Neoplasia

NEOPLASIA
Neoplasia is defined as: "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 that evoked the change."
Neoplasia has genetic and environmental causes. It is important to note that both play parts in causing   neoplasia.
Carcinogens/process of carcinogenesis
Chemical carcinogenesis

Initiation
  * Results from interaction of chemical with DNA to activate a proto-oncogene or inactivate a tumor suppressor gene by formation of covalent adducts.
  * Chemicals that can form adducts (direct acting) are usually electrophiles.
  * Many chemical carcinogens require activation by metabolic pathways (pro-carcinogens or indirect acting carcinogens) an example of a metabolic pathway is the p-450 cytochrome mono-oxygenase.
  * Initiation alone does not result in tumours.
Promotion
  * Promotors are usually irritants or substances that produce cell activation and proliferation.
  * Effects of promotors are reversible.
Promotors cannot induce neoplasia: i) alone, ii) if applied before initiator, iii) if applied in too small an amount for effect, or iv) if too much time elapses between applications.  

Ultraviolet Light
  * Strong epidemiologic relationship to squamous cell, basal cell, and melano-carcinoma in fair skinned people.
  * Causes formation of pyrimidine dimers in the DNA leading to mutations.
  * Activation of T-suppressor cells facilitates emergence of tumor clones.
  * Individuals with defects in the enzymes that mediate DNA excision-repair are especially susceptible.
Ionizing Radiation
  * Ionizing radiation includes: X-rays, gamma rays, as well as particulate radiation; alpha, beta, positrons, protons, neutrons and primary cosmic radiation.   All forms are carcinogenic with special sensitivity in:
      * Bone Marrow: Acute leukemia occurs before other radiation-induced neoplasia (Seven year...