic process that is associated with the development of the initiated cell into a biologically malignant cell population. Proliferation The reproduction or multiplication of similar forms, especially cells. Promotion Phase The second step in the carcinogenesis process in which specific agents (referred to as promoters) enhance the further development of the initiated cells. Proteins The most diverse and abundant of organic compounds in the body.  There are about 100,000 different kinds of proteins that perform a large variety of important functions, such as the protein pores in cell membranes, keratin in skin and hair, collagen in ligaments and tendons, myosin in muscles, and hemoglobin in RBCs. The building blocks for proteins are the 20 amino acids. Proto-oncogenes Normal or good cellular genes that instruct the production of the regulatory proteins and growth factors within the cell or its membrane. Activation of a proto-oncogene can cause alteration in the normal growth and differentiation of cells, which leads to neoplasia. Pulmonary Fibrosis Changes in the lining of the pulmonary alveoli in which the normal epithelial cells are replaced by fibrous tissue.  Gases poorly diffuse across the fibrous tissue and thus gas exchange is drastically reduced in the lungs. Pyknosis A degenerative change in a cell in which it thickens with a shrinking of the nucleus and the chromatin condenses to a solid, structureless mass or masses.