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Here's a nice, concise description of the phenomenon of programmed
cell death, which I had brought up in my earlier post.
From : http://www.acs.ucalgary.ca/~browder/apoptosis.html Programmed Cell Death in Development Programmed cell death (PCD) is an important mechanism in both development and homeostasis in adult tissues for the removal of either superfluous, infected, transformed or damaged cells by activation of an intrinsic suicide program. One form of PCD is apoptosis, which is characterized by maintenance of intact cell membranes during the suicide process so as to allow adjacent cells to engulf the dying cell so that it does not release its contents and trigger a local inflammatory reaction. Cells undergoing apoptosis usually exhibit a characteristic morphology, including fragmentation of the cell into membrane-bound apoptotic bodies, nuclear and cytoplasmic condensation and endolytic cleavage of the DNA into small oligonucleosomal fragments (Steller, 1995). The cells or fragments are then phagocytosed by macrophages. Signals that can trigger apoptosis can include lineage information, damage due to ionizing radiation or viral infection or extracellular signals. Extrinsic signals may either suppress or promote apoptosis, and the same signals may promote survival in one cell type and invoke the suicide program in others (Steller, 1995). Invocation of the suicide program involves the synthesis of specific messenger RNA molecules and their translation. PCD can sometimes be suppressed by inhibiting transcription or translation (Steller, 1995), which provides evidence that cell death is mediated by intrinsic cellular mechanisms. Cell death is currently the subject of considerable research activity. This interest stems, in part, from the potential for understanding oncogenesis and the possibility of exploiting the cell death program for therapeutic purposes. For example, inhibition of cell death might contribute to oncogenesis by promoting cell survival instead of death. Likewise, triggering cell death might provide the means for eliminating unwanted cells (e.g., tumor cells). This might be accomplished by harnessing tumor necrosis factor (TNF), which triggers apoptosis in some target cells. Recognition of PCD as a developmental mechanism dates back to the 1930's. Developmental processes that involve PCD include: Elimination of transitory organs and tissues. Examples include phylogenetic vestiges (pronephros and mesonephros
in higher vertebrates), anuran tails and gills and larval organs of
holometabolous insects.
Tissue remodeling. Vertebrate limb bud development (Fig. 11.42, Saunders, 1982; Fig. 1,
Saunders, 1966) is an example. If PCD fails, in formation of the digits, digits
remain joined by soft tissue. Compare, for example, the situation in the chick
and duck hind limbs. If chick limb mesoderm is combined with duck ectoderm, PCD
fails and the digits remain joined (Saunders, 1966). This observation implicates
the ectoderm in providing the signal to trigger PCD. Another example is
formation of heart loops during vertebrate development. Depletion of cells in
spinal ganglia occurs during development of the chick embryo. As shown in Table
11.1 (Saunders, 1982), there is precise chronological and spatial control over
this process. Interestingly, injections of nerve growth factor reduce the
frequency of cell death in the spinal ganglia. This observation provides a link
between growth control and PCD.
----- Original Message ----- From: Jerry Zhu To: *** Sent: Wednesday, July 06, 2005 7:40 PM Subject: Re: [ROSEN] What causes aging? Hi Judith, Enjoy your post. Thanks! I bought a book by MIT prof. about cancer. A two hour quick skim leads me totally disagree. I will get into that book in detail later time. Aging process is very important phenomenon to understand life. I doubt that science can privide the answer. I believe it belongs to the realm of philosophy. (I need to define science and phiolosphy here to support my arguments but I am lazy to do that here) How long cancer reserach has been done? any reasonable achievements? Science and philosophy are not related in scientific way as we see today. Some scientists carry their research with philosophyical sense unconsciously. But I doubt this will address major issues like aging and cancer. the disappearance of "webing" tissue between fingers probably not killed themselves. It maybe that they split in the middle to attach to both sides in the process of cell differentiation under the impact of substrates. I also think that aging is self caused. External cause maybe different chemical processes from aging's eventhough they may demontrate similar symptons. Later soon Jerry Zhu |