FIBROBLASTS AND THEIR TRANSFORMATIONS: THE CONNECTIVE-TISSUE CELL FAMILY - 정리해야

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Many of the differentiated cells in the adult body can be grouped into families

whose members are closely related by origin and by character. An important

example is the family of connective-tissue cells, whose members are not only

related but also unusually interconvertible. The family includes fibroblasts, cartilage

cells, and bone cells, all of which are specialized for the secretion of collagenous

extracellular matrix and are jointly responsible for the architectural

framework of the body. The connective-tissue family also includes fat cells and

smooth muscle cells. Figure 23–52 illustrates these cell types and the interconversions

that are thought to occur between them. Connective-tissue cells contribute

to the support and repair of almost every tissue and organ, and the

adaptability of their differentiated character is an important feature of the

responses to many types of damage.

Fibroblasts Change Their Character in Response to Chemical Signals

Fibroblasts seem to be the least specialized cells in the connective-tissue family.

They are dispersed in connective tissue throughout the body, where they secrete

a nonrigid extracellular matrix that is rich in type I or type III collagen, or both,

as discussed in Chapter 19. When a tissue is injured, the fibroblasts nearby proliferate,

migrate into the wound <TGAT>, and produce large amounts of collagenous

matrix, which helps to isolate and repair the damaged tissue. Their ability

to thrive in the face of injury, together with their solitary lifestyle, may explain

why fibroblasts are the easiest of cells to grow in culture—a feature that has

made them a favorite subject for cell biological studies (Figure 23–53).

As indicated in Figure 23–52, fibroblasts also seem to be the most versatile of connective-tissue cells, displaying a remarkable capacity to differentiate into other members of the family. There are uncertainties about their interconversions,

however. Fibroblasts in different parts of the body are intrinsically different, and there may be differences between them even in a single region.

Figure 23–53 The fibroblast. (A) A phase-contrast micrograph of fibroblasts in culture. (B) These drawings of a living fibroblastlike cell in the transparent tail of a tadpole show the changes in its shape and position on successive days. Note that while fibroblasts flatten out in culture, they can have more complex, process-bearing morphologies in tissues. See also Figure 19–54. (A, from E. Pokorna et al., Cell Motil. Cytoskeleton 28:25–33, 1994; B, redrawn from E. Clark, Am. J. Anat. 13:351–379, 1912. Both with permission from Wiley-Liss.)

“Mature” fibroblasts with a lesser capacity for transformation may, for example,

exist side by side with “immature” fibroblasts (often called mesenchymal cells)

that can develop into a variety of mature cell types.

The stromal cells of bone marrow, mentioned earlier, provide a good example

of connective-tissue versatility. These cells, which can be regarded as a kind

of fibroblast, can be isolated from the bone marrow and propagated in culture.

Large clones of progeny can be generated in this way from single ancestral stromal

cells. According to the signal proteins that are added to the culture medium,

the members of such a clone can either continue proliferating to produce more

cells of the same type, or can differentiate as fat cells, cartilage cells, or bone

cells. Because of their self-renewing, multipotent character, they are referred to

as mesenchymal stem cells.

Fibroblasts from the dermal layer of the skin are different. When placed in

the same culture conditions, they do not show the same plasticity. Yet they, too,

can be induced to change their character. At a healing wound, for example, they

change their actin gene expression‎ and take on some of the contractile properties

of smooth muscle cells, thereby helping to pull the wound margins together;

such cells are called myofibroblasts. More dramatically, if a preparation of bone

matrix, made by grinding bone into a fine powder and dissolving away the hard

mineral component, is implanted in the dermal layer of the skin, some of the

cells there (probably fibroblasts) become transformed into cartilage cells, and a

little later, others transform into bone cells, thereby creating a small lump of

bone. These experiments suggest that components in the extracellular matrix

can dramatically influence the differentiation of connective-tissue cells.

We shall see that similar cell transformations occur in the natural repair of

broken bones. In fact, bone matrix contains high concentrations of several signal

proteins that can affect the behavior of connective-tissue cells. These

include members of the TGFb superfamily, including BMPs and TGFb itself.

These factors regulate growth, differentiation, and matrix synthesis by connective-

tissue cells, exerting a variety of actions depending on the target cell type

and the combination of other factors and matrix components that are present.

When injected into a living animal, they can induce the formation of cartilage,

bone, or fibrous matrix, according to the site and circumstances of injection.

TGFb is especially important in wound healing, where it stimulates the conversion

of fibroblasts into myofibroblasts and promotes the formation of the collagen-

rich scar tissue that gives a healed wound its strength.

The Extracellular Matrix May Influence Connective-Tissue Cell Differentiation by Affecting Cell Shape and Attachment

The extracellular matrix may influence the differentiated state of connective-tissue cells through physical as well as chemical effects. This has been shown in

studies on cultured cartilage cells, or chondrocytes. Under appropriate culture

conditions, these cells proliferate and maintain their differentiated character,

continuing for many cell generations to synthesize large quantities of highly distinctive

cartilage matrix, with which they surround themselves. If, however, the

cells are kept at relatively low density and remain as a monolayer on the culture dish, a transformation occurs. They lose their characteristic rounded shape, flatten

down on the substratum, and stop making cartilage matrix: they stop producing

type II collagen, which is characteristic of cartilage, and start producing

type I collagen, which is characteristic of fibroblasts. By the end of a month in

culture, almost all the cartilage cells have switched their collagen gene expression‎

and taken on the appearance of fibroblasts. The biochemical change must

occur abruptly, since very few cells are observed to make both types of collagen

simultaneously.

The biochemical change seems to be induced, at least in part, by the change

in cell shape and attachment. Cartilage cells that have made the transition to a

fibroblast-like character, for example, can be gently detached from the culture

dish and transferred to a dish of agarose. By forming a gel around them, the

agarose holds the cells suspended without any attachment to a substratum,

forcing them to adopt a rounded shape. In these circumstances, the cells

promptly revert to the character of chondrocytes and start making type II collagen

again. Cell shape and anchorage may control gene expression‎ through intracellular

signals generated at focal contacts by integrins acting as matrix receptors,

as discussed in Chapter 19.

For most types of cells, and especially for a connective-tissue cell, the opportunities

for anchorage and attachment depend on the surrounding matrix,

which is usually made by the cell itself. Thus, a cell can create an environment

that then acts back on the cell to reinforce its differentiated state. Furthermore,

the extracellular matrix that a cell secretes forms part of the environment for its

neighbors as well as for the cell itself, and thus tends to make neighboring cells

differentiate in the same way. A group of chondrocytes forming a nodule of cartilage,

for example, either in the developing body or in a culture dish, can be

seen to enlarge by the conversion of neighboring fibroblasts into chondrocytes.

Osteoblasts Make Bone Matrix

Cartilage and bone are tissues of very different character; but they are closely related in origin, and the formation of the skeleton depends on an intimate partnership between them.

Cartilage tissue is structurally simple, consisting of cells of a single type—

chondrocytes—embedded in a more or less uniform highly hydrated matrix consisting

of proteoglycans and type II collagen, whose remarkable properties we

have already discussed in Chapter 19. The cartilage matrix is deformable, and the

tissue grows by expanding as the chondrocytes divide and secrete more matrix

(Figure 23–54). 

Bone, by contrast, is dense and rigid; it grows by apposition—that

is, by deposition of additional matrix on free surfaces. Like reinforced concrete,

the bone matrix is predominantly a mixture of tough fibers (type I collagen fibrils),

which resist pulling forces, and solid particles (calcium phosphate as

hydroxylapatite crystals), which resist compression. The collagen fibrils in adult

bone are arranged in regular plywoodlike layers, with the fibrils in each layer

lying parallel to one another but at right angles to the fibrils in the layers on either

side. They occupy a volume nearly equal to that occupied by the calcium phosphate.

The bone matrix is secreted by osteoblasts that lie at the surface of the

existing matrix and deposit fresh layers of bone onto it. Some of the osteoblasts

remain free at the surface, while others gradually become embedded in their own

secretion. This freshly formed material (consisting chiefly of type I collagen) is called osteoid. It is rapidly converted into hard bone matrix by the deposition of

calcium phosphate crystals in it. Once imprisoned in hard matrix, the original

bone-forming cell, now called an osteocyte, has no opportunity to divide,

although it continues to secrete further matrix in small quantities around itself.

The osteocyte, like the chondrocyte, occupies a small cavity, or lacuna, in the

matrix, but unlike the chondrocyte it is not isolated from its fellows. Tiny channels,

or canaliculi, radiate from each lacuna and contain cell processes from the

resident osteocyte, enabling it to form gap junctions with adjacent osteocytes

(Figure 23–55). Although the networks of osteocytes do not themselves secrete or

erode substantial quantities of matrix, they probably play a part in controlling the

activities of the cells that do. Blood vessels and nerves run through the tissue,

keeping the bone cells alive and reacting when the bone is damaged.

A mature bone has a complex and beautiful architecture, in which dense

plates of compact bone tissue enclose spaces spanned by light frameworks of

trabecular bone—a filigree of delicate shafts and flying buttresses of bone tissue,

with soft marrow in the interstices (Figure 23–56). 

The creation, maintenance, and repair of this structure depend not only on the cells of the connective-tissue family that synthesize matrix, but also on a separate class of cells called osteoclasts that degrade it, as we shall discuss below.

Most Bones Are Built Around Cartilage Models

Most bones, and in particular the long bones of the limbs and trunk, originate

from minute “scale models” formed out of cartilage in the embryo. Each scale

model grows, and as new cartilage forms, the older cartilage is replaced by bone.

The process is known as endochondral bone formation. Cartilage growth and

erosion and bone deposition are so ingeniously coordinated that the adult bone,

though it may be half a meter long, is almost the same shape as the initial cartilaginous

model, which was no more than a few millimeters long.

The process begins in the embryo with the appearance of hazily defined

“condensations”—groups of embryonic connective tissue cells that become

more closely packed than their neighbors and begin to express a characteristic

set of genes—including, in particular, Sox9 and, after a slight delay, Runx2. These

two genes code for gene regulatory proteins that are critical for cartilage and

bone development, respectively. Mutant cells lacking Sox9 are unable to differentiate

as cartilage but can form bone (and in some parts of the body will make

bone where cartilage should be). Conversely, animals lacking functional Runx2

make no bone and are born with a skeleton consisting solely of cartilage.

Soon after expression‎ of Sox9 has begun, the cells in the core of the condensation

begin to secrete cartilage matrix, dividing and enlarging individually as

they do so. In this way, they form an expanding rod of cartilage surrounded by

more densely packed non-cartilage cells. The cartilage cells in the middle segment

of the rod become hypertrophied (grossly enlarged) and cease dividing;

and at the same time, they start to secrete Indian Hedgehog—a signal molecule

of the Hedgehog family. This in turn provokes increased production of certain

Wnt proteins, which activate the Wnt pathway in cells surrounding the cartilage

rod. As a result, they switch off expression‎ of Sox9, maintain expression‎ of

Runx2, and begin to differentiate as osteoblasts, creating a collar of bone around

the shaft of the cartilage model. Artificial overactivation of the Wnt pathway tips

a larger proportion of cells into making bone rather than cartilage; an artificial

block in the Wnt signaling pathway does the opposite. In this system, therefore,

Wnt signaling controls the choice between alternative paths of differentiation,

with Sox9 expression‎ leading the way toward cartilage, and Runx2 expression‎

leading the way toward bone.

The hypertrophied cartilage cells in the shaft of the cartilage model soon die,

leaving large cavities in the matrix, and the matrix itself becomes mineralized,

like bone, by the deposition of calcium phosphate crystals. Osteoclasts and

blood vessels invade the cavities and erode the residual cartilage matrix, creating

a space for bone marrow, and osteoblasts following in their wake begin to

deposit trabecular bone in parts of the cavity where strands of cartilage matrix

remain as a template. The cartilage tissue at the ends of the bone is replaced by

bone tissue at a much later stage, by a somewhat similar process, as shown in

Figure 23–57. 

Continuing elongation of the bone, up to the time of puberty, depends on a plate of growing cartilage between the shaft and the head of the

bone. Defective growth of the cartilage in this plate, as a result of a dominant

mutation in the gene that codes for an FGF receptor (FGFR3), is responsible for

the commonest form of dwarfism, known as ach[안내]태그제한으로등록되지않습니다-ondroplasia (Figure 23–58).

The cartilage growth plate is eventually replaced by bone and disappears.

The only surviving remnant of cartilage in the adult long bone is a thin but important layer that forms a smooth, slippery covering on the bone surfaces at joints, where one bone articulates with another (see Figure 23–57). Erosion of

this layer of cartilage, through aging, mechanical damage, or autoimmune attack, leads to arthritis, one of the commonest and most painful afflictions of old age.

Bone Is Continually Remodeled by the Cells Within It

For all its rigidity, bone is by no means a permanent and immutable tissue. Running

through the hard extracellular matrix are channels and cavities occupied by

living cells, which account for about 15% of the weight of compact bone. These

cells are engaged in an unceasing process of remodeling: while osteoblasts

deposit new bone matrix, osteoclasts demolish old bone matrix. This mechanism

provides for continuous turnover and replacement of the matrix in the

interior of the bone.

Osteoclasts (Figure 23–59) are large multinucleated cells that originate, like macrophages, from hemopoietic stem cells in the bone marrow. The precursor cells are released as monocytes into the bloodstream and collect at sites of bone

resorption, where they fuse to form the multinucleated osteoclasts, which cling to surfaces of the bone matrix and eat it away. 

Osteoclasts are capable of tunneling deep into the substance of compact bone, forming cavities that are then

invaded by other cells. A blood capillary grows down the center of such a tunnel, and the walls of the tunnel become lined with a layer of osteoblasts (Figure 23–60). 

To produce the plywoodlike structure of compact bone, these osteoblasts lay down concentric layers of new matrix, which gradually fill the cavity, leaving only a narrow canal surrounding the new blood vessel. Many of the osteoblasts become trapped in the bone matrix and survive as concentric rings of osteocytes. At the same time as some tunnels are filling up with bone, others are being bored by osteoclasts, cutting through older concentric systems. The consequences of this perpetual remodeling are beautifully displayed in the layered patterns of matrix observed in compact bone (Figure 23–61).

Osteoclasts Are Controlled by Signals From Osteoblasts

The osteoblasts that make the matrix also produce the signals that recruit and

activate the osteoclasts to degrade it. Two proteins appear to have this role: one

is Macrophage-CSF (MCSF), which we already encountered in our account of

hemopoiesis (see Table 23–2); the other is TNF11, a member of the TNF family (also called RANKL). The behavior of the osteoblasts in attracting their opponents

may seem self-defeating, but it has the useful function of localizing osteoclasts

in the tissue where they are needed.

To prevent excessive degradation of matrix, the osteoblasts secrete, along

with MCSF and TNF11, another protein, osteoprotegerin, that tends to block the

action of TNF11. The higher the level of Wnt activation in the osteoblasts, the

more osteoproteregin they secrete and, consequently, the lower the level of

osteoclast activation and the lower the rate of bone matrix degradation. The Wnt

signaling pathway thus seems to have two distinct functions in bone formation:

at early stages, it controls the initial commitment of cells to an osteoblast fate;

later, it acts in the differentiated osteoblasts to help govern the balance between

matrix deposition and matrix erosion.

Disturbance of this balance can lead to osteoporosis, where there is excessive

erosion of the bone matrix and weakening of the bone, or to the opposite condition,

osteopetrosis, where the bone becomes excessively thick and dense. Hormonal

signals, including estrogen, androgens, and the peptide hormone leptin,

famous for its role in the control of appetite (discussed below), have powerful

effects on this balance. At least some of these effects are mediated through influences

on the osteoblasts’ production of TNF11 and osteoprotegerin.

Circulating hormones affect bones throughout the body. No less important

are local controls that allow bone to be deposited in one place while it is

resorbed in another. Through such controls over the process of remodeling,

bones are endowed with a remarkable ability to adjust their structure in

response to long-term variations in the load imposed on them. It is this that

makes orthodontics possible, for example: a steady force applied to a tooth with

a brace will cause it to move gradually, over many months, through the bone of

the jaw, through remodeling of the bone tissue ahead of it and behind it. The

adaptive behavior of bone implies that the deposition and erosion of the matrix

are in some way governed by local mechanical stresses (see Figure 23–56). Some

evidence suggests that this is because mechanical stress on the bone tissue activates

the Wnt pathway in the osteoblasts or osteocytes, thereby regulating their

production of the signals that regulate osteoclast activity.

Bone can also undergo much more rapid and dramatic reconstruction when

the need arises. Some cells capable of forming new cartilage persist in the connective

tissue that surrounds a bone. If the bone is broken, the cells in the neighborhood

of the fracture repair it by a sort of recapitulation of the original embryonic

process: cartilage is first laid down to bridge the gap and is then replaced by

bone. The capacity for self-repair, so strikingly illustrated by the tissues of the

skeleton, is a property of living structures that has no parallel among presentday

man-made objects.