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Unit2. Cells and Aging
Aging is a normal process accompanied by a progressive alteration of the body’s homeostatic adaptive responses; the specialized branch of medicine that deals with the medical problems and care of elderly person is called geriatrics.
衰老是一种正常的生理过程，伴有机体内环境稳定适应性应答的进行性改变。研究老年人的健康问题和保健的一门特殊医学分支被称作老年病学
The obvious characteristics of aging are well known; graying and loss of hair, loss of teeth, wrinkling of skin, decreased muscle mass, and increased fat deposits. The physiological signs of aging are gradual deterioration in function and capacity to maintain a constant internal environment (homeostasis) in response to changes in temperature, diet, and oxygen supply. These sings of aging are related to a net decrease in the number of cells in the body and to the dysfunctioning of the cells that remain.
衰老的明显特征众所周知：头发花白和脱落，牙齿脱落，皮肤起皱，肌肉减少，脂肪积存增加。衰老的生理征兆是机体对环境刺激反应的功能和能力的逐渐退化如同维持机体内环境稳定来应对温度，饮食和氧气供应改变的能力减退一样，机体新陈代谢也减慢了。衰老的这些迹象与机体中细胞数目的净减少以及存余细胞的功能紊乱有关。
The extracellular components of tissues also change with age. Collagen fibers,
responsible for the strength in tendons, increase in number and change in quality with aging. These changes in the collagen of arterial walls are as much responsible for their loss of extensibility as are the deposits associated with atherosclerosis, the deposition of fatty materials in the arterial walls. Elastin, another extracellular component, is responsible for the elasticity of blood vessels and skin. It thickens, fragments, and acquires a greater affinity for calcium with age – changes that may also be associated with the development of atherosclerosis.
组织的细胞外成分也随着年龄的变化而变化。负责肌腱力量的胶原纤维的数量增加，而质量却随着衰老降低。动脉壁上的胶原质的变化与动脉壁伸展性丧失有关，正如动脉粥样硬化，即动脉壁脂肪物质堆积造成动脉壁伸展性缺失一样。弹性蛋白是另一种细胞外成分，与血管和皮肤的弹性有关，随着年龄的变化，它变粗，易断裂并且与钙有很大的亲和力，这些变化可能也与动脉粥样硬化的发生有关。
Glucose, the most abundant sugar in the body, may also play a role in the aging process. According to one hypothesis, glucose is added, haphazardly, to proteins inside and outside cells, forming irreversible cross-links between adjacent protein molecules. As a person ages, more cross-links are formed, and this probably contributes to the stiffening and loss of elasticity in aging tissues.
葡萄糖是机体中含量最多的糖类，它在衰老的过程中可能也起作用。根据一个假说，随机地在细胞内外蛋白质上添加葡萄糖，结果会在相邻蛋白质分子之间形成不可逆的交联。随着人的衰老，会形成更多的交联，这很可能就是造成衰老的组织僵硬及弹性丧失的原因。
Although many millions of new cells normally are produced each minute, several kinds of cells – heart cells, skeletal muscle fibers, nerve cells - cannot be replaced, Experiments have shown that many other cell types have only a limited capability to divide. Cells grown outside the body divide only a certain number of times and then stop. The number of divisions correlates with the donor/s age and with the normal life span of the different species from which the cells are obtained. These observations provides strong evidence for the hypothesis that cessation of mitosis is a normal, genetically programmed event.
According to this view, an “aging” gene is part of the genetic blueprint at birth, and it turns on at a preprogrammed time, slowing down or halting processes vital to life.
虽然正常情况下，每分钟会有好几百万的新细胞产生，但有几种细胞：心脏细胞，骨骼肌纤维细胞，神经细胞是无法替代的。实验显示，许多种类的细胞分裂能力有限。在机体外生长的细胞仅仅分裂几代就停止了。细胞分裂数与捐献者的年龄和这些细胞来自的不同物种的正常寿命有关。这些发现为这种假说提供了有力的证据，即（细胞的）有丝分裂的终止是正常的，是由基因决定的。根据这个观点，衰老基因是出生时就存在的基因蓝图的一部分，在预定的时间里开启，减慢或停止对生命至关重要的生理过程。
Another theory of aging is the free radical theory. Free radicals are electrically charged molecules that have an unpaired electron. Such molecules are unstable and highly active and can easily damage proteins. Some effects are wrinkled skin, stiff joints, and hardened arteries. Free radical may also damage DNA. Among the factors that produce free radicals are air pollution, radiation, and certain foods we eat. Other substances in the diet such as vitamin E, vitamin C, beta-carotene, and selenium are antioxidants and inhibit free radical formation. The free radical theory of aging is bolstered by two recent discoveries. Strains of fruitt flies bred for longevity produce larger-than-normal amouts of an enzyme called superoxide dismutase, which functions to neutralize free radicals. Also, injection of genes that lead to production of superoxide dismutase into fruit fly embryos prolongs their average lifetime.
衰老的另一个理论即自由基理论，自由基是含有未配对电子的带电荷分子。这种分子性质不稳定，具有高反应活性，容易损害蛋白质。自由基的影响有：皮肤起皱，关节僵直，动脉硬化。自由基也可以损害DNA。产生自由基的因素有：空气污染，放射线，以及我们吃的某些食物。饮食中的其他物质如维生素E，维生素C，β-胡萝卜素以及硒都是抗氧化剂，可以抑制自由基的形成。最近的两个研究支持了衰老的自由基理论。孕育长寿的果蝇株从而产生超正常量的酶：过氧化物歧化酶，它可以中和自由基。同样，把产生过氧化物歧化酶生成的基因注入果蝇胚胎中会延长其平均寿命。 Whereas some theories of aging explain the process at the cellular lever, others
concentrates on regulatory mechanisms operating within the entire organism. For example, the immune system, which manufactures antibodies against foreign invaders, may start to attack the body’s own cells. This autoimmune response might be caused by changes in the surfaces of cells, causing antibodies to attach and mark the cell for destruction. As surface changes in cells increase, the autoimmune response intensifies, producing the well-known signs of aging.
然而，关于衰老的理论，有些是在细胞水平上解释其过程，有的则强调整个机体内运作的调节机制，比如免疫系统产生各种抗抗外来异物的抗体，也可能会对自体细胞发
起攻击。这种自身免疫应答可能是细胞表面的变化造成，引起抗体附着并标记细胞进行破坏。当细胞表面变化增加，自身免疫应答加强，产生众所周知的衰老的标志。