终末分化（Terminal cell differentiation）

　　细胞质的多少，形态及特征性分化反映了肿瘤细胞的分化程度和恶性程度。

　　1.高分化恶性肿瘤：胞质丰富骨有特征性分化，例如鳞状细胞癌，癌细胞胞质丰富可出现圆形，梭形、纤维形和蝌蚪形癌细胞。其特征分化表现为鳞癌细胞 胸质骨角化物，呈深红染色;腺癌细胞胞质内有分泌空泡，横纹肌肉瘤瘤细胞质内出现横纹等。

　　2.低分化恶性肿瘤：肿瘤细胞分化愈差，其胞质愈少，电镜下细胞器内质网。线粒体、高尔复合体、中心体愈少。

　　3.恶性肿瘤细胞胞质：呈嗜酸碱性染色即红中带蓝，全深染色这是由于癌细胞繁殖迅速，合成自身蛋白质较多，致甩质呈深红色;又由于合成蛋白质时核蛋白体增多，故胞质为嗜碱性略呈蓝色。

　　4.肿瘤细胞胞质内有时可见吞噬的异物，如红细胞、细胞碎片等。有时见癌细胞胞质内含有另一胩癌细胞，称封入细胞。

 癌的生物学基础 - Google 图书结果

Terminal cell differentiation http://www.sbs.utexas.edu/shankland/BIO349/lc23term.htm

April 20, 2003

I. Embryos rely on specialized nutrient sources (e.g. yolk), and have very little interactio with the external environment. However, the embryo must construct a postembryonic body that can successfully feed, locomote, and defend itself in the world at large. Thus, during the latter stages of embryogenesis the organism begins to build functional organs composed of cells that are highly specialized for some mature physiological function - e.g. epidermal cells that minimize evaporation; red blood cells that transport O2; muscle cells that contract on command.

II. Differentiation is a general term the process by which the various cells of an organism develop their unique (or 'differentiated') properties. The phrase terminal differentiation refers to the latter part of this process in which a cell develops those properties that are associated with its mature function.

• As suggested by the name, terminal differentiation is usually the final stage in the maturation of a cell. There are some situations in which a terminally differentiated cell can 'dedifferentiate' and transform into another type of cell [see "transdifferentiation", in lecture 2/6], but such events are the exception, not the rule.
• Terminally differentiated cells fall into general tissue-types (or `histotypes'). In animals, the four main tissue-types are: epithelial cells, neural cells, muscle cells, and connective tissue. There are many, many subtypes within each category.
• There are certain general features that are true or at least typical of terminal differentiation for most types of cell.
  1. During terminal differentiation, a cell actively begins to express hundreds or thousands of previously silent differentiation genes. These genes encode proteins and/or RNAs that are specifically required for the mature cell function - e.g. hemoglobin in red blood cells; proteins of the contractile apparatus in muscle cells.
  2. In addition, terminal differentiation often involves a compartmentalization of particular functions to different parts of the cell, typically reflected by the subcellular localization of particular gene products and in some cases by elaborate changes in cell shape.
  3. In some tissues, terminal differentiation is also accompanied by cessation of the cell cycle, i.e. the terminally differentiated cell is postmitotic. This is especially true of cells such as neurons that would have to disassemble their elaborate morphology in order to undergo cell division. [Note that some terminally differentiated cells are polyploid, i.e. they continue to replicate their genomic DNA without undergoing cell division.]

II. Some features of terminal differentiation are exemplified by the formation of blood cells, a process called hematopoiesis.

• Terminally differentiated blood cells have a life-span much shorter than that of the organism, e.g. average life-span of a human erythrocyte (= red blood cell) is 120 days. Hence, hematopoesis involves a continual cell turnover in which old erythrocytes die (and are removed from the blood) and new erythrocytes are produced.
• In vertebrate animals, hematopoeisis occurs in two phases.
• During the first or embryonic phase of hematopoesis differentiated blood cells are generated by the ventral mesoderm, specifically the blood islands of the yolk sac. Few if any of these earliest blood cells live past birth/hatching.
• The second or definitive phase begins late in embryogenesis and continues postembryo-nically. The key element in this second phase is the hematopoietic stem cell. These stem cells arise from mesoderm around the aorta, but move from there (through the bloodstream) to other organs and eventually take up permanent residence in the bone marrow.
• Like the spermatogonium [see lecture, 1/21], the hematopoietic stem cell is a self-renewing stem cell capable of dividing to produce both (1) more differentiated cell types [i.e. blood cells] and (2) more stem cells like itself. Because of this potential for self-renewal, the stem cell population established in the embryo is never exhausted and continues to produce new blood cells throughout the animal's life.
• There are certain advantages to having a stem cell population that is capable of replenishing a population of differentiated cells.
  • The differentiated cells can be 'designed' for short life-spans. This is particularly important for cells that are damaged or destroyed during their normal function, e.g. epithelial cells lining the wall of the stomach.
  • A stem cell population can rapidly compensate for traumatic tissue loss. For example, a human normally replaces 0.8% of his/her erythrocytes every day. But significant blood loss (say 10% of total volume) will cause the release of a hormone (erythropoietin) that causes the stem cells in the bone marrow to increase their production of new erythrocytes. This increased formation continues until total number of erythrocytes has returned to normal.
  • A stem cell population also allows for adaptation to different environments. For example, someone who moves from sea-level to high altitudes will initially suffer from hypoxia due to the reduced O2 content of the air. Here as well, the hematopoetic system responds to this situation by increasing the rate of erythrocyte production, increasing the erythrocyte population until it is once again sufficient to meet the O2 demands of the body.
• The hematopoietic stem cell is pluripotent, and has the potential to generate any of the dozen or more types of differentiated blood cell.
  1. The first experimental demonstration that hematopoietic stem cells are pluripotent came from the 'spleen colony assay' in which bone marrow was transplanted into mice whose own hematopoietic cells had been eliminated by one of several techniques.
  2. Some of the transplanted bone marrow cells colonized the recipient's spleen, which formed large colonies of developing blood cells [see Gilbert, Fig. 15.22]. When there were only a few such colonies, the researchers felt confident that each colony was a clone descended from a single transplanted marrow cell.
  3. Many clones contained only one blood cell type. However, some clones contained multiple different cell types, suggesting that the founder cell was pluripotent.
  4. To be certain that these mixed colonies were truly clones, researchers genetically marked the marrow cells by X-ray irradiation prior to transplantation. X-rays will cause a unique pattern of chromosomal rearrangements within each irradiated cell, and the multiple blood cell types within a single spleen colony shared the same chromosomal markers and therefore arose from the same bone marrow stem cell.

IV. Another instructive example of terminal cell differentiation is myogenesis or the formation of muscle. We will here focus on the formation of skeletal muscles from the myotome of vertebrates.

• Myotome cells go through a series of stages to generate a mature skeletal muscle cell.
  1. The first step is transformation into a myoblast. Myoblasts remain mitotically active, but are committed to the formation of muscle to the exclusion of other cell types.
  2. The second step is that the myoblasts cease mitosis, and fuse into a large, tubular, multinucleate cell called a myotube. The first definitive evidence that myotubes arise by cell fusion (rather than nuclear division) came from studies in which chimeras were generated by fusing mouse embryos carrying two distinguishable alleles of a muscle cell protein [see the handout, Fig. 9.7].
  3. The third step is the differentiation of the myotube into a functional myofiber or muscle fiber. Myofibers express an ensemble of proteins that permit them to contract when stimulated by nerves. In the case of skeletal muscles, these contractile proteins arrange themselves in a precise cytoskeletal organization that lends the fiber a striped or 'striated' appearance.
• The process of myogenesis is regulated by a family of related gene products called the myogenic proteins. The myogenic proteins are transcription factors of the basic-helix-loop-helix (bHLH) type that activate the transcription of muscle-specific differentiation genes.
  1. Two of these proteins - MyoD and myf5 - are both expressed in the myotome prior to myogenesis, and initiate the process. The function of these two gene products is largely redundant - i.e. homozygous knockout of either gene alone causes a modest reduction in muscle formation; whereas homozygous knockout of both genes completely blocks the formation of somitic muscle.
  2. MyoD was first discovered because it is capable of causing the muscle differentiation when misexpressed in cultured cell lines that would not otherwise form muscle.
• Vertebrate animals produce their full complement of skeletal myofibers early in development (by birth, in mammals), and in the absence of injury these fibers survive and function through the remainder of the animal's life. However, skeletal myofibers grow considerably in length and width during postembryonic development, and normal growth depends upon a special subset of myoblasts called satellite cells.
  1. Unlike other myoblasts, the satellite cells do not fuse to form myotubes during the embryonic phase of myogenesis. Rather, the satellite cells persist interspersed between the myofibers, where they play no part in muscle contraction.
  2. During postembryonic growth, satellite cells fuse at the ends of the already existing myofibers to increase their length and the number of nuclei/fiber. The unfused satellite cells undergo mitosis (i.e. self-renewal), so that the population of satellite cells is never exhausted as the muscle grows.
  3. In addition to their role muscle growth, the satellite cells of skeletal muscle can function as stem cells if the original myofibers are destroyed by injury or disease. Under these pathological conditions, satellite cells can fuse with one another to form new muscle fibers. However, in mammals the ability of satellite cells to produce new myofibers is limited, and muscles that have lost myofibers to injury or disease can only undergo partial regeneration.

Learning goals

1. What is distinctive about the phase of cellular development that is called terminal differentiation?

2. What is the spleen colony assay? How was it used to demonstrate that hematopoietic stem cells are pluripotent?

3. How does the hematopoietic stem cell system allow the body to alter the size of its erythrocyte population in response to blood loss or changes in environmental conditions?

4. Learn the sequence of stages by which myotome cells give rise to skeletal muscle fibers.

5. What is the developmental function of the myogenic proteins? Why must both MyoD and myf5 be inactivated to prevent the formation of skeletal muscle?

6. What are satellite cells, and how do they contribute to the postembryonic changes in skeletal muscles?

Panagiotis A. Tsonis http://molinterv.aspetjournals.org/content/4/2/81.full

Urodele amphibians, such as salamanders and newts, are largely known for their extraordinary regenerative capabilities. These animals can regenerate amputated limbs, tail, removed retina and lens, heart, spinal cord, brain––virtually anything as long as the animal is left alive (1). What is truly striking is that regeneration occurs in adult amphibians and involves the usage of already existing terminally differentiated cells, rather than undifferentiated stem cells. For example, after limb amputation, the tissues in the stump, such as muscle and cartilage, lose their tissue characteristics that distinguish them. This “dedifferentiation” process leads to the formation of the blastema (2, 3). Cells within the blastema are undifferentiated, but soon after a period of proliferation, these cells redifferentiate to build a faithful replica of the lost part of the limb (Figure 1⇓). Likewise, when the eye lens is removed from urodeles, the pigment epithelial cells (PECs) of the dorsal iris dedifferentiate, thereby losing their characteristic pigmentation and producing a lens vesicle, which, in turn, differentiates to lens epithelium at the anterior and to lens fibers at the posterior (Figure 1⇓). Even isolated single PECs can transdifferentiate to lens in vitro (4). From these two examples, we can firmly conclude that regeneration occurs via “transdifferentiation,” because one terminally differentiated cell has been reprogrammed to become another. This reprogramming has virtually no parallel in other normal differentiation processes.

In the past few years, however, many studies have shown that stem cells––cells that are reserved and used for repair––might have greater potential for regeneration than originally thought. Stem cells can be local (i.e., located in the brain and involved in nervous tissue repair), or nonlocal (i.e., hematopoieitic and involved in repair of several different tissues, such as liver, nervous, or cardiac) (5–8). Despite some spectacular results, a direct role for non-local stem cells in repair has been disputed, mainly because stem cells can fuse with the cells at the site of repair (9). One should keep in mind that, unlike the strategy used by urodeles, mammalian stem cells do not undergo dedifferentiation, but rather pre-exist as multipotent cells. Therefore, this repair strategy is not the same as the one used in urodeles.

It has been hypothesized, however, that some similarities between stem cells and transdifferentiating cells, at the molecular level, are inevitable (2, 10). For example, mesenchymal stem cells located in the bone marrow can differentiate to chondrocytes, myocytes, osteoblasts, or adipocytes. Therefore, at some particular stage these cells might not differ from blastema cells. Such common signatures of stem cells and dedifferentiated cells might provide very important insights about the biology of stem cells and of transdifferentiation of terminally differentiated cells.

A recent paper provides the first strong support that mammalian myotubes can transdifferentiate by generating progenitor cells (11). The murine C2C12 myogenic cell line, which upon serum withdrawal fuses into characteristic multinucleated myotubes, was used in this study. Such myotubes cannot normally revert to single-celled myoblasts. Chen et al. (11) screened this cell line with individual samples from a library of 50,000 discrete small molecules for four days, after which the cells were transferred to medium containing either osteogenesis-inducing substances (such as ascorbic acid, dexamethasone and glycerophosphate) or to medium containing adipogenesis-inducing substances. The cultures were maintained for seven additional days and then assayed for osteogenesis or adipogenesis using specific markers. A 2-(4-morpholinoanilino)-6-cyclohexyaminopurine analog was found to inhibit the formation of myotubes, resulting in mononucleated cells. Under conditions favoring osteogenesis, 35% of the cells stained positive for the alkaline phosphatase, a marker for osteogenic activity. Similarly, this 2,6-disubstituted purine compound, under conditions that favor adipogenesis, induced fat-cell morphology, with oil droplets accumulating inside the cytoplasmic membrane and with 40% of the cells positive for the adipose marker oil red O. The compound was named reversine because of its ability to reverse terminally differentiated cells to progenitor cells, which in turn were able to differentiate to different cell types. What Chen et al. have discovered is that murine cells are able to dedifferentiate and create multipotent progenitor cells. In this sense, they have created for the first time stem cells from differentiated cells. Conceivably, these cells could be regarded as analogous to the blastema cells created by dedifferentiation during limb regeneration (Figure 1⇓).

These finding are of enormous importance. First, they show that such screening can identify small molecules with transdifferentiation properties. These molecules can be used for therapeutic applications, since similar strategies can be devised for other cell types as well. Secondly, the generation of multipotent progenitor cells from differentiated cells can provide useful information and comparison with the transdifferentiation events seen in urodele amphibians. Such comparisons at the molecular level will eventually lead to the understanding of mechanisms used by the two strategies of regeneration, that of the urodeles and that of stem cell recruitment.

It will also be interesting to see whether such small molecules can induce dedifferentiation in a variety of cell types. In other words, can we identify a common signal for dedifferentiation, or are signals unique to different cell types? In urodeles, for example, it would be logical to assume that common signals have prevailed. Other implications of the identification and usage of these small molecules are in their application to induce regeneration in mammals. Can they work also in vivo? The findings by Chen et al. will generate much interest and will stimulate research to answer these questions.

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Figure 1.

Transdifferentiation in urodele limb and lens regeneration and in a committed murine cell line. After amputation of the newt limb (or tail) the intact terminally differentiated cells (such as, mesenchymal cells, myotubes, nervous tissue, bone, or cartilage) dedifferentiate by losing the characteristics of their origin. This dedifferentiation process produces the blastema cells, which then redifferentiate to reconstitute the lost limb. After lentectomy, the dorsal iris pigment epithelial cells lose their pigments and become dedifferentiated cells, which consequently regenerate a perfect lens. A small molecule, reversine, is able to induce dedifferentiation of myotubes formed by murine C2C12 cells. This dedifferentiation produces mesenchymal progenitor cells that are able to differentiate to different cell types, such as adipocytes and osteoblasts. According to this scheme, the reversine produced progenitor cells could be analogous to the blastema cells or other dedifferentiated cells used during regeneration in urodeles.

References

1. ↵
   Gross, J. Principles of regeneration (1969). Academic Press, New York, N.Y. An excellent book outlining the major regenerative capabilities in invertebrates and vertebrates.
2. ↵
   Tsonis, P.A. Regeneration in vertebrates. Dev. Biol. 221, 273–284 (2000).

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3. ↵
   Brockes, J.P. and Kumar, A. Plasticity and reprogramming of differentiated cells in amphibian regeneration. Nat. Rev. Mol. Cell Biol. 3, 566–574 (2002).

   CrossRefMedline
4. ↵
   Del Rio–Tsonis, K. and Tsonis, P.A. Eye regeneration at the molecular age. Dev. Dyn. 226, 211–224 (2003). A review outlining the process or transdifferentiation of the pigment epithelial cells during urodele lens and retina regeneration as well as regeneration of the retina in other animals via progenitor cells.

   CrossRefMedline
5. ↵
   Blau, H.M., Brazelton, T.R., and Weimann, J.M. The evolving concept of a stem cell. Entity or function? Cell 105, 829–841 (2001).

   CrossRefMedline
6. Tsonis, P.A. Regenerative biology: The emerging filed of tissue repair and regeneration. Differentiation 70, 397–409 (2002). This paper presents an overview of the potential of all vertebrate organs that can be repaired by transdifferentiation or stem cells.

   CrossRefMedline
7. Johansson, C.B., Momma, S., Clark, D.L. et al. Identification of a neural stem cell in the adult mammalian CNS. Cell 96, 25–34 (1999). Ependymal cells act as neural stem cells.

   CrossRefMedline
8. ↵
   Orlic, D., Kjstura, J., Chimenti, S. et al. Bone marrow cells regenerate infracted myocardium. Nature 410, 701–705 (2001). Stem cell-derived myocytes occupy up to 70% of an infracted portion of the ventricle.

   CrossRefMedline
9. ↵
   Vassilopoulos, G., Wang, P.R., and Russell, D.W. Transplanted bone marrow generates liver by fusion. Nature 422, 901–904 (2003). Hepatocytes with bone marrow markers are not the result of stem cell contribution but rather of fusion between bone marrow stem cell and hepatocytes during regeneration.

   CrossRefMedline
10. ↵
    Tsonis, P.A. and Del Rio–Tsonis, K. Lens and retina regeneration: Transdifferentiation, stem cells and clinical applications. Exp. Eye Res. 78, 161–172 (2004). Transdifferentiation versus stem cells. Similarities, differences and possible common signatures.

    CrossRefMedline
11. ↵
    Chen, S., Zhang, Q., Wu, X., Schultz, P.G., and Ding, S. Dedifferentiation of lineage-committed cells by a small molecule. J. Am. Chem. Soc. 126, 410–411 (2004). In this paper the small molecule reversine has been identified to induce mammalian myotubes to dedifferentiate to multipotent progenitor cells, which have then the capacity to differentiate to osteoblasts or adipocytes.

    CrossRefMedline

Reactivation of the Cell Cycle in Terminally Differentiated Cells (Molecular Biology Intelligence Unit, 17) [Hardcover]  Marco Crescenzi (Editor)  

Direct Reprogramming of Terminally Differentiated Mature B Lymphocytes to Pluripotency

http://www.cell.com/abstract/S0092-8674(08)00447-9

Jacob Hanna1, Styliani Markoulaki1, Patrick Schorderet1, Bryce W. Carey1, 2, Caroline Beard1, Marius Wernig1, Menno P. Creyghton1, Eveline J. Steine1, John P. Cassady1, 2, Ruth Foreman1, 2, Christopher J. Lengner1, Jessica A. Dausman1 and Rudolf Jaenisch1, 2, ,  

1 The Whitehead Institute for Biomedical Research, Cambridge, MA 02142, USA
2 Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02142, USA

Corresponding author

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• Summary
• Pluripotent cells can be derived from fibroblasts by ectopic expression of defined transcription factors. A fundamental unresolved question is whether terminally differentiated cells can be reprogrammed to pluripotency. We utilized transgenic and inducible expression of four transcription factors (Oct4, Sox2, Klf4, and c-Myc) to reprogram mouse B lymphocytes. These factors were sufficient to convert nonterminally differentiated B cells to a pluripotent state. However, reprogramming of mature B cells required additional interruption with the transcriptional state maintaining B cell identity by either ectopic expression of the myeloid transcription factor CCAAT/enhancer-binding-protein-α (C/EBPα) or specific knockdown of the B cell transcription factor Pax5. Multiple iPS lines were clonally derived from both nonfully and fully differentiated B lymphocytes, which gave rise to adult chimeras with germline contribution, and to late-term embryos when injected into tetraploid blastocysts. Our study provides definite proof for the direct nuclear reprogramming of terminally differentiated adult cells to pluripotency.