hepatic stem cell in liver regeneration

8/11/2019 Hepatic Stem Cell in Liver Regeneration

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LIVER REGENERATION 9

0 89 2. 66 38 /9 6/ 00 10 -1 2 49 / 0 1 . 50 .0 FASEB

1249

Hepatic stem cells in liver regeneration

SNORRI S . THORGE IRS SON

Laboratory of E xperim ental C arcinogenesis, D ivision of B asic S ciences, N ational C ancer Institute, N ational

Institutes of H ealth, B ethesda, M aryland 20892-4255, U SA

The key to understanding liver development, hepato-

carcinogenesis, and the ability to regenerate lies in es-

tablishing the existence of a liver stem cell population

(or populations). By now we should be readily con-

vinced that hepatocytes them selves have the ability to

exit quiescence and replicate. But the fundamental

concept that adult m ammalian livers contain stem cells

that can replace hep atic parenchym a has been the sub-

ject of an ongoing controversy , which continues to be

fueled by debate over the role of stem cells in liver

growth

and repair. Early studies of liver cell lineages

assumed the ro le of stem cells as progenitors for bo th

normal and transformed hepatocytes, including hepato-

cellu lar carcinoma. In recent years, significant ad-

vances have furthered our understanding of liver stem

cell bio logy: we are beginning to realize the role of

growth factors, tran scriptional activators, and cell-cell

communication in the regulation of stem cell activa-

tion. With the development of in vitro assays, in vivo

genetic marking studies, and identification and charac-

terization of the critical trans-activating factors respon-

sible for cell lineage, we are able to address these

issues. We now recognize and distinguish the unipo-

tential stem cell system of the hepatocyte from the

bipotential hepatoblast from the multipotentia l facul-

tative stem cell system or oval cell. D o you believe

that the adult liver contains stem cells? After all, w e

can induce the formation of pancreatic acini and intes-

tinal metaplasia in hepatic tissue. M oreover, pancreatic

ductules can give rise to cells that exhibit all the phe-

notypic traits and biological properties of differentiated

hepatocytes. Most liver injuries are not due to simple

70 partial hepatectomies, but rather to toxic injuries

from viruses, chem icals, and other agents. It is impera-

tive to understand the role of stem cells in order to ul-

timately control the ability of the liver to regenerate. I

believe that adult liver stem cells exist, and I wager

that you w ill, too , after reading this last review in the

series.

-Clifford

J.

Steer, Coordinating Editor

ABSTRACT The concept that the liver contains

epithelial celia that

share

som e of

th e

m ajor p rop er-

ties of stem cells of the well-characterized s tem

cell-fed lineages found in bone marrow intestinal

epitheium

an d

epiderm is is now well supported.

Nevertheless the population dynam ics of the m ajor

types

of liver ep ith elial cells hep ato cytes

an d

bile

epithelia

display

a strildng difference from the popu-

lation dynam ics of the classic stem cell system s. The

focus of this review is on recent studies of the

activation and expansion of liver stem cells in vivo

and the role these cells may play in regeneration of

the liver. The requirement for a selective and sus-

tained expression of growth factors during th e early

stages of stem cell activation is highlighted. In addi-

tion results ar e presented supporting the hypothesis

that after loss of liver mass, both the quiescent stem

cells as well as the residual differentiated hepatocytes

an d bile duct epitheial cells ar e activated to prolif-

erate. H ow ever significant contribution of the stem

cells to the regeneration process only occurs under

circum stances in which the residual differentiated

cells

a re fu nc tio nally

compromised

and/or

cannot

p ro lifer ate.-T ho rg eir sso n S. S . H ep atic stem c ells

in l iv er

regeneration. FASEBJ . 10,1249-1256(1996)

Key

Words:

growth

factors

.

liver stem cell biology stem cell

activation hepatocytes b ile duct epithelia l cells . l iv er p a re n-

chym a oval cells - p ar tia l h ep ate cto m y . prolferw ion dfferen-

tiation

THE UVER IS M ITOTICALLY a quiescent organ in adult hu-

mans and animals. In spite of this slow cellu lar turnover,

the two parenchyinal cell lineages, hepatocytes and bile

epithelia l cells, have a remarkable capacity to meet re-

placement demands of cellular loss. The best example of

the capacity of adult hepatocytes and bile epithelia l cells

to proliferate is seen after partial hepatectomy (PH )2 in

rats and mice, in which the compensatory hyperplasia of

these cells in the remaining lobes restores the liver mass.

M oreover, th is process of liver regeneration can be re-

1 A dd re sa c or re sp on de nc e to Dr. Thorgeusson, at :N ational C ancer

Institu te, B ldg. 37, Room 3C28, 37 ConventDr.MSC4255, Bethesda, MD

20892-4255 USA.

2Abbre viatio ns:FGF, acidic fib ro bla st g ro wth fac to r, TGFa , trans-

forming growthfactor-alpha;CF , hepatocyte growthfactorSCF, stem

cellfactor;FP, a-fetoprotein;AF , 2-acetylaminofluorene; EGF, epi-

th elia l g ro wth factor.


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THORGEIRSSON

peated several tim es in experim ental animals. The in-

creased use and success of liver transplantation in clin i-

cal m edicine have shown that these animal models

correctly reflect the capacity of the human liver to regen-

erate (1). Due to this well-established trait of adult hepa-

tocytes and bile epithelial cells to repeatedly regenerate

the liver after either surgically or toxically induced loss

of the tissue, the existence of hepatic stem cells that may

partic ipate in liver regeneration has been somewhat con-

troversial. The major part of the hepatic stem cell contro-

versy may, however, be due to the failure to recognize

that the adult organism contains many kinds of stem

cells, and that these may exist at d ifferent stages of dif-

ferentiation and have very different capacities for gener-

ating m ultilineage progeny.

Self-m aintenance is a fundamental and common trait of

all stem cells. A cell population that has an extensive

self-maintain ing capacity is perhaps the only definition

that applies to all stem cells (2). In this context, the adult

liver, having the extensive capacity for maintaining

parenchymal cell number throughout the life span of the

organism , can be viewed as a single lineage stem cell

system in which the hepatocyte is the stem cell. Recent

data from hepatic cell transplantation experim ents in a

transgenic mouse model (3) have demonstrated the tre-

mendous growth potential of adult hepatocytes (12-16

doublings per donor cell), further supporting the notion of

the liver parenchyma as a single lineage (or unipotentia l)

stem cell system . On the other hand, if the classic defi-

n ition of stem cells proposed by Potten and Loeffler (4) is

used, one would differentiate between actual stem cells,

po ten tial stem cells, and comm itted stem celLs. A ctual stem

cells are defined as undifferentia ted cells capable of 1)

proliferation,

2)

self-maintenance,

3)

production of a

large number of differentiated progeny, 4) regeneration of

the tissue after in jury , and 5) flexibility in the use of

these options. Potten and Loeffler (4) consider that po-

tential stem cells are la tent or quiescent counterparts of

the actual stem cells that may be reactivated to become

functional stem cells, and that comm itted stem cells may

correspond to the dividing transit cells (e.g ., dividing-

transit enterocytes), sharing w ith the term inally differenti-

ated cells the ability to execute tissue-specific functions.

Applying the definition of stemness used by Potten and

Loeffler (4), the hepatocytes appear to be comm itted

stem cells that are normally quiescent, but can be acti-

vated to produce progeny whose only differentia tion op-

tio n is h ep ato cy tic .

The fact that the early fetal hepatocytes or hepatoblasts

are progenitors for both adult hepatocytes and bile epi-

thelial cells suggests that the hepatoblasts are at least

b ipotential precursors (5). The question then arises

whether either or both of the cell lineages derived from

the hepatoblast reta in the bipotential capacity of the

precursor cells. There is at present no substantial evi-

dence indicating that adult hepatocytes are more than a

unipotential comm itted stem cell system . The possible

exception to this generalization is the concept of ductular

metaplasia or transformation of hepatocytes in to ductules.

This topic has been extensively discussed by Desmet and

his colleagues (for review , see ref 6). S tudies of chronic

cholestatic diseases in humans using both enzyme histo-

chem istry and cytokeratin immunohistochem istry have

provided evidence for gradual transformation of hepato-

cytes into bile duct-type cells (7). However, evidence

showing that these bile duct-type cells also exhibit func-

tional characteristics of normal bile epithelium is still

lacking. It is therefore questionable, but still possible ,

that ductular metaplasia of hepatocytes seen in choles-

tatic diseases may reflect a multipotential (or at least

bipotentia l) capacity of the hepatocytes.

In contrast to the hepatocyte system , there is strong

evidence indicating that the bile epithelium harbors a

compartm ent of cells that can produce progeny (oval

cells) capable of differentiating into several lineages in-

cluding bile epithelia, hepatocytes, in testinal epithelia ,

and possibly exocrine pancreas (8-11). A lso , as pointed

out by Sell (10), there may exist a periductal system of

stem cells capable of differentia ting into all the hepatic

lineages. These ductular/periductal cells are frequently

referred to as the hepatic stem cell compartm ent. There

is also strong evidence that oval cells, the early progeny

from the hepatic stem cell compartm ent (8-11), are more

prim itive and more poorly differentiated than are hepato-

cytes or bile epithelial cells, and the multip le differentia-

tion options of oval cells are firm ly established.

Therefore, oval cells meet more of the stem cell criteria

of Potten and Loeffler (4) than do hepatocytes: 1) they

are poorly differentiated, 2) they demonstrate extensive

proliferation, and 3) they have m ultip le differentiation

options, including hepatocytes and bile ductular epi-

thelia l cells. Whether oval cells possess the capacity to

proliferate and differentiate throughout the life span of an

animal is still uncertain , as appropria te studies have not

been done. Nevertheless, the immediate precursors of

oval cells appear to approximate Potten and Loefflers

defin ition of potentia l stem cell, whose functions of stem -

ness are latent but can be reactivated by appropriate con-

ditions. This defin ition of a potential stem cell

corresponds to the hypothesis of faculta tive liver stem

cell proposed earlier by G risham (12), whose activation

he predicted would lead to the proliferation of oval cells.

In light of these observations, I should like to propose

that the liver be viewed as composed of two stem cell

systems: the unipotentia l committed stem cells (hepato-

cytes) and the multipotential nonparenchymal epithelial

(ductular) system s (Fig. 1). A comprehensive discussion

of the founding of hepatocytes and bile duct epithelial

cells from hepatoblasts has recently been published (5),

and w ill not be included in this review .

HEPATIC STEM CELL COMPARTMENT

The existence of cells w ith stem -like properties that may

differentiate in to hepatocytes was first postulated nearly


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HEPATIC STEM CELLS

1251

Figure 1. Schem atic diagram show ing the development of cell lineages in the liver.

60 years ago by Kinosita (13), and later suggested as a

possibility by other investigators (14-16). W ilson and

Leduc (16) postulated, based on experim ents involving

liver regeneration in the mouse after chronic injury, that

indifferent cholangiole cells proliferated to form oval

cells that m ight differentiate to form both hepatocytes and

biliary epithelial cells.

The major support for the existence of hepatic stem

cells has, perhaps not surprisingly in light of the earlier

work by W ilson and Leduc (16), come from extensive

TABLE

1.

Markers

commonly used to assess differentionnd tot racelineag eofli verepithe lial

ell?

M arkers Hepatob lasts Oval cells H epatocy tes Bil educ tcel ls References

CK7 - -

-

+

17,56,57

CK 8

+

+ + +

17,56

CK18

+

+ + +

17,56

CK19

-1- + -

+

17,56,58

CK14 [+] [+1

- -

17,59,60

1UJ3 +

+1- +

- 17,56,58

AFP

+

+

-

-

17,56,58

GGT

+ + +

17,56

OV-6

+

+

.

+

61

OV-1

(+ )

+

- 4- 61

BDS7

+ 4-

-

+

58,62

BD 1 - -

+

63

BPC5

+ - -

64

HES6 -

+

58,62

OC.2

4- + - 4

27,65,66

OC.3

+ + 4-

27,65,66

H. 1

4 -

27,65,66

H.2

+(Transient) - -

27,65,66

HBD.1

4-

4-

4

27,65,66

A6

AFro ,, , C r i sh a i n

+1 -

a nd T ho rge irsso n (ref

5 Wi

+

t h p e rmis s io n .

+

67,68


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THORGEIRSSON

Activat ionof Facultat ive

jj StemCells

Figure 2.Schematicre pre se nta tio n illu stra tin g th e hypothesis that oval

c el ls d ev e lo p from facultative stem cells located in the canal of Hering

(b iliary ductule). The schem e shows thedev elopme ntal ptionsthatoval

cells

ossess, includ ing differentiation in to hepatocy tes and

biliarypi-

thelium,ormationfintestinalpithel ium,nd pancre aticcinarepithe -

hum bydifferentiationnd/or metaplasia,nd death.

studies in hepatic carcinogenesis (9-11, 17). Several im -

portant experim ental models, mostly in the rat, have

emerged from these studies. The most commonly used

models in the rats are produced by 1) treatment w ith azo-

dyes (18), 2) feeding of a choline-deficient diet, w ith or

w ithout supplements of ethionine (19) or 2-acety-

lam inofluorene (20), 3) tr ea tm en t w ith 2 -a ce ty la min oflu o-

rene and partial hepatectomy (AAF/PH) (21), and 4)

treatm ent w ith D -galactosam ine (22). Central to all these

experim en.tal models is the extensive destruction and/or

comprom ised function of hepatocytes, coupled with the

apparent inability of the residual hepatocytes to prolifer-

ate. A common cellular response in rats subjected to the

experim ental systems listed above is the proliferation of

small periportal cells w ith scant cytoplasm and ovoid nu-

clei, which have been termed oval cells (17, 23, 24). The

existence of sim ilar cells has been reported in the human

liver (25, 26). These oval cells are thought to represent a

progeny of the hepatic stem cell compartm ent, and at

least in some instances to be a precursor for the hepatic

tumors (10, 11, 27). The precise anatom ical location of

the hepatic stem cell compartment in normal liver is still

unclear, but present data suggest that the term inal duc-

tu le cells connecting the canals of Hering w ith the bile

canaliculi and/or a distinct population of periductal cells

constitute the hepatic stem cell compartm ent (vide infra;

9-11, 16, 28, 29). Recent studies in the rat liver have

shown that oval cells are capable of differentiating, in ad-

dition to bile epithelium , into at least two lineages in

vivo, including hepatocytes (24, 30) and intestinal type

epithelium (31). Furthermore, isolated oval cells in cul-

ture can be induced to differentiate in to both hepatocyte-

like and biliary type of cells (32-34). M arkers commonly

used to assess differentiation and to trace lineages of oval

cells are listed in Table 1. These data and other results

not reviewed here have supported the notion that oval

cells have lineage options sim ilar to those displayed by

hepatoblasts in early stages of liver development. A s

such, the oval cells ca n be regarded as bipotential pre-

cursors for the tw o hepatic parenchymal cell lineages.

However, oval cells may, particularly when the hepatic

m icroenvironment is drastically disrupted, exhibit a ca-

pacity to differentiate into nonhepatic lineages (F ig . 2)

(35).

LIV ER REGEN ERA TIO N

VIA HEPATIC STEM

CELLS

The AAF/PH model in the rat w ill be used to illustrate

both the cellu lar biology and growth factor/receptor sys-

tem s involved in stem cell-energized liver regeneration.

In this experim ental system , a rapid and extensive prolif-

eration of oval cells takes place after PH , first in the

periportal area; la ter, these cells expand into the liver ac-

inus and differentiate into small basophilic hepatocytes

(Fig.

3

(24, 36). The powerful activation of the stem

cell compartm ent seen in the AAF/PH model is a conse-

quence of a close to complete m itoinhibitory effect of

AAF on the adult rat hepatocytes that prevents the regen-

eration from the remaining liver tissue (21, 24).

LOCALIZATION OF HEPATIC STEM CELLS

It has been established that proliferation of desmin-posi-

tive Ito cells is closely associated with the early stages of

oval cell proliferation in the AAF/PH model (37). Early

population of oval cells can be identified by the use of

the monoclonal antibody OV-6 (37) and distinguished

F igure 3. Tim e course of the developm ent and expansion of the oval cell

population in the AAF/PH model. Oval cells are id en tifie d b y y -G T

staining. The tim e of partial hepatectom y is day 0 (Od); 6d , 9d , and 13 d

indicate number of days after the operation. Note the rapid expansion of

the oval cell population after 6 days and the formation by 13 daysofthe

basophih ic focus com posed of d iphoid hepatocytes that are losing the y-G T

staining.


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ThNN

HGF

aFOF

TGFa

TGFI

SC F

c-kit

Stem Transitional

Cells

HEPATIC STEM CELLS 1253

- 2 9 II

14y {149}

Differentiated

Hepatocytes

Figure 4 . Schem atic diagram show ing the tem poral relationsh ip between

expression of A FP, growth factors , and differen tiation stages of the hepatic

stem cell progeny during

liverregenerationntheAAF/PH model.

from proliferating desm in-positive Ito cells (38). Results

from a detailed tim e course study of activation of hepatic

stem cells in the AAF/PH model, utilizing a combination

of immunohistochem istry with OV-6 and desm in antibod-

ies and autoradiography after [3H ]thym idine adm inistra-

tion shortly after the PH, indicate that the earliest

population of proliferating OV-6 positive cells is located

in the small b ile ductules (38). In addition , these early

populations of OV-6-positive cells express album in and

a-fetoprotein (AFP) (36). These data clearly show that

the majority of thym idine-labeled , OV -6-positive cells

first observed after PH in the AAF/PH model reside in

the bile ductules. Moreover, at the time when few of the

OV-6-positive cells in the large bile ducts become la-

beled with thym idine, the ductular-derived OV-6-positive

and thym idine labeled oval cells expressing both albu-

m in and AFP have already started to infiltrate into the

liver acinus (36, 38). It therefore seems likely that the

major source of oval cells, at least in the AAF/PH model,

is derived from the lin ing cells of the biliary ductules and

that these cells constitu te the dormant/facultative hepatic

stem cell com partm ent.

GROWTH FACTORS INVOLVED IN HEPATIC

STEM C ELL A CTIV ATIO N

AN D EX PA NSION

During normal hepatic regeneration as well as during re-

newal from the stem cell compartment, several grow th

factors appear to affect the proliferation and differentia-

tion of hepatic cells (37, 39, 40). The question therefore

arises as to whether the same growth factors known to be

involved in normal hepatic regeneration are also involved

in regeneration from the stem cell compartm ent.

There are three prim ary grow th factors associated

w ith normal liver regeneration: transform ing grow th fac-

tor-alpha (TGF-a), hepatocyte grow th factor (HGF), and

acidic fibroblast grow th factor (aFGF) (41). Each of these

grow th factors is also capable of inducing replication of

prim ary hepatocytes in vitro (41). In addition, transform -

ing grow th factor-beta 1 (TGF-1) is also expressed dur-

ing hepatic regeneration , and it has been proposed that

TGF-1 may provide at least part of the negative growth

signals controlling liver size after the compensatory hy-

perplasia that occurs after loss of liver mass (42). The

first cells entering DNA synthesis after PH in the

AAF/PH model are the OV-6 and desm in-positive bile

duetular and Ito cells, respectively , in the periportal area

(38). Coincident w ith the appearance of these cells, an

increase in the expression of TGF-a, HGF , and TGF-1

is observed, whereas increased expression of aFGF is

first seen 24 h later (29). A ll the grow th factors are then

expressed at high levels throughout the period of expan-

sion and differentiation of the oval cells and return to lev-

els seen in normal liver at the end of the regeneration

process. The cellu lar distribution of the grow th factor

transcripts differs: TGF-a and aFGF transcrip ts are

found both in Ito cells and oval cells (37, 40), whereas

the HGF transcripts are only found in Ito cells (39). The

TGF-1 transcrip ts are located mainly in Ito cells, but

the early population of oval cells also contain the TGF-f31

transcripts (43). The cellular distribution of the tran-

scrip ts for all the receptors corresponding to the grow th

factors has revealed that all are located on oval cells (22,

44, 45). These data suggest that the same primary grow th

factors involved in liver regeneration from existing differ-

entiated parenchyma are also involved in regeneration

from the stem cell com partm ent.

Recently, a novel liganci/receptor system , the stem cell

factor (SCF)/c-kit system , which may be uniquely in-

volved in the earliest stages of hepatic stem cell activa-

tion, was discovered (46). In the AAF/PH model, the

expression of both SCF and c-kit is seen before the ex-

pression of AFP (46), and the levels of both the SCF and

the c-kit transcripts decline before those of TGF-a,

aFGF, HGF , and TGF-31 (46). It has also been shown

that in contrast to TGF-a, HGF , aFGF , and TGF-f31, the

SCF /c-kit system is only slightly and transiently activated

in regeneration after PH in normal liver (46). The SCF /c-

kit signal transduction system is believed to play a funda-

mental role in the survival, proliferation , and migration of

stem cells in hematopoiesis, m elanogenesis, and gameto-

genesis (47). It appears that in all cases, SCF and c-kit

are involved in the early stages of stem cell activation . In

the hemopoietic stem cell system , it has also been dem -

onstrated that SCF in combination w ith selective multipo-

tentia l colony-stimulating factors can influence the

relative frequency of progenitor cells comm itted to vari-

ous lineages (48). Whether the SCF /c-kit system in the

early hepatic stem cell population interacts w ith other he-

patic g row th fa cto rs so as to influence the frequency of

lineage comm itment of progenitor cells is not known at

present. However, the hepatic expression pattern and cel-

lular location of the SCF /c-kit system indicate that th is


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THORGEIRSSON

signal transduction system is required only during the

early activation and transitional phase of the oval cell dif-

ferentiation . Once the oval cells have differentiated into

the small basophilic hepatocytes, the expression of both

SCF and c-kit is abolished (49). This concept is sche-

matically illustrated in F ig . 4 .

PARTICIPATION OF LIVER STEM CELLS IN

REGENERATION

After the hepatocyte population is reduced by, for exam -

ple, PH , the residual hepatocytes proliferate promptly,

continue to cycle until the deficit is repaired , and con-

tinue to function while proliferating . Under these condi-

tions, no apparent contribution to the regeneration

process is provided by the stem cell compartment. A cti-

vation of oval cell proliferation and differentiation by in-

jury, which is more severe and/or qualitatively different

from the simple loss that triggers only hepatocyte prolif-

eration, results in transient reestablishment of a hepato-

cytic lineage that has all the characteristics of a potential

or facultative stem cell system (5). Cells in the normally

quiescent stem cell compartm ent are activated to produce

poorly differentiated oval cell progeny. Oval cells prolif-

erate extensively to yield a large population of cells that

m igrate throughout the parenchyma, some of which differ-

entiate as they m igrate. H epatocytic progeny of oval cells

merge into the functional compartm ent of mature hepato-

cytes and help restore the parenchyma. S im ilar to the

generation of new hepatocytes after simple loss, the pro-

duction of hepatocytes via the stem cell (oval cell)

m echanism is also episodic and transient. These two dis-

tinct mechanisms of hepatocyte formation are both sub-

jected to several points of stringent control (5). Controls

are required to regulate the reinitia tion of hepatocyte for-

mation from the normally quiescent hepatocytes, as well

as to regulate the activation of potential stem cells that

energizes cell flow through the entire lineage. A lthough

the controls may differ between the two mechanism s of

hepatocyte formation, it is probable that both pathways

are simultaneously activated after loss of liver mass, in-

cluding that after simple PH.

One of the earliest phenotypic indications of liver stem

cell activation is the expression of AFP (5). A transient

expression of AFP is also s ee n a ft er s i mp le PH an d simi-

lar to that seen in stem cell activation in the A AF/PH

model, the AFP transcripts ar e located in the bile ductu-

les (36, 50). Expression of both SCF and c-kit is also

transiently elevated after standard PH (46). However,

there is no evidence indicating that the stem cell-derived

hepatocytes significantly contribute to regeneration of

liver mass after simple PH (5). These observations sug-

gest that the activation, and in particular, the expansion

of liver stem cells, are stringently controlled during hepa-

tocyte-driven liverregeneration.

Recent studies of the early activation of oval cell pro-

liferation in the AAF/PH model have provided important

clues toward defin ing the mechanism controlling the early

e xp ans io n o f o va l c el ls (51, 52). It has been shown that a

low dose of AAF (and its analogs 2-AF and N-OH-2-

A AF) elicited , in th e absence of PH, a m it og en ic re -

sponse in ductular and periductular cells w ithin 24 h

after adm inistration . The compounds also induced the ex-

pression of HNF1, HNF3y, AFP , and album in in the

ductular cells, sim ilar to the observations shown earlier

in the complete AAF/PH model (53). In contrast, initia-

tion of bile duct proliferation by ligation of the common

bile duct had no effects on the expression of these genes

in ductal cells. In addition to eliciting a m itogenic re-

sponse, adm inistration of AAF also induced apoptosis of

cells in the portal areas, a process that contributed to the

overall re tention of liver morphology. As no significant

increase in the cellu larity of the portal areas was ob-

served at this tim e, it is probable that an equilibrium be-

tween mitosis and apoptosis exists to maintain a constant

number of ductal cells. This observation is in striking

contrast to the condition seen in the complete AAF/PH

Figure 5 . Schematic dep iction of liver regeneration illustrating the

contribution of hepatic stem cells. Induction of reparative renewal of the

epithelial cell populations in the liver after bo th simple PH and certain

types of more severe liver inju ry involves activation of both the normally

quiescen t hepatocytes and the poten tial (facultative) liver stem cells . As

discussed in the text, the essential requirem ent for the sustained activa-

tion of the stem cells and their p rogeny required for generating the

d ifferen tiated cell lineages needed for liver regeneration is a prolonged

expression of a set of grow th factors, includ ing those known to be involved

in liver regeneration after sim ple partial hepatectom y.


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HEPATIC STEM CELLS

1255

model, in which the oval cells continuously proliferate

and expand into the liver acinus. Furthermore, adm ini-

stration of AAF alone results in only a modest and tran-

sient increase in expression of hepatotrophic growth

factors, again in contrast to the intense and sustained ex-

pression of growth factors seen in the AAF/PH model

( vid e s up ra ).

The importance of some of the grow th factors that are

highly expressed during the activation and expansion of

oval cells in the

AAF/PH

model is beginning to emerge.

It has now been demonstrated that infusion of epidermal

growth factor (EGF) and HGF, either alone or in combi-

nation, can partia lly substitute for the PH in the AAF/PH

model (52). The results from this study demonstrate that

although both EGF and HGF increase the number of pro-

liferating cells after AAF adm inistration , they preferen-

tia lly act on different cell populations. A lthough

adm inistration of AAF alone or in combination with infu-

sion of HGF resulted in proliferation of almost equal

numbers of ductal and Ito cells, infusion of EGF and/or

combination of EGF and HGF resulted in 75-80 of th e

proliferating cells having a ductal phenotype. A lso , infu-

sion of EGF and HGF resulted in a decreased number of

cells undergoing apoptosis in response to AAF. These re-

sults have important implications for our understanding of

both hepatic stem cell activation and expansion as well

as for the mechanism of liver regeneration . Based on the

data discussed above, the follow ing model of liver regen-

eration can now be proposed (F ig. 5). Induction of re-

parative renewal of the epithelia l cell populations in the

liver after simple PH and certain

types of

m ore severe

liver injury involves activation of both the normally qui-

escent hepatocytes and the potential (facultative) liver

stem cells. In a healthy liver, the reparative renewal of

the hepatocyte and biliary epithelial cell populations is

accomplished in most instances by proliferation of resid-

ual differentiated cells of each types, resulting in only a

transient activation of the stem cells. However, under

conditions in which the hepatocytes are unable to re-

spond to the regenerative stimuli and/or are functionally

comprom ised, a susta ined activation of the stem cells and

their progeny ensues, generating the differentiated cell

lineages needed for the liver regeneration. An essential

requirement for the stem cell-driven liver regeneration is

a sustained expression of a set of grow th factors, includ-

ing those known to be involved in liver regeneration after

simple PH .

Several implications emerge from this in tegrated model

of liver regeneration. Perhaps the most important predic-

tion arising from the model suggests that in chronic liver

diseases such as viral hepatitis, a condition in which the

hepatocytes are comprom ised and a continuous regen-

eration exists, the stem cells would be activated and

might contribute to the regenerative process. The obser-

vations that oval-type cells occur in livers harboring

hepatitis B virus-associated liver tumors (26) and that

these cells can

be infected by the hepatitis B virus at

early stage of differentiationhave significantimplications

for human hepatocarcinogenesis (54, 55). A lso , the possi-

b ility of isolating liver stem cells from which hepatic

lineages can be generated in vitro holds promise for fu-

ture approaches to gene therapy of liver-related diseases.

REFERENCES

1 Van

Thiel,. H.,Gavaler,.S.,Kam, L,

Francavilla, A .. Polim eno , L ,

Schade P. R . , Sm i th , J. , D iven, W ., Penkm t, R . J., and Staral, T . E . (1989)

H apid grow th of an intact hum an liver transplan ted in to a recipien t larger

th an th e d on or .

Castroenterology 93 ,

1414-1419

2. Lajtha, L . C. 1970 temcelloncepts.

N o uv . R ev .

Fr .

Hematol.

21,59-65

3 . Rhim , J. A ., Sandgren , E . P ., Degen, J. L ., Palm iter, R . D ., and Brinster, R .

L (1 99 4) R epla ce me nt of dis ea sed m ouse live r b y he pa tic c ell tra ns pla nts .

tion.

Science 263, 1149-1152

4.

P otte n, C . S ., a nd L oe ffler, M . (19 90 ) Stemc el ls : a tt ri bu te s, c yc le s, s pi ra ls ,

p it fa ll s a n d u n ce rt ai nt ie s .

Development

110, 1001-1020

5. Grisham, J. W. , a n d Th or ge ir ss o n,

S.S.

(19 96 ) L iv er stem c ells . In

T he S te m

Cel l Handbook

(Potten , C . S ., ed) A cadem ic Press, N ew Y ork In press

6 . V an Eyken, P ., and Desm et, V . J. ( 19 92 ) D ev elo pm e nt o f i ntr ah ep atic bile

d uc ts , d uc tu la r m e ta pl as ia o f h e p at oc yt es , an dc yt ok er at in p at te rn s i n v ar io us

typ es o f h um an h epa tic n eop la sm s. In T he R ole o f C ell T yp es i n H ep at oc ar -

cinogenesis

( Sir ic a, A . E ., e d) p p. 2 27 -2 63 , CR C P re ss , B oc a H ato n, F lo rid a

7 . V an Eyken , P ., Scio t, R ., Callea, F ., and D esm et, V .

J.

( 19 8 9) A cy to k er a ti n-

imm un oh is to ch em ic al s tu dy o f f o ca l n od ula r h yp erp la sia

of

th e liv er f urth er

evidence that ductular m etaplasia of hepatocy tes contributes to ductular

proliferation.

Liver

9,372-377

8 . Thorgeirsson , S . S . (1993) C om mentary. H epatic stem cells . Am . I. Pathol.

1 42 , 1 33 1- 13 33

9 . Fausto, N . (1990) O val cells and liver carcinogenesis: an analysis of cell

line age s in he pa tic tu mo rs u sin g on cog en e tra ns fe ction tec hniq ue s. Prog.

C lin . B io l. R et. 331 325-334

10. Sell, S . (1990) Is there a liver stem cell?

C a nc er R et .

50,3811-3815

11. S igal, S . H ., B rill, S ., and R eid , L M . (1992) The liver as a stem cell and

l in ea ge s ys tem .

Am . J. Physiol. 263 C

13 9-C 1 48

1 2. C ris ha m,J . W. ( 19 80 ) C ell ty pe s in lo ng -te rm p ro pa ga ble c ultu re s o f r at liv er .

An n. N .Y . A ce d. S ci. 349, 128-137

1 3. K in osita, R . (1 93 7) S tu die s on th e c arc ino ge nic c hem ic al su bsta nc es .

Trans.

S o c. P a th o l. Jpn. 27 665-727

14. Price,

J.M.,

Harman , J. W ., M ille r, E . C ., a nd M ill er ,J . A . ( 19 52 ) P ro gre ss iv e

m icroscopic alterations in the livers of rats fed the hepatic carcinogens

3 - m et hy l4 -d im e th yl am in oa zo be nz en e a nd 4 - fl uo ro -4 -d im e th yl am i no az o-

benzene.

C a nc er R et .

12,192-200

15 .

F arb er, E . (19 56 ) S im ila ritie s in th e se que nce of e arly h istolo gic al c han ge s

induced in the liver of the rat by eth ion ine, 2 -acety lam inofluorene. and

3-methy l4-d imethy laminoazobenzene.

C a nc er R e,.

16,142-148

16 .

Wilson ,

J.W.,and

L educ, E . H . (1958)

Roleofcholangioles

n rest o ra t io n

of the liver of the m ouse after dietary inju ly. J.

Pathol.

icterioL

76 ,

441-449

17 . M arceau, N . (1990) B iology

ofdi sease.ell

l in ea ge a nd d if fe re nt ia ti on

program s in epiderm al; u ro thelial and hepatic tissues and

their

eoplasms.

La b. I n ve st . 63 4-20

18 . Inaoka, Y . (1967) S ign ificance of the so-called oval cellr ol if e ra ti on d u ri ng

azo -dy e h ep at o car c in og en es is . Cairn 58,355-366

19. Shinozuka,.,Lom bard i, B ., Sell, S ., and lam marino, R . M . 1978 Early

h isto log ica l an d fu nc tio na l altera tion s o f ethio nin e liv er c arc in og ene sis in

r ats f ed s c ho lin e- de fic ie nt d ie t.

C an ce r R et.

38 , 1092-1098

20 . Sell, S ., and Salm an, J. ( 19 84 ) L ig ht - a nd e le ctr on -m ic ro sc op ic a uto ra dio g-

raph ic analysis of proliferating cells during the early stages of chem ical

h ep at oc av ei no ge ne si s i n t he r at i nd uc ed b y f ee di ng N -2 -f lu or en yl -a ce tam id e

in a c ho lin e- de fic ie nt d ie t.

Am .

J.

Pathol.

1 14 , 2 87 -3 00

21. Tatematsu, M ., Ho, R . H ., Kaku, T ., Ekem , J. K ., and Farber, E . (1984)

Studies on the pro liferation and fate of oval cells in the liver of rats treated

w ith 2 -a ce ty la m in ofl uo re ne a nd p artia l h ep ate cto my .

Am .

I.

Pathol.

114,

418-430

22. L esch, R ., R eutter, W ., K eppler, D ., and D ecker, K . (1970) L iver restitution

a fte r a cute g ala cto sa mine he pa titis: a utorad iog rap hic a nd bio che mic al stu d-

ie s in r ats .

Exp.

Mol.

Pathol.

1.2,58-69

23. Farber, E . (1984) Cellu lar biochem istry of the stepw ise developm ent of

c an ce r w it h c hem ic al s. C an ce r R e,. 4 4, 5 46 3- 54 74

24. Evarts , H . P ., N agy, P ., M arsden, E ., and Thorgeirsson, 5. 5 . (1987) A

pre curso r-pro duc t re la tio nsh ip e xists b etw een o va l c ells a nd h epa toc yte s in

r at l iv e r.

Carcinogenesit

8 , 1 73 7 -1 7 40

25. Gerber, M . A ., Thung , S . N ., Shen , S ., S tmm eyer, F . W ., and Ishak , K . C.

(1 98 3) P he noty pic c ha ra cte riz ation of h epa tic p ro lifera tion : a ntig enic e x-

pression of proliferating epithelial cells in fetal liver, m assive hepatic ne-

c ro sis , a nd no du la r tra ns fo rm ation of th e liv er. Am . J. Pathol. 110,70-74

26. H sia, C . C ., Evarts, H . P ., N akatsukasa, N ., M araden , E . H ., and Thorgeira-

s on , S .S . ( 19 92 ) O cc ur re nc e o f o va l-ty pe c ells in h ep atitis B v ir us -a ss oc ia te d

human hepat o car c in og en es is .

Hepatology 16,1327-1333


8/8

1256 Vol. 10 S eptem ber 1996

Th e

FASEBjournal

THORGEIRSSON

27 . Hixson, D .C ., Fans, R . A ., and Thom pson, N . L. (1990) An a n ti ge n ic p o rt ra it

o f t he l iv er d ur in g c ar ci no ge ne si s.

Pashobiology

58,65-77

28 . Factor, V . M ., R adaeva, S . A .,

an d

T horge irsso n, S .S . (1 99 4) O rig in a nd

fate

of oval cells in flipin -induced hepatocarcinogenesis in the m ouse. Am . J.

Pathol.

145,409-422

29 . Thorgeirsson , S . S ., Evarts , R . P ., Fu jio, K ., and Hu, Z . (1994) C ellular

biology oftherat hepatic stem cell com partm ent. In L ive r Carc inogene s is

(Skouteris,

C. C.,ed

p p. 1 29 -1 45 ,

NATO

A SI S er ie s, V ol. H 88 , S prin ge r-

Ver lag , Ber li n /He id e lb e rg

30 .

Lem ire, i. M ., Shio jin, N ., and Fausto , N . (1991) O val cell p roliferation and

the o rig in of sm all he pa to cyte s in liv er inju ry in du ce d by D -ga la ctos am in e.

Am .

I.

Pathol.

139,535-552

31 . Tatem atsu, M ., K aku, T., M edline, A ., and Farber, E . (1985) In testinal

m eta pla sia as a co mm on op tion o f ov al ce lls in re la tio n to c ho lan giofibros is

in th e liv ers of ra ts e xpo se d to 2-a cetyla min oflu ore ne .

L ab . I nv es t.

52 ,

354-362

32 . Hayner, N . T., B raun, L, Yaswen , P ., B rooks, M ., an d F austo, N . (19 84 )

Iso zy me p ro files of o va l c ells, pa re nc hy ma l c ells, a nd bilia iy c ells isola te d

b y c en trifu ga l elu ina tio n fro m n orm al a nd pre ne opla stic liv ers .

C a nc er R et .

44,332-338

33 . G erm ain, L ., C oye tte , R ., a nd M arc ea u, N . (1 98 5) D iffe re ntia l c ytok era tin

and x-fetoprotein expression in m orphologically d istinct epithelial cells

e me rg in g a t th e e ar ly s ta ge s o f ra t h ep ato ca rc in og en es is . C ancer R et. 45

673-681

34 . Cerm ain, L ., N oel, H ., G ourdeau, H ., and M arceau, N . (1988) Prom otion of

g ro wth a nd d if fe re ntia tio n o f r at d uc tu la r o va l c ells in p rim a ry c ultu re .

Cancer

Ret.

48,368-378

35 . Thorgeirason, S . S ., and Evarts, H . P . (1992) Growth and differen tiation of

stem cells in adult liver. In

T he R ol e o f C el l T yp es o f H ep at oc ar ci no ge ne si s

(S irica, A . E ., ed) pp. 100-120, C RC Press, R oes R aton , Florida

36. Golding,..Sarraf, C . E ., Lalan i, E. N ., A nilkum ar, T . V ., Edw ards, R . J. ,

N agy , P ., T horg eirss on , S . S ., a nd A liso n, M .R . (19 95 ) O va l c ell diffe re ntia -

tio n in to he patoc yte s in th e a ce tyla mino -flu ore ne -tre ate d re ge ne rating ra t

liver.

Hepatology

22 ,

1243-1253

37 . E varts, R . P ., N akatsukasa, I-I., M a raden, E. H ., H u, Z ., and Thorgeirsson, S .

S . (1 99 2) E xpre ssio n of tra nsform in g grow th fa ctor-a in re gen era tin g live r

an dd u ri ng h epa ti c d if fe re n ti at io n . MoE.

Carcinogen.

5,25-31

38 . Evarts, R . P ., Hu Z. F ujio , K ., M ar sd en , E . R .,a nd Th or ge ir ss on ,S . S . ( 19 93 )

A ctiva tio n o f he patic ste m c ell c om pa rtm en t in th e ra t: R ole of tra nsform in g

g ro wth f ac to r-U , h ep ato cy te g ro wth f ac to r, a nd a cid ic f ib ro bla st g ro wth f ac to r

i n e a rl y p ro li fe ra ti on . Cel l G ro wth & D (ffer . 4, 555-561

3 9. H u,Z .,

E va rts , H .P ., F ujio, K ., M arsd en , E . R ., a nd T ho rge irsso n, S . S . (19 93 )

Expression of hepatocy te grow th factor and c-m et genes during hepatic

differentiation and liver developm ent in the rat.

A m. J . P athol. 142,

1823-1830

40 .

M arsden, E . H .. H u, Z ., Fujio, K ., N akatsukasa, H ., T horgeirsson , S . S ., and

E va rts, R . P . (1 99 2) E xpres sio n of a cidic fibrob la st gro wth fa cto r in re ge n-

e ra ti ng l iv er a nd d ur in g h ep at ic d if fe re nt ia ti on .

La b. I nv es t.

67,427-433

41 . M ichalopoulos, C . (1990) L iver regeneration : m olecu lar m echanism s of

g row th c o nt ro l.

FASEBJ. 4,

176-187

42 . M ead, J . E., and Fausto , N . (1989) T ransform ing grow th factor

a

may be a

p hy siolo gic al reg ula to r of live r re ge ne ra tion b y m ea ns of a utoc rine m ec ha -

n isms.

P ro c. N a t .

Acad.

S ci. U SA 86 ,

1558-1562

43 . Evarts, R . P ., Nakatsukasa, H ., M araden , E . R ., H sia, C . C ., Dunsford , H .

A ., and Thorgeirsson , S . S . (1990) C ellular and m olecular changes in the

early stages of chem ical hepatocarcinogenesis in the rat.

C a nc er R et .

50 ,

3439-3444

44 . Lenzi, R ., L iu, M . H., Tarsetti, F ., S lo tt, P . A ., A lpin i, C ., Xhai, W . R .,

Paronetto , F ., Lenzen , R ., and Tavolim , N . (1992) H istogenesis of bile

d uc t- lik e c ells p ro lif era tin g d ur in g e th io nin e h ep ato ca rc in og en es is .

Lab.

Invest.

66,390-402

45 . Hu, Z ., Evarts , R . P ., Fujio, K ., Omon, N ., Om on, M ., M arsden, E . H ., and

T ho rg ei ra so n, S .S . ( 19 96 ) E xp re ss io n o f t ra ns fo rm in g g ro w th f ac to r a lp ha /e pi -

derm al grow th factor receptor, hepatocy te grow th factor/c-m et and acid ic

f ib ro bla st g ro wth fa cto r/f ib ro bla st g ro wth fa cto r r ec ep to rs d ur in g h ep ato ca r-

cinogenesis.

Carcinogenesi.c

17,931-938

46 . Fujio, K ., E varts, H . P ., H u,Z ., M arsden , E. R .,andThorgeirsson,S . 5. (1994)

E xp re ss io n o f s te m c ell f ac to r a nd its r ec ep to r, c -k it, d ur in g liv er r eg en er atio n

from putative stem cells in adult rat.

La b. I nv es t.

70,511-516

47 . M orrison-G raham , K ., and Takahashi, Y . (1993) S teel factor and c-kit

rec ep tor fro m m uta nts to a g row th fac to r sy ste m.

BioEssays

15 ,

77-84

48 . M etcalf, D . (1991) L ineage com mitm ent of hem opoietic progen itor cells in

d ev elo pin g b la st c ell c olo nie s: in flu en ce o f c olo ny -s tim ula tin g fa cto rs . Proc .

N at . A ced. Scj. U SA

88, 11310-1 1314

49. Fujio,.,Hu,Z.,

Evans,

R.

P ., M arsd en, E .

H.,Niu,C.-H., andTho rgeira son

S .S . (19 96 ) C oex pre ssio n o f s te m c ell fac to r a nd c-k it in e mbryo nic a nd a dult

liver.

Exp.

C e ll R e t. 224,243-250

50 . Lem ire, J. M ., and Fausto, N . (1991) M ultiple alpha-fetoprotein RN As in

adult rat liver cell type-specific expression and differential regu lation .

C a nc er R et .

51,4656-4664

51 . B isgaard , H . C ., N sgy P., San ton i-R ugiu E ., and T horgeirason , S . S . (1996)

P ro life ra tio n, a po pto sis, an d ind uc tio n of he patic tra ns crip tion fac to rs a re

characteristic of the early response of biliary epithelial (oval) cells to

c h em i c al c a rc in o gen s.

Hepatology 23,62-70

52. N agy, P ., B isgaard , H . C ., Santoni-R ugiu , E ., an dT ho rg ei rs so n, S . S . ( 19 96 )

In v ivo infusion of grow th factors enhances the m itogen ic response of rat

h ep at ic d uc ta l oval ellsollowi ngdmin is t ra t io n o f 2 - ace t y laminof l uo r ene .

Hepatology

23,71-79

53. Nagy, P ., B isgaard , H . C ., and Thorgeirsson, S . 5 . (1994) Expression of

hepatic liven transcription factors during liver developm ent and oval cell

differentiation. J . C ell

Biol.

126,223-233

54. H sia, C .-C ., T horgeirsson, S . S .,

an d

la bo r, E . ( 19 94 ) E xp re ss io n o f h ep atitis

B surfa ce a nd c ore a ntige ns a nd tra ns fo rm ing g ro wth fa cto r-a in ov al c ells

of the liver in patients w ith hepatocellu lar carcinom a.

M ed. V iroL

43 ,

216-221

55. Thorgeirsson , 5. 5 . (1995) Target cell populations in v irally associated

he pato ca rc in og en esis. P roc ee din gs o f th e 2 5th Intl. S ym po siu m of th e P rin -

cess Takamatsu Cancer Research Fund on Hepatitis C V irus and its

Involvem ent in the D evelopm ent of C arcinom a, pp. 163-170, Princeton

S cie nti fic P ub lis hin g C o., P ri nc eto n, N ew J ers ey

56. Shio jiri, N ., L em ire, J. M ., and Fauston , N . ( 1991) C ell lineages and oval cell

p ro ge ni to rs i n r at l iv er d ev el op m en t.

C a nc er R e, .

51,2611-2620

57. V an Eyken, P ., aiid D esm et, V . J. (1993) Cytokeratins and the liver.

Liver

13 , 113-122

58. Cerm ain, L ., B lou in, M .-J., and M arceau, N . (1988) B iliazy epithelial and

hepatocytic

cell

ine age rela tio ns hip s in e mb ry on ic ra t liv er a s de te rm in ed

b y th e d iff er en tia l e xp re ss io n o f c yto ke ra tin s, a ip ha -f eto pr ote in , a lb um in , a nd

c el l s ur fa ce -e xp os ed c omp on en ts .

Cancer

Ret . 48,4909-4918

59. B isgaard, H . C ., N agy , P ., Ton, P .1 ., H u, Z., and Thorgeirsson , S .S . (1994)

M odulation of keratin 14 and a-fetopm tein expression during hepatic oval

c el l p ro li fe ra ti on a nd l iv er r eg en er at io n. J . C e ll .

Physiol.

159,475-484

60. B isgaard, H . C ., Ton,P. T.,N agy, P ., an d T horgeirsson . S . S . (1994)

P h en o ty p ic mo d ul at io n

of

k er at in s, v im e ntin , a nd a -fe to pr ote in in c ul tu re d

ra t liv er e pith elia l c ells a fte r c he m ic al, o nc og en e, a nd s po nta ne ou s tra ns fo r-

mat ion .

J Cell. Physiol.

159,485-494

61. D unsford , H . A ., and Sell, S . (1989) Production of m onoclonal antibod ies to

pre ne opla stic live r c ell p op ula tio ns in du ced b y c he mic al ca rc ino ge ns in ra ts

a nd tr an sp la nta ble M or ris h ep ato ma s.

Cancer Ret .

49,4887-4893

62. M arceau , N ., G erm ain , L ., Coyette, R ., N oel, M ., and G ourdeau , H . (1986)

Cell o f origin of d istinct cultu red rat liver ep ithelial cells, as typed by

c yto ke ra ti n a nd s ur fa ce c om po ne nt s ele ctiv e e xp re ss io n.

Biochem. Cell. Rio .

6 4, 7 88 -8 02

63 . Y ang , L ., Fans, H . A ., and H ixson , D . C . (1993) Phenotyp ic heterogeneity

w ith in c lo no ge nic d uc ta l c ell p op ula tio ns is ola te d fr om n orm a l a du lt ra t liv er.

P ro c. S oc . Exp. Biol. Med .

204,280-288

64 . M arc ea u, N ., B lo uin , M .-J., N oe l, M ., T { 24 6} ro k,., a nd L ora nge r, A . (1 99 2) T he

ro le of bip ote ntia l prog en ito r c ells in liv er on tog en esis a nd ne opla sia . In Th e

R ol e o f C el l T yp es i n R ep at oc ar ci no ge ne si s

(S iric a, A . E ., e d) pp . 12 1-14 9,

CR C

P re ss , B oc a R ato n, F lo rid a

65. H ixson, D . C ., and A llison , J. P . ( 19 85 ) M on oc lo na l a ntib od ie s r ec og niz in g

ov al c ells ind uc ed in th e liv er o f r ats by N -2 -flu ore ny la ce ta mide o r e th iom ne

in a c ho lin e- de fic ie nt d ie t. Cancer Ret . 45,3750-3760

66. Fans, H . A ., M onfils, B . A ., D unsford , H . A ., and H ixson , D . ( 1991) A ntigenic

re la tio ns hip b etw ee n o va l c ells a nd a s ub po pu la tio n o f h ep atic fo ci, n od ule s,

a nd c arc ino ma s in du ced b y the re sista nt he pa to cyte m ode l s yste m.

Cancer

Ret . 5 1, 1 30 8- 13 17

67. Engelhard t, N . V ., Factor, V . M ., Y asova, A . K ., P olto ran ina, V . S ., B azanov ,

V . N ., a nd L asa re va , M . N . (19 90) C om m on an tige ns of m o use o va l a nd bilia iy

e pit he lia l c ells . E xp re ss io n o n n ew ly f orm e d h ep ato cy te s.

Differentiation

45 ,

29-37

68. Engelhard t, N . V ., Factor, V . M ., M edvinsky , A . L , B aranov , V . N ., Lazareva,

M. N.,and

P oltora nin a, V . S . (19 93 ) C om m on a ntige n

ofov aland

biliary

e pithe lia l c ells (A 6) is a d iffe re ntia tion m ark er o f e pithe lia l a nd e ryth ro id

c ell lin ea ge s in e arly de ve lop me nt o f th e m ou se .

Differentiation

55, 19-26

hepatic stem cell in liver regeneration

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