Specialized Tissues, Stem Cells, and Tissue Renewal 그림만 보고 있어도 통찰력이 생기는 자료

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stem cell로 분화되는 각종 cell, tissue에 관한 분자생물학적 논문

참고할 수 있는 좋은 그림이 많음.

Cells evolved originally as free-living individuals, but the cells that matter most to us, as human beings, are specialized members of a multicellular community. They have lost features needed for independent survival and acquired peculiarities that serve the needs of the body as a whole. Although they share the same genome, they are spectacularly diverse: there are more than 200 different named cell types in the human body (see our web site for a list). These collaborate with one another to form many different tissues, arranged into organs performing widely varied functions. To understand them, it is not enough to analyze them in a culture dish: we need also to know how they live, work, and die in their natural habitat, the intact body.

- 세포는 원래 free-living 개체로서 진화했으나, 인간을 만드는 세포들은 multicellular community의 특별 집합임. 

- 세포들은 독립적 삶을 위해 필요한 특성을 잃어왔고, 인체 전체로서 필요한 습성을 얻어왔음. 

- 세포는 같은 유전자를 나누어가졌을지라도, 그들은 완전히 다름. 인체에는 200개의 서로 다른 이름을 가진 세포가 있음. 

- 세포들은 서로 협력하려 많은 다른 조직을 구성하고, 여러기관으로 배열하여 다양한 기능을 수행함. 

- 이를 이해하기 위해, 세포배양 접시에서 그것을 분석하는 것으로는 충분치 않음. 

- 우리는 또한 그들이 손상되지 않은 인체 자연적인 환경내에서 어떻게 사는지, 작용하는지, 죽는지를 알아야 함. 

 panic bird...

Chapter23 Specialized Tissues, Stem Cells, and tissue renew.pdf

Cells evolved originally as free-living individuals, but the cells that matter most to us, as human beings, are specialized members of a multicellular community. They have lost features needed for independent survival and acquired peculiarities that serve the needs of the body as a whole. Although they share the same genome, they are spectacularly diverse: there are more than 200 different named cell types in the human body (see our web site for a list). These collaborate with one another to form many different tissues, arranged into organs performing widely varied functions. To understand them, it is not enough to analyze them in a culture dish: we need also to know how they live, work, and die in their natural habitat, the intact body.

- 세포는 원래 free-living 개체로서 진화했으나, 인간을 만드는 세포들은 multicellular community의 특별 집합임. 

- 세포들은 독립적 삶을 위해 필요한 특성을 잃어왔고, 인체 전체로서 필요한 습성을 얻어왔음. 

- 세포는 같은 유전자를 나누어가졌을지라도, 그들은 완전히 다름. 인체에는 200개의 서로 다른 이름을 가진 세포가 있음. 

- 세포들은 서로 협력하려 많은 다른 조직을 구성하고, 여러기관으로 배열하여 다양한 기능을 수행함. 

- 이를 이해하기 위해, 세포배양 접시에서 그것을 분석하는 것으로는 충분치 않음. 

- 우리는 또한 그들이 손상되지 않은 인체 자연적인 환경내에서 어떻게 사는지, 작용하는지, 죽는지를 알아야 함. 

In Chapters 7 and 21, we saw how the various cell types become different in the embryo and how cell memory and signals from their neighbors enable them to remain different thereafter. In Chapter 19, we discussed the building technology of multicellular tissues—the devices that bind cells together and the extracellular materials that give them support. In this chapter, we consider the functions and lifestyles of the specialized cells in the adult body of a vertebrate. We describe how cells work together to perform their tasks, how new specialized cells are born, how they live and die, and how the architecture of tissues is preserved despite the constant replacement of old cells by new. We examine in particular the role played in many tissues by stem cells—cells that are specialized to provide an indefinite supply of fresh differentiated cells where these are lost, discarded, or needed in greater numbers.

- 이 챕터에서 우리는 척추동물의 특별 세포의 생활사와 기능을 탐구함. 

- 세포들이 그들의 일을 수행하기 위해서 어떻게 함께 작용하는지, 세로운 세포는 어떻게 생겨나는지, 세포가 어떻게 살고 죽는지를 설명함. 그리고 나이든 세포가 새로운 세포로 완전대치됨에도 불구하고 어떻게 조직의 구조와 기능이 유지되는지에 대해서 알아봄. 

We discuss these topics through a series of examples—some chosen because they illustrate important general principles, others because they highlight favorite objects of study, still others because they pose intriguing problems that cell biology has yet to solve. Finally, we shall confront the practical question that underlies the current storm of interest in stem cells: How can we use our understanding of the processes of cell differentiation and tissue renewal to improve upon nature, and make good those injuries and failings of the human body that have hitherto seemed to be beyond repair?

EPIDERMIS AND ITS RENEWAL BY STEM CELLS

We begin with a very familiar tissue: the skin. Like almost all tissues, skin is a complex of several different cell types. To perform its basic function as a barrier, the outer covering of the skin depends on a variety of supporting cells and structures, many of which are required in most other tissues also. It needs mechanical support, largely provided by a framework of extracellular matrix, mainly secreted by fibroblasts. It needs a blood supply to bring nutrients and oxygen and to remove waste products and carbon dioxide, and this requires a network of blood vessels, lined with endothelial cells. These vessels also provide access routes for cells of the immune system to defend against infection: macrophages and dendritic cells, to phagocytose invading pathogens and help activate lymphocytes, and lymphocytes themselves, to mediate more sophisticated adaptive immune system responses (discussed in Chapter 24). 

Nerve fibers are needed too, to convey sensory information from the tissue to the central nervous system, and to deliver signals in the opposite direction for glandular secretion and smooth muscle contraction.

Figure 23–1 illustrates the architecture of the skin and shows how it satisfies all these requirements. An epithelium, the epidermis, forms the outer covering, creating a waterproof barrier that is self-repairing and continually renewed. Beneath this lies a relatively thick layer of connective tissue, which includes the tough collagen-rich dermis (from which leather is made) and the underlying fatty subcutaneous layer or hypodermis. In the skin, as elsewhere, the connective tissue, with vessels and nerves running through it, provides most of the general supportive functions listed above. The epidermis, however, is the fundamental, quintessential component of the skin—the tissue that is peculiar to this organ, even though not the major part of its bulk. Appendages such as hairs, fingernails, sebaceous glands, and sweat glands develop as specializations of the epidermis (Figure 23–2). 

Complex mechanisms regulate the distribution of these structures and their distinctive patterns of growth and renewal. The regions of less specialized, more or less flat epithelium covering the body surface between the hair follicles and other appendages are called interfollicular epidermis. This has a simple organization, and it provides a good introduction to the way in which tissues of the adult body are continually renewed.

Epidermal Cells Form a Multilayered Waterproof Barrier

The interfollicular epidermis is a multilayered (stratified) epithelium composed largely of keratinocytes (so named because their characteristic differentiated activity is the synthesis of keratin intermediate filament proteins, which give the epidermis its toughness) (Figure 23–3). These cells change their appearance from one layer to the next. Those in the innermost layer, attached to an underlying basal lamina, are termed basal cells, and it is usually only these that divide. Above the basal cells are several layers of larger prickle cells (Figure 23–4), whose numerous desmosomes—each a site of anchorage for thick tufts of keratin filaments—are just visible in the light microscope as tiny prickles around the cell surface (hence the name). Beyond the prickle cells lies the thin, darkly staining granular cell layer (see Figure 23–3). It is at this level that the cells are sealed together to form a waterproof barrier. Mice that fail to form this barrier because of a genetic defect die from rapid fluid loss soon after birth, even though their skin appears normal in other respects.

The granular layer, with its barrier to the movement of water and solutes,  marks the boundary between the inner, metabolically active strata and the outermost layer of the epidermis, consisting of dead cells whose intracellular organelles have disappeared. These outermost cells are reduced to flattened scales, or squames, filled with densely packed keratin. The plasma membranes of both the squames and the outer granular cells are reinforced on their cytoplasmic surface by a thin (12 nm), tough, cross-linked layer of proteins, including a cytoplasmic protein called involucrin. The squames themselves are normally so compressed and thin that their boundaries are hard to make out in the light microscope, but soaking in sodium hydroxide solution (or a warm bath tub) makes them swell slightly, and their outlines can then be seen (see Figure 23–3).

Differentiating Epidermal Cells Express a Sequence of Different Genes as They Mature

Let us now set this static picture in motion to see how the epidermis is continually renewed. While some basal cells are dividing, adding to the population in the basal layer, others (their sisters or cousins) are slipping out of the basal cell layer into the prickle cell layer, taking the first step on their outward journey.

When they reach the granular layer, the cells start to lose their nucleus and cytoplasmic organelles, through a degradative mechanism that involves partial activation of the machinery of apoptosis; in this way, the cells are transformed into the keratinized squames of the keratinized layer. These finally flake off from the surface of the skin (and become a main constituent of household dust). The time from birth of a cell in the basal layer of the human skin to its loss by shedding from the surface is about a month, depending on body region.

As the new keratinocyte in the basal layer is transformed into the squame in the outermost layers (see Figure 23–4), it steps through a succession of different states of gene expression‎, synthesizing a succession of different members of the keratin protein family. Meanwhile other characteristic proteins, such as involucrin, also begin to be synthesized as part of a coordinated program of terminal cell differentiation—the process in which a precursor cell acquires its final specialized characteristics and usually permanently stops dividing. The whole program is initiated in the basal layer. It is here that the fates of the cells are decided.

Stem Cells in the Basal Layer Provide for Renewal of the Epidermis

Humans renew the outer layers of their epidermis a thousand times over in the course of a lifetime. In the basal layer, there have to be cells that can remain undifferentiated and carry on dividing for this whole period, continually throwing off descendants that commit to differentiation, leave the basal layer, and are eventually discarded. The process can be maintained only if the basal cell population is self-renewing. It must therefore contain some cells that generate a mixture of progeny, including daughters that remain undifferentiated like their parent, as well as daughters that differentiate. Cells with this property are called stem cells. They have so important a role in such a variety of tissues that it is useful to have a formal definition.

The defining properties of a stem cell are as follows:

1. It is not itself terminally differentiated (that is, it is not at the end of a pathway of differentiation).

2. It can divide without limit (or at least for the lifetime of the animal).

3. When it divides, each daughter has a choice: it can either remain a stem cell, or it can embark on a course that commits it to terminal differentiation(Figure 23–5).

Stem cells are required wherever there is a recurring need to replace differentiated cells that cannot themselves divide. The stem cell itself has to be able to divide—that is part of the definition—but it should be noted that it does not necessarily have to divide rapidly; in fact, stem cells usually divide at a relatively slow rate.

The need for stem cells arises in many different tissues. Thus, stem cells are of many types, specialized for the genesis of different classes of terminally differentiated cells—epidermal stem cells for epidermis, intestinal stem cells for intestinal epithelium, hemopoietic stem cells for blood, and so on. Each stemcell system nevertheless raises similar fundamental questions. What are the distinguishing features of the stem cell in molecular terms? What factors determine whether it divides or stays quiescent? What decides whether a given daughter cell commits to differentiation or remains a stem cell? And where the stem cell can give rise to more than one kind of differentiated cell—as is very often the case—what determines which differentiation pathway is followed?

The Two Daughters of a Stem Cell Do Not Always Have to Become Different

At steady state, to maintain a stable stem-cell population, precisely 50% of the

daughters of stem cells in each cell generation must remain as stem cells. In

principle, this could be achieved in two ways—through environmental asymmetry

or through divisional asymmetry (Figure 23–6). In the first strategy, the division

of a stem cell could generate two initially similar daughters whose fates

would be governed by their subsequent environment or by some random process

with an appropriate environmentally controlled probability; 50% of the

population of daughters would remain as stem cells, but the two daughters of an

individual stem cell in the population might often have the same fate. At the

opposite extreme, the stem cell division could be always strictly asymmetric,

producing one daughter that inherits the stem-cell character and another that

inherits factors that force it to embark on differentiation. The neuroblasts of the

Drosophila central nervous system, discussed in Chapter 22, are an example of

cells that show this type of divisional asymmetry. This strategy in its strict form

has a drawback, however: it means that the existing stem cells can never

increase their numbers, and any loss of stem cells is irreparable, unless by

recruitment of some other type of cell to become a stem cell. The strategy of control

by environmental asymmetry is more flexible.

In fact, if a patch of epidermis is destroyed, the surrounding epidermal cells

repair the damage by migrating in and proliferating to cover the denuded area.

In this process, a new self-renewing patch of epidermis is established, implying

that additional stem cells have been generated to make up for the loss. These

must have been produced by symmetric divisions in which one stem cell gives

rise to two. In this way, the stem cell population adjusts its numbers to fit the

available niche.

Observations such as these suggest that the maintenance of stem cell character

in the epidermis might be controlled by contact with the basal lamina,

with a loss of contact triggering the start of terminal differentiation, and maintenance

of contact serving to preserve stem cell potential. This idea contains a

grain of truth, but it is not the whole truth. As we now explain, not all the cells in

the basal layer have the capacity to serve as stem cells.

The Basal Layer Contains Both Stem Cells and Transit Amplifying

Cells

Basal keratinocytes can be dissociated from intact epidermis and can proliferate

in a culture dish, giving rise to new basal cells and to terminally differentiated

cells. Even within a population of cultured basal keratinocytes that all seem

undifferentiated, there is great variation in the ability to proliferate. When human

keratinocytes are taken singly and tested for their ability to found new colonies,

some seem unable to divide at all, others go through only a few division cycles

and then halt, and still others divide enough times to form large colonies. This

proliferative potential directly correlates with the expression‎ of the b1 subunit of

integrin, which helps mediate adhesion to the basal lamina. Clusters of cells with

high levels of this molecule are found in the basal layer of the intact human epidermis

also, and they are thought to contain the stem cells (Figure 23–7). We still

do not have definitive markers for the stem cells themselves, and we still do not

understand in molecular terms what it is that fundamentally defines the stemcell

state. This is one of the key problems of stem-cell biology, and we shall say

more about it in later sections of the chapter.

Paradoxically, many if not all of the epidermal cells that generate large

colonies in culture seem to be cells that themselves as a rule divide rarely. One

line of evidence comes from experiments in which a pulse of the thymidine analog

bromodeoxyuridine (BrdU) is given to a young animal, in which the epidermis

is growing rapidly, or to a mature animal following an injury that provokes

rapid repair. One then waits for many days or weeks before fixing the tissue and

staining with an antibody that recognizes DNA in which BrdU has been incorporated.

The BrdU is taken up by any cell that is in S phase of the division cycle at

the time of the initial pulse. Because the BrdU would be expected then to be

diluted by half at each subsequent cell division, any cells that remain strongly

labeled at the time of fixation are assumed to have undergone few or no divisions

since replicating their DNA at the time of the pulse. Such label-retaining cells can

be seen scattered among unlabeled or lightly labeled cells in the basal layer of the

epidermis even after a period of several months, and large numbers are visible in

hair follicles, in a region called the bulge (see Figure 23–2). Ingenious labeling

procedures indicate that the label-retaining cells, in the hair follicle at least, are

in fact stem cells: when a new cycle of hair growth begins after an old hair has

been shed, the label-retaining cells in the bulge at last divide and contribute the cells that go to form the regenerated hair follicle. Although it is not certain that all

the stem cells of the hair follicle have this label-retaining character, some clearly

do, and the same seems to be true of the stem cells in the interfollicular epidermis.

Moreover, basal cells expressing b1 integrin at a high level—the cells that can

give rise to large colonies in culture —are rarely seen dividing.

Mixed with these cells there are others that divide more frequently—but only

for a limited number of division cycles, after which they leave the basal layer and

differentiate. These latter cells are called transit amplifying cells—“transit”,

because they are in transit from a stem-cell character to a differentiated character;

and “amplifying”, because the division cycles they go through have the effect

of amplifying the number of differentiated progeny that result from a single stemcell

division (Figure 23–8). In this way, a small population of stem cells that divide

only rarely can generate a plentiful supply of new differentiated cells.

The Rate of Stem-Cell Division Can Increase Dramatically When New Cells Are Needed Urgently

- stem cell division은 손상 등으로 새로운 수요가 있을때 드라마틱하게 증가함

Whatever the mechanism of stem-cell maintenance may be, the use of transit amplifying divisions brings several benefits. 

First, it means that the number of stem cells can be small and their division rate can be low, even when terminally

differentiated cells have to be produced rapidly in large numbers. This reduces the cumulative burden of genetic damage, since most mutations occur in the course of DNA replication and mitosis, and mutations occurring in cells that are

not stem cells are discarded in the course of tissue renewal. The likelihood of cancer is thus reduced. If the immortal strand hypothesis is correct, so that stem cells always retain the original “immortal” template DNA strands, the risk is still

further reduced, since most sequence errors introduced during DNA replication will be in the newly synthesized strands, which the stem cells ultimately discard. 

Second, and perhaps more important, a low stem-cell division rate in normal circumstances allows for dramatic increase when there is an urgent need -for example, in wound repair. The stem cells can then be roused to divide

rapidly, and the additional division cycles can both amplify the stock of stem cells and increase steeply the production of cells committed to terminal differentiation. 

Thus, for example, when a patch of hairy skin is cut away, the slowly dividing stem cells in the bulge region of surviving hair follicles near the wound are switched into rapid proliferation, and some of their progeny move out as new stem cells to form fresh interfollicular epidermis to cover the wounded patch of body surface.

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The Rate of Stem-Cell Division Can Increase Dramatically When New Cells Are Needed Urgently