The Four Planes of Development

â€Å"Development is a series of re-births. There comes a time when one psychic personality ends and another begins†Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦ â€Å"Our work as adults does not consist teaching, but in helping the infant mind in its work of development† (Dr Montessori, The Absorbent Mind, Chap 3) What did Dr Montessori mean by the four planes of development? Describe each plane of development. Explain how we use this knowledge about the child in the Montessori classroom, with the main focus at the age group of 0-6 years. THE FOUR PLANES OF DEVELOPMENTThe life of the child that will become tomorrow’s adult is basically divided into four planes or stages. Each plane consists of a period of six years. Within these stages the development of the child is quite intense at the beginning, then it consolidates and finally trickles into the next. The first & third planes of development are periods of intense creation, whereas the second & fourth planes of development are the ca lm periods of consolidation. First plane of development (0-6years) â€Å"Development is a series of re-births.There comes a time when one psychic personality ends and another begins. The first of these periods goes from birth to six years of age and the child’s mentality basically remains the same. It includes two sub-phases, from birth to three years and three to six years. In the first of these, the child has a type of mind that adults cannot exert upon to influence. In the second sub-phase (3-6years), they are still mentally the same but the child becomes susceptible to adult influence and their personality undergoes great changes. † (The Absorbent Mind, chapter 3, Pg 17).The first plane of development (0 – 6 years) is a period of intense creation. This period is of very great fundamental importance for the formation of the child or the foundation of the personality of the child. This is the period of transformation. This plane of development is further divid ed in to two sub phases, (0 – 3) and (3 – 6) years. The first sub plane is known as ‘The unconscious absorbent mind’. The infant during this period is also identified as a ‘spiritual embryo’ as the infant has within himself the potentialities which determine his future development.The child can learn subconsciously, and effortlessly, through observations and explorations. The child is a sensorial explorer at this stage, that is, the child basically learns through his senses. During the absorbent mind stage, the sensitive periods are at their strongest and help the child’s learning process as well as the child’s initial adaptation. During this first plane of development various physical abilities develop in the young child. Physically the body develops from head to toe. Between the age of zero to three years, these abilities develop separately and independently of each other.Hand and leg movements are not guided by the mind. At th is stage, the child needs to create himself. It is a period of rapid development for the child and the child develops physically, mentally, socially as well as emotionally. As his physical body becomes more defined, he learns both consciously and unconsciously as his mind easily absorbs his environment. He becomes more sensitive to things adult take for granted and learning for him is easy and fast. At this stage, he also learns to care for himself, dress himself, feed himself etc.

Limitations of International law in protecting human rights essays

Limitations of International law in protecting human rights essays The term International Law refers to the principles and rules of conduct that nations regard as binding and, therefore, are expected to and usually do conform to, in their relations with one another and their conduct toward their own people. The chief rights recognized in international law correlate to human rights. Fundamental Human rights include: protection against slavery, the right to self-determination (determining your own fate), freedom from torture, freedom of thought, and the right to be presumed innocent until proven otherwise. Now many human rights are protected by international law, but the question of how effective it is, is itself questionable. While nations choose to participate in international law, it is extremely effective. However, the second when a nation feels it neednt adopt an international agreement, such as when Israel failed to ratify the U.N Convention for the Ban on Torture, then very little can be done. Even when a nation has signed an agreement (which she is under no obligation to do) there is virtually no way the international community can ensure that it abides by the agreement. An international judicial organ exists (International Court of Justice), but its powers are limited by the concept of state sovereignty. Which is to say a nation has the authority of being independent and in charge of the conditions in which it choose to live. In this case, the sovereign right not to part take in a court case which would potentially deem it guilty of refusing rights to its citizens, however, even then there is no way to force the verdict on the guilty party. In addition, the formal process for regular review of human rights in states is a self-reporting system in which the state in expected to disclose all abuses. However, as one can imagine no nation would want to incriminate themselves, and taint their image in the international community by doing so. Consequently, most abuses go unreported an ...

Types of crimes Essay Example | Topics and Well Written Essays - 250 words

Types of crimes - Essay Example UCR measures this crime using National Incident-Based Reporting System. Offenders of this crime include enemies of politicians and other famous personalities whereas victims of this crime include government employees, police officers, and service workers. Offenders commit this crime wherever and whenever they find some opportunity to kill. This crime is also on rise in the United States. I think that reporting system of this crime is good as news channels are doing their job perfectly in reporting these incidents. Vehicle Theft Vehicle theft refers to the act of taking another person’s vehicle without informing that person. The intention of the offender is to keep that vehicle permanently. UCR measures this crime by analyzing the number of cases reported to the police by the victims. Offenders of this crime include thieves whereas victims include any person who posses some vehicle of value. Thieves usually steal vehicles from car garages present in the houses and from streets. This crime seems to be on decline because of effective law enforcement from police and law enforcement agencies. Police reporting system and National Incident-Based Reporting System are working well for this crime so nothing needs to be changed.

Evaluate the role of religion in the perpetuation and resolution of Research Paper

Evaluate the role of religion in the perpetuation and resolution of conflict - Research Paper Example ry religion has been a major engine to war, bloodshed, hatred and intolerance, in most case we have seen religion acting as an arbitrator between the conflicting parties. Therefore, to resolve conflicts between conflicting parties, religion has used empathy, openness to and even encouraged love for strangers, the suppression of unbridled ego and acquisitiveness, the communicative of human rights, unilateral gestures of forgiveness and humility, interpersonal repentance and acceptance of responsibility in past errors as a means of reconciliation and the drive of social justice. Religion has emphasized that people should view life as sacred and a gift offered to humankind by God. Therefore, each person should value others life by avoiding hurting the inner feeling either by messing with ones peace of heart by conflict or by murder. Many religion has encourage to practice the virtue of love to each other by practicing forgiveness in case one wrong the other instead of engaging themselves in a tug of war as a means of conflict resolution. Therefore, the sanctity of life should be observed at personal, social and political level. Religion has practiced the aspect of interiority by observing disciplines even in societies that are quite communally oriented. Through prayer, meditation, the experience of divine love, ecstasy, guilt and repentance all reflect the central importance of inner life of a person. When religion influences all these virtues in inner life of people it encourages people to have conflict resolution techniques. Conflicting parties through application of religious virtues are able to have self-control of their ego through the practice of love and kindness to each other. The role of empathy in western religion and traditions is critically essential. Religion has impacted the experience of empathy in terms of religious contexts to people either in terms of advocacy and long-term education or more directly in the workshop setting. For example, there is

10 Legal Case briefs Study Example | Topics and Well Written Essays - 2500 words

10 Legal briefs - Case Study Example 2. Procedural History: The case was first heard in a Chicago court, before being appealed in the Illinois appellate court and later the Illinois Supreme Court. The case was passed further to US Supreme Court for a final determination. 3. Facts: Terminiello, a suspended catholic priest made a speech that was injurious to the personality of certain racial groups in an auditorium if the state of Chicago. this caused a public disturbance which forced the police to arrest him and present him to court. 5. Reasoning: The reasoning applied under this case is that inflammatory speeches and fight words are against the provisions of the first amendment of the US constitution. Therefore, the law does not protect the right of speech which creates anger and dispute. Additionally, the provisions of the amendment are against any speech or words that cause public unrests and disputes. 6. Rule: The trial court ruled that Terminiello was guilty of a breach of peace, because his conduct entailed a breach of public peace and decorum. The both the Appellate and the Supreme courts of Illinois affirmed the ruling. The US Supreme Court ruled that the statute used to charge him was unconstitutional. 7. Holding: the Supreme Court held that the speech made by Terminiello was protected under the first amendment of the US constitution. It also held that the statute applied to convict him by the Chicago trial court was unconstitutional. 8. Dissent: Chief justice Vinson opposed the opinion, observing that the statute applied was suitable for punishing fight words. The same sentiments was echoed by justices Frankfurter and Jackson, who observed that reversing the ruling granted by the trial court and affirmed by both the appellate and the supreme courts of Illinois was a breach of balance of power of the state and the federal courts. 2. Procedural History: The case was first held in a California trial court. The appeal was presented to the

Corporate Social Responsibility Essay Example for Free

Corporate Social Responsibility Essay Corporate Social Responsibility (CSR) means that a corporation should be held accountable for any of its actions that affect people, their communities, and their environment. Actually, MTR has been implementation of CSR. Since 1993, MTR staff will use their spare time to contribute to society, spontaneously organize and participate in volunteer activities. In 2005, MTR has also set up a volunteer program, to provide support for employees. The the MTR at â€Å"railway people, railway Heart† volunteers plan to participate in 86 community projects and participate in volunteer up to 1,800 people and 12,000 recipients. The target includes the elderly, low-income families, the physically disabled, mentally retarded persons and other people in need. MTR not only set up a volunteer program, but also provide free rides promotions for elderly. Moreover, MTR organized 17 work-life balance seminars and 8 value creation seminars. In addition, MTR in 2005 to help establish the first Charter of Corporate Social Responsibility (CSR Charter), served as one of its founder members. It formulates the first MTR corporate social responsibility guidelines, as well as the establishment of sustainable development and corporate social responsibility policy Steering Committee. You can see that, MTR held accountable for Hong Kongers of its actions. The vision of MTR is that care services, linking and building community to become an internationally recognized business pioneer. And, we think MTR been efforts to implement. However, MTR can do it better. Some of peoples said that MTR held accountable for Hong Kongers of its actions is insufficiency. Although MTR is Limited Corporation, the largest shareholder still is government. Thus, MTR belong public goods, it must be emphasized that the public welfare at first. Because of it, parts of public think that MTR should do more to maintain or improve our public welfare. The most people displeasure MTR is the fare should decrease or reduce to provide cheaper ticket for public as MTR earn much money each year. It should give some feedback to public. MTR owns along the real estate development projects, the total assets more than three hundred billion dollar. MTR offer concessions to passengers, such as short-term fare concessions ten get one free, but this small favors obviously not been accepted by the public. We think that MTR should think more about public and to listen to the views of the public, and make the appropriate action or respond. MTR is a large public company; it should not just focus on economic profit and ought to look out the needs of public. If not, public will procession to express their dissatisfaction because MTR do not held accountable for Hong Kongers of its actions. The impact of CSR on the company’s economic profit is very small, or not even. The most popular debate of CSR is that CSR will make the company lowers economic efficiency and profit as the business need to use extra resources for social purposes. But, it is not be applicable on MTR. It is because MTR not only provide transport services, it also have other business, such as investment in housing. Therefore, the profit of MTR is very large. Even though it use some resource to implement CSR, it will not affect its profit. Conversely, it will aroused public discontent if MTR have not implement its CSR as MTR is a big company and earn a lot of money from public, it should do something for public. On the other hand, it may affect its image as public think that MTR just focus on economic profit without their public welfare. Although MTR always build its image via TV, but parts of people think that it is not enough. They do not just want the promotion and they want the real action, like decrease the fare. People are not easy to deceive. If MTR not listen and ameliorate its actions, plan or behavior, it will increase the displeasure of public and affect its image. People will adopt a more radical behavior to express their dissatisfaction when it does not have any improve. And, it will affect its reputation.

Research into Cancer Stem Cells

Research into Cancer Stem Cells Cancers are composed of a heterogeneous mix of cells with varying differentiation, proliferation and tumourigenic properties. In vivo studies have demonstrated that within a cancer population, only percentage of cells are able to initiate tumour development [1]. It is widely believed that the heterogeneous groups of cells include a small population of cancer cells with stem cell properties: the cancer stem cell (CSC). These cells have the capacity to self-renew and differentiate asymmetrically and give rise to bulk populations of nontumourigenic cancer cells. Current cancer treatments may eradicate the tumour bulk but spare the populations of stem cells which are able to restore tumour tissue causing recurrence of the cancer. This may explain why initial tumour regression does not necessarily translate to improved patient survival in many clinical trials. Identification and characterisation of these stem cells may offer means of targeting cancer at its root. Cancer Stem Cell Definition The AACr workshop in 2006 defined a cancer stem cell as: A cell within a tumour that possesses the capacity to self-renew and to cause the heterogeneous lineages of cancer cells that comprise the tumour. Cancer stem cells can thus only be defined experimentally by their ability to recapitulate the generation of a continuously growing tumour.[2] Therefore the stem cell definition requires that cell possess 2 fundamental properties. Self renewal, the process whereby at least one daughter cell of a dividing stem cell retains stem cell properties Potency, the ability of cells to differentiate into diverse cells that comprise the tumour. [3]. It was agreed that defined CSCs may not necessarily derive from normal tissue stem cells, indeed one important and unanswered question is whether tumours derive from organ stem cells that retain self renewal properties or whether tumour stem cells are proliferative progenitors that acquire self-renewal capacity [2]. Normal Tissue Heterogeneity The continuous replacement of differentiated, functional cells by proliferation of more primitive cells in human tissue is a normal homeostatic process. Organs are composed of collections of differentiated cells that perform discrete functions [4]. The total cell population is regarded as constituting a cell division hierarchy [5]. The stem cell is central in the renewal hierarchy and has two functions within this model. It can act as the initiating cell in a cell division and differentiation process, producing a large family of differentiated descendants, a process known clonal expansion. Another function is for the cells to undergo division to produce two stem cell daughters identical to the initial stem cell and to replace the stem cells used in clonal expansion. This process is called self-renewal [6] and is shown diagrammatically in Figure 1. As cells move down the hierarchy they acquire the differentiated features associated with tissue function and the proportion of differenti ated cells increases. In this way the stem cell has the ability to maintain organ life [4]. This concept predicts the existence of three categories of cell within the population: Proliferating, self renewing stem cells; Proliferating non-renewing transitional cells (transit amplifying); Non-proliferating, differentiated end cells. Following division the stem cell can give rise to a transit amplifying cell that will undergo further rapid proliferation to produce offspring which expand the populations of cells arising from the initial division and progressively commit irreversibly to differentiation along one or several lineages[4]. An important feature of a stem cell is their ability to undergo asymmetric cell division giving rise to a progenitor cell and to a new stem cell. Somatic SCs reside in confined tissue compartments referred to as the niche. Here the microenvironment suppresses SC proliferation, resulting in a quiescent SC population. This population maybe triggered to proli ferate and differentiate in response to injury (Ghotra, 2009). Seven common and distinguishing features of stem cells have been described [4]: Stem cells comprise a small subpopulation of a given tissue. Stem cells are ultra-structurally unspecialized, with a large nuclear-to-cytoplasmic ratio and few organelles Stem cells can be pluripotent Stem cells are slow cycling but may be induced to proliferate more rapidly in response to certain stimuli Stem cells have a proliferatve reserve that exceeds an individuals lifetime An intermendiate group of transit amplifying cells exists The microenvironment plays a critical role in the homeostasis of the stem cell and the differentiation of its progeny. The stem cell is capable of division and clonal expansion. As cells differentiate they lose their proliferative potential. The stem cell can self renewal or divide to produce proliferative transitional cells. Tumour Heterogeneity It has been recognised for many years that tumours exhibit morphologic heterogeneity but they are also functionally heterogeneous in terms of cell proliferation and tumour forming capacity based on transplantation assays [7]. Heterogeneity within tumours is seen within individual tumours in terms of morphology, cell surface markers, cell proliferation kinetics and response to therapy. In vitro and in vivo observations suggest that most cancer cells do not proliferate and that loss of capacity to divide is a feature of the tumour. Only a small proportion of cells have the ability to form tumours in vivo, referred to as tumourgenicity. The cancer stem cell theory posits that neoplasms, like physiological tissue can be hierarchically organised, and that CSCs at the apex of this of this cellular hierarchy and seem to comprise of only a subpopulation of tumour cells are essential for its initiation [8, 9]. Two models have been proposed to explain tumours heterogeneity Stochastic and Hiera rchy, summarised in Figure 2. Both models account for the existence of a cell with stem cell properties, but only the hierarchy model predicts the existence of a stem cell at the top of a hierarchy, which the potential to produce all other cell types within the tumour. Stochastic Model The stochastic model predicts that a tumour is biologically homogeneous and the behaviour of the cancer cells is influenced by intrinsic (eg signalling pathways, levels of transcription factors) or extrinsic factors (eg host factors, immune response, and microenvironment). It is suggested that the randomness and unpredictability of these factors result in heterogeneity in many aspects of marker expression and tumours initiation capacity [10]. A key requirement of the stochastic model is that all cells are equally sensitive to such influences and that the cells can revert from one state to another. For this model to be functional all tumour cells are not permanently affected and all have equal capacity to be induced to one state or another and the changes upon the cell are not permanent [11]. A growth fraction of Hierarchy Model The second model is the hierarchy model which predicts that the tumour is a caricature of normal tissue development and a hierarchy where the stem cell is at the tops is maintained (Pierce) [7]. The cancer stem cell maintains itself and its clones by self-renewal. The cells also mature to produce differentiated offspring which form the bulk of the tumour and lack stem cell properties. As in normal tissue only a small percentage of the tumour population maintain the capacity for long term proliferation while most cells proceed forward down the differentiation pathway resulting in aberrant terminal differentiation [4]. Due to differences in characteristics, stem cells can be selected and enriched for. Variations in tumour growth rates may be due the effects of normal homeostatic mechanisms that regulate stem cells and transit amplifying cell reproduction or alterations of the stem cell niche microenvironment [4]. Much of the evidence for this comes from clonogenic and tumourgenic assay s, which will be discussed further. Hierarchy model contains cells that are composed of biologically distinct cells including cancer stem cells which are all have different functional properties. The stochastic model predicts that all cells are equal the cell heterogeneity is due to intrinsic and extrinsic influences upon the cells which result in heterogeneity of cell function. Experimental Evidence Early Work The first evidence for the existence of cancer stem cells came from functional cell proliferation studies in the1940s 1960s. Radiolabelling cells and autoradiography enabled measurements into the proliferation, lifespan and hierarchical relationships in normal and neoplastic tissues [10, 12]. From these studies came the proposal that tumours are caricatures of normal development including the existence of stem cells [7]. Much early work was on the cancer of the haematopoietic system. In the 1970s Clarkson and other groups carried out pioneering studies that established cancers exhibited functional heterogeneity [10, 13]. These include cytokinetic studies carried out in cell lines, murine models of the acute leukaemias and in vivo examination of leukaemia blast proliferation kinetics in human AML and ALL patients. The data showed that the majority of leukemic blasts were post mitotic and needed to be continuously replenished from a relatively small proliferative fraction. Only a smal l number of leukemic blast cells were cycling in vivo and of these two proliferative fractions were observed: a larger, fast cycling subset with a 24 hour cell cycle time and a smaller, slow cycling, with a dormancy of weeks to months. From this data it was suggested that the slow cycling fraction was generating the fast cycling fraction thought to be the leukemic stem cell population because they had similar kinetic properties to those observed for normal haematopoietic stem cells. This was a clear suggestion that tumours exhibit functional heterogeneity in terms of proliferative potential. Following the identification of these slow cycling cells it was predicted the inability to kill the leukaemic stem cells (LSCs) was the cause of relapse and failure of chemotherapeutic therapies. Whilst combining treatment with in vivo cytokinetic studies, investigators observed that LSCs respond to the depletion of the of the leukemic cell mass by go into cycle after chemotherapy. It was sugges ted the way to eliminate dormant LSCs was to find the window when they are cycling. Identifying and assaying the potential LSCs was a major stumbling block and characterising them was impossible. This was when attention focused on the clonogenic assay was adapted by several groups to assay AML which identified phenotype of AML cultures in vitro with differing proliferative potential, providing the further proof for hierarchy in AML [14-16]. Clonogenicity Definition of a clone A clone is an operationally defined as a group of cells derived from a single ancestor cell. Clonogenicity is the ability of a given cell population, when plated as single cells, to produce one or more clones. This can be measured by the clonogenic assay which can quantify the proportion of colony forming cells, as a percentage of plated population, referred to as colony forming efficiency (CFE). It has been suggested that colony-forming cells possess two fundamental properties of progenitor cells: the ability to give rise to differentiated descendents and the capacity for self-perpetuation [17]. Therefore the ability to measure the capacity of cells to form clones is a useful tool in the study of the cancer stem cell concept. Quantitative measurement of clonogenicity Development of the clonogenic assay. Puck and Marcus The first clonogenic assay In 1956 Puck and Marcus published a paper describing a cell culture technique for assessment of colony forming ability of single mammalian cells [18]. Plated in culture dishes with a suitable medium human cervical carcinoma cells (HeLa) were supplemented with a large number of irradiated feeder cells and the number of colonies formed was counted. Their technique was a simple rapid method for growing single mammalian cells into macroscopic colonies with a colony forming efficiency of 80 100% . The authors developed this assay further to enable quantification of the effects of high energy radiation on cell populations in vitro [18-20]. They plated HeLa cells and measured their response to x-rays, producing the first in vitro radiation cell survival curve [21]. This assay has since been used for a wide variety of studies with many cell types using improved culture conditions, and for the testing of many potential chemotherapeutic agents. Till and McCulloch Following the work of Puck and Marcus, Till and McCulloch generated the first in vivo survival curves [22, 23]. They showed that when mouse bone marrow cells were injected into recipient mice that had been given total body irradiation to suppress endogenous haematopoiesis, visible colonies developed in the spleens that derived from cells in the graft. This work demonstrated that the cells injected into the mice were capable of self-renewal and it was speculated that these cells were stem cells. The evidence for this conclusion was that the curve from the number of marrow cells transplanted proportional to the number of colonies developed within the spleen. In addition, the radiation survival curve of cells that form colonies closely resembled survival curves developed by Puck and Marcus for in vitro cells [21]. This, however, was only indirect evidence and did not prove that the colonies originated from single cells, so the group carried out further experiments to determine the singl e cell origin on the colonies within the spleens [24]. Heavily irradiated bone marrow was transplanted into heavily irradiated recipient mice. The idea was that some cells containing genetic abnormalities caused by irradiation in the donor bone marrow cells would retain the ability to proliferate and produce clones containing this abnormality [24]. This worked to some extent, with a small number of colonies containing cells which all showed the same chromosome abnormality within that colony. It was hypothesised that if the capacity to form colonies is to be considered as a criterion to identify stem cells, then cells must lose this capacity upon undergoing differentiation. This hypothesis was tested by applying hypoxia as a differentiating pressure to mouse bone marrow, which resulted in a reduction in colony formation in the spleens of hypoxic mice [17]. They described how the number of colonies form in the spleens of mice in hypoxic conditions is reduced. This was thought to be du e to hypoxia stimulating erythropoiesis which stimulates erythropoietin, indicating that erythropoietin reducing colony forming production in the spleen. This data suggested that an increased demand for differentiated cells reduces the number of stem cells, resulting in the reduction of colony forming ability. Later Developments Since its development, the in vitro clonogenic assay has become a valuable tool in the study of cell growth and differentiation. [25]. Several adaptations to the original method have been made including immobilising cells in a top layer of 0.3% agar to avoid formation of tumor cell aggregates by random movement which might be confused with colony growth [26]. Agar has also been replaced by some groups with agarose, which is easier to handle (Laboise 1981) or methylcellulose which allows better recovery of the colony for replating. Others have simplified the culture medium and omitted the need for feeder cells. The exact protocol depends largely on cell type, but the basic system remains the same. The development of a protocol for secondary plating efficiency has proved a useful tool for the measurement of self-renewal and has the advantage of being able to identify cells that are able to undergo a large number of cell divisions [26]. This involves selecting specific colonies to deter mine their proliferative potential over a number of passages. Clonogenicity and Cell Renewal Hierarchy Clonogenic assays have been used to identify and morphologically characterise the three cell types above. Barrandon and Greens [27] work identified the clonal types of keratinocytes and linked this to their capacity for multiplication. They defined colonies as Holoclone, Meroclone or Paraclone. The Holoclone was described as a colony with a larger smooth nearly circular perimeter containing many small cells, which it has been suggested that these cells represent the proliferating self renewing stem cells. Paraclones were described as differentiated end cells which are more elongated and flattened in appearance, however paraclones can divide quite rapidly therefore classification of clonal type cannot be deduced form the study of growth rates alone or morphology alone. Meroclones were described as a combination of holoclones and paraclones. Relating morphology and colony size to clonogenicity can be used to further identify potential stem cells within the clonogenic assay and give mor e detail to the fate of their descendents. The differences in growth unit size may reflect several properties including different proliferative capacities and clonogenic cell kinetics. However, clonogenicity in vitro alone, does not define a stem cell, and other subpopulations, such as transit amplifying cells may also be able to produce a colony size of 32 or more cells. Although ability of a cell to form a colony implies substantial proliferative capacity, this does not unambiguously identify a stem cell [28]. Tumor Cell Heterogeneity and Hierarchy Certain characteristics have emerged from clonogenic studies on cells derived from human tumors. It was noticed that a few cells in each tumor were able to give rise to colonies in culture, whilst some colonies contained transit amplyifing cells undergoing a limited number of terminal divisions. Other cells (usually the majority) were non-proliferating stem cells. Looking at CFE and colony size of human tumors and replating experiments has demonstrated the heterogeneity of a wide range of tumor types including neoplastic human urothelium [29], melanoma [30, 31] and squamous carcinoma [32]. This supports the idea that cells within solid tumors consist of cellular hierarchies, which will be discussed further. The cancer stem cell model accounts for heterogeneity within a primary cancer by proposing that each cancer consists of a small population of cancer stem cells and a much larger population of cells which have lost their self-renewal capacity [5]. The clonogenic assay has been used explore this cellular heterogeneity present in human tumors, lending support to the stem cell model of tumor growth. Multiple myeloma has served as a valuable model in early clonogenic assay development. This was studied by Hamburger and Salmon in 1977 [33], who created an essentially selective system which restrict proliferation to cells capable of anchorage independent growth, thought to be a characteristic of stem cells [34]. They described an in vitro bioassay for human myeloma colony-forming units in culture which was applied to the study of patients with multiple myeloma and related monoclonal bone marrow derived B cell neoplasm. Bone marrow samples from patients with multiple myeloma and normal volun teers were cultured in the presence of an agar feeder layer prepared by either human type O+ washed erythrocytes or adherent spleen cells of BALB/c mice. They found a linear relationship between colony formation and the number of nucleated bone marrow cells plated. Multiple myeloma patients exhibited much higher numbers of colonies formed compared to normal volunteers. It was shown that the number of colonies was proportional to the number of colonies plated, suggesting that colonies were derived from single myeloma stem cells. This was the development of the human tumor stem cells assay. The Human Tumor Stem Cell Assay clonogenicity and cancer stem cells The ability to grow human solid tumors in two-layer soft agar culture was developed for the clinical application of testing in vitro tumor sensitivity or resistance to chemotherapeutic agents. It is a possible means by which anticancer drugs can be selected for activity against tumor cells from a patient [35] as a way of tailoring chemotherapeutic regimes to individual patients and of testing new cytostatic agents [36]. The assay assesses treatment effects of stem cells by a testing their ability to reproduce and form a colonies of cells. Using semi-solid agar with enriched medium supports colony growth from cell suspensions from a variety of human tumors. A semi-solid medium suppresses the growth of most normal cells and there is evidence of the malignant nature of these colonies [33] . An important consideration is the relationship between the response of clonogenic cells to drugs in vitro and the response of the tumor to the same drug in the patient [10]. The stem cell model of human cancer suggests that cure or duration of remission after clinical treatment should correlate only with killing of stem cells. Assessment of treatment effects on an unselected cell population (eg on the basis or morphological criteria) is therefore likely to be misleading since the effects on a small population of stem cells will be masked by those on the large population of stem cells. Human tumors of a single histological type appear to have a pattern of response in vitro that is similar to their clinical behaviour. Within a histological type, tumours are heterogeneous in response both in vitro and in vivo. Studies directly comparing the response in vitro with the subsequent clinical response have shown important correlations. The proportion of human tumors that grow with a plating efficiency sufficient for assessment of drug activity (à ¢Ã¢â‚¬ °Ã‚ ¥30 colonies per 500,000 cells plated is frequently less that 50%. Usually only a proportion of these tumors will manifest in vitro sensitivity [37]. There have been a wide range of predictive value positives reported for the human clonogenic tumour cell assay when applied to a patient population with an expected clinical response rate of 15-49% [38]. This value could be misleading and in practice may only be workable for cytotoxicity testing for only one third of specimens tested. The limitation exits that not all sam ples will produce clones in vitro so those that do may exhibit a treatment bias [35]. Other problems with the use and interpretation of human tumor clonogenic assays include low plating efficiency and small proportion of tumors available for testing; difficulty in preparing single cell suspensions, production of only small quantities of data, and problems defining drug sensitivity and response criteria [35]. Factors influencing size of sub-populations It has been proposed that as in normal cell populations, human tumor cell populations are also heterogeneous and comprise stem cells, non-stem transitional cells with limited proliferative capacity and end cells [6]. MacKillop suggested that four factors may influence the relative size of these subpopulations: The probability of self-renewal (Psr) of stem cells (producing two daughter stem cells). The distribution of cells within the system can be treated mathematically by assuming probability functions. The potential of the transitional cells for further cell division, as defined by clonal expansion number (n=number of generations between the first generation non-stem cells and the end cells.) The relative effect of cell loss on each subpopulation (Stem cells, transit amplifying, end cells) as described by cell loss factors (ÃŽÂ ¦s, ÃŽÂ ¦t ÃŽÂ ¦ ec). The number of generations of cell proliferation following initiation of the tumor cell population for individual stem cell. Stem cell division in normal tissue must provide a supply of differentiated functional cells to compensate for physiological losses and at the same time maintain a constant stem cell population. A probability of self-renewal in which two stem cells daughters Psr =0.5, would yield a steady state [28]. If no cell loss occurs, it has been modelled that the number of stem cells will increase exponentially with Psr > 0.5 [6]. For the simplest case in which all non-stem cells are end cells (n=0) the proportion of stem cells increases linearly with increasing Psr. and the proportion of stem cells in a tumor decreases as the extent of multiplication of the transitional cell compartment. This results in the stem cell being the less common cell type numerically than transit amplifying and differentiated end cells. These scenarios are affected by cell loss which may occur through necrosis, migration or differentiation, of which only differentiation is selective of cell type. A selective loss th rough differentiation increases the population of stem cells. The modelling of tumor cell growth has implications for the use of clonogenic assays as predictors of the stem cell fraction on human tumors, especially in regards to cut-off points in terms of colony size and determining which cells represent the stem cell fraction [6]. Between studies there are differences between how colonies are scored morphologically and numerically and how long cells are allowed to grow [31] and considering this evidence may be an important issue when comparing data between different studies. Clonogenicity in cell lines and stem cells in cell lines Clonogenicity has recently been used to identify stem cell properties of cells in long term culture cancer cell lines. The colony forming efficiency and secondary plating efficiency of carcinoma derived cell lines including head and neck squamous, breast [39] and prostate [39-42] were investigated and considered to contain potential stem cells. These studies show that cell lines show clear differences between clonal types (holoclone, meroclone, paraclone) and have similar properties in this respect to normal epithelial cells [39]. The proportions of clonal types between the carcinoma cell lines vary greatly. DU145 colonies were evenly spread in number between the clonal types, whereas PC3 cells produced mainly meroclones and LNCaP cells produced mainly paraclones [41], all based on colony morphology. These studies have also looked at the relationship between potential cancer stem cell markers and clonogenicity. CD133 enriched DU145 cells were assayed for clonogenicity, but no difference was found between the positive and negative cells [41], but when isolated CD44+ integrin ÃŽÂ ±2ÃŽÂ ²1+ CD133+ sorted cells were compared against CD44+ integrin ÃŽÂ ±2ÃŽÂ ²1low CD133low a higher CFE was observed in conjunction with a marked difference in morphology to CD44+ integrin ÃŽÂ ±2ÃŽÂ ²1-/low CD133- in DU145 MACS sorted cells [40]. Immunocytochemistry demonstrated that different clonal types showed varying levels of expression of CD44, ÃŽÂ ±2ÃŽÂ ²1 integrin and ÃŽÂ ²-catenin in PC-3 [42] and DU145 clones [39]. There is further evidence to suggest the presence of cells with stem cell behaviour such as dye-exclusion and higher clonogenicity, in several human epithelial cell lines [39, 43-45], which further supports the idea that cell lines contain stem cells. The ad vantage of cancer cell lines that contain cells displaying stem cell characteristics would facilitate the study of molecular pathways and the properties that define the cancer stem cells in vitro. Recent Developments Much progress has been made in the modelling of the leukemic diseases, where the level of heterogeneity was first and most thoroughly explored. Human cells fulfilling the properties expected of drug resistant cancer stem cells were initially isolated from blood cancers [2]. Improvements in the genetics of recipient mice have led to the definition SCID-repopulating cell (SRC). Many improvements to the NOD/SCID murine model continue to be made by using recipient mice that are engineered to be deficient in natural killer (NK) and macrophage activity; part of that innate immune system. It has been demonstrated that a small subpopulation of acute myeloid leukaemia cells with an immature immunophenotype possess the ability colonise immune deficient NOD/SCID mice to give rise to more differentiated leukaemia cells and to recapitulate the heterogeneous phenotype of the bulk tumour [46]. The phenotypically more mature cells failed to engraft in mice, suggesting the presence of an identifiable tumour cell hierarchy. These cells are referred to as tumour initiating cells. Cancer Stem Cell Identification CSCs have been defined on the basis of their ability to seed tumours in animal hosts, to self renew and to spawn differentiated progeny (non-CSCs)[47]. Pioneering work in this area originated from studies on leukaemia stem cells and later included demonstrations of CSCs in solid tumours, particularly breast and brain cancers. However, work in solid tumours has proved challenging. The frequency of CSCs in solid tumours is highly variable [48]. Difficulties with tumour CSC identification Evidence for the existence of cancer stem cells in solid tumours has been more difficult than in the haematopoietic system to obtain for several reasons: 1) The cells within the tumour are less accessible. Tissue has to undergo mechanical or enzymatic digestion to obtain a single cell suspension which can be analysed. 2) There is a lack of functional assays suitable for detecting and quantifying normal stem cells from many organs. 3) Only a few cell surface markers have been identified and characterised. Of these there is no one marker which is specific for a stem cell or cancer stem cells and for selection they often have to be used in combination. Cancer Stem Cell Markers Stem cells are most commonly identified by staining for cell surface markers, exclusion of fluorescent dyes or labelling with tritiated thymidine [3] . The technology to develop monoclonal antibodies to specific molecules and flow-cytometery based sorting and analysis has been a big driving force in recent CSC developments. Much work has been done to define cell surface markers. It has been shown that two distinct subpopulations can be separated from a single tumour that differ in their cell surface markers and their ability to seed new tumours in vivo. Most of the currently used markers do not recognise functional stem cell activity. By using combinations of cell surface markers, the homogenous purification of stem cells can be obtained [3]. Table 1 below reviews the current suggested markers for some tumour types. The use of animal models has allowed identification and assessment of markers that are expressed by cancer stem cells. The most convincing demonstration of identity CSC s elected by biomarkers comes from serial transplantation of cellular populations into animal models. The CSC containing fraction should re-establish the phenotypic characteristics of the original tumour [48]. In 1997 Bonnet et al showed that the ability to transfer human leukaemias into NOD/SCID mice was retained by a small proportion of cells with the CD34+, CD38- phenotype [46]. The CD44 and CD133 markers have emerged as potential markers of immature epithelial cells for isolating CSCs in several tissue types including brain and prostate. Cells have been isolated from several tumour types and serially transplanted in xenograft models: Breast CD44+ CD24-/low established tumours in recipient mice. Brain CD133+ enriched cells. Prostate Side population CD44+ enriched. In these experiments small numbers of selected cells produced tumours in recipient mice. In this instance CSCs can on