推荐回答（2个）
人体肿瘤的形成，即由正常细胞转变为癌细胞，是一个相当长的过程。通常需要许接触致癌物多年之后，使组织、器官的细胞引起进行性的重度不典型增生．才演变成癌。这一期间体力诱导期．一般长达15—30年。
人体发生肿瘤的原因很多，但总的来说肿瘤的发生既与外源性致癌的性质、强度和作用时间有着—定的关系，同时也与人体的内在因素有重要的关系。外源性致病因素包括物理性致癌因素、化学性致癌因素等。内源性因素包括内分泌功能紊乱、神经精神因素、免疫状态和遗传因素等.尽管外源件致癌因素的存在容易发生癌肿，但是处于同样条件下接触同质、同量致癌因素，有的人发病，有的人则不发病，可见外因虽然很重要．但必须在内因的基础上才能起作用，
细胞分子学研究的进展，使人们了解到细胞是个复杂的精细的生命结构。许多致癌物质，有的影响细胞核的基因使之发生改变，有的作用于细胞膜表面，通过膜上的物理和生物化学的变化，影响细胞的代谢环节。由于这些影响，细胞内酶系统和细胞核功能发生紊乱，使得处于静止或正常分裂的细胞出现分裂启动和去氧核糖核酸合成促进现象,导致细胞分裂、分化和生长行为的一系列改变，引起癌变。
最新的癌基因理论认为，肿瘤的发生是由于细胞的增殖与分化失常。当细胞分裂增加，而分化或死亡减少时，未分化的终极细胞数增多，即出现了恶性生长现象。细胞的增殖常由调控失去平衡引起。癌基因学说阐明了一些影响到细胞增殖或癌变关键控制点的基因变化，并认为有两类基因直接参与肿瘤的发生，它们是癌(肿瘤)基因和肿瘤抑制基因。癌(肿瘤)基因是指在自然或实验条件下，具有潜在诱导细胞恶性转化的基因。癌(肿瘤)基因的表达产物对细胞的增殖起正调节，当它们发生结构改变或表达过度，促生长的作用过强，会引起细胞的过度增生，而另一类肿瘤抑制基因的产物，则对细胞的增殖起抑制作用，当它们的结构与功能改变时，失去了对细胞增殖的负调节作用，也会发生使细胞增生的信息。在两种基因中的任何一种或共同的变化下，即有可能导致肿瘤的发生。
肿瘤是机体在各种致癌因素作用下，局部组织的某一个细胞在基因水平上失去对其生长的正常调控，导致其克隆性异常增生而形成的新生物。一般认为，肿瘤细胞是单克隆性的，即一个肿瘤中的所有瘤细胞均是一个突变的细胞的后代。
一般将肿瘤分为良性和恶性两大类。所有的恶性肿瘤总称为癌症
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