MINIREVIEW SERIES FOR THE 50TH
VOLUME
Professor Toshio Ito: a clairvoyant in pericyte biology
Makoto Suematsu and Sadakazu Aiso1
Department of Biochemistry and Integrative Medical Biology,1 Department of Anatomy, School of Medicine
Keio University, Tokyo, Japan(Received for publication on March 23, 2001)

Abstract. Ito cells are liver-speci®c pericytes which were ®rst described as Fett Speicherung Zellen,
the fat-storing cells encircling outside sinusoidal endothelial cells, in 1951 by the late professor Toshio
Ito. His pioneering approaches for morphological characterization of the cells stimulate investigators
to further examine their functional roles in liver homeostasis: a body of evidence has been accumulated
in recent years showing that the cells play a crucial role in storage and delivery of vitamin A, regulationof sinusoidal tone and local blood supply, and tissue repair and ®brosis. It is now widely accepted thatmicrovascular pericytes including Ito cells serve as a key player that controls angiogenesis. Furthermore, recent studies support a concept that Ito cells constitutes a bridging apparatus mediating bidirectional metabolic interactions between sinusoids and hepatocytes, utilizing prostanoids and/or gaseous
mediators such as nitric oxide and carbon monoxide as signaling molecules. This article reviews
researches on this liver-speci®c pericyte and its leading roles in recent development of pericyte biology.
(Keio J Med 50 (2): 66±71, June 2001)
Key words: pericytes, microcirculation, vitamin A, carbon monoxide, soluble guanylate cyclase

the next year.3 The cells are characterized by welldeveloped rough endoplasmic reticulum and lipid
droplets containing vitamin A which are mainly stored
in the perikaryon. Several thick cytoplasmic processes
are protruded directly from the perikaryon being called
the primary processes; they extend their processes on
the outer surface of sinusoidal endothelial cells.4 The
cell was therefore named Ito cell after his discovery,
while the same cell has also been called by a variety
of different names such as pericytes, fat-storing cells,
parasinusoidal cells, lipocytes and hepatic stellate cells.
Since then, varied functions of this liver-speci®c pericytes were examined extensively through molecularbiological techniques, and the results suggest an important role of Ito cells in homeostasis of hepatobiliary
functions. Furthermore, his discovery of this unique cell
greatly contributed to extensive advances in biology of
microvascular pericytes, the de®nition of which was
not fully established in his era. This article reviews
physiologic and pathophysiologic roles of Ito cell in

Introduction
Besides hepatocytes in its parenchyma, the vertebrate liver consists of several different kinds of nonparenchymal cells, such as endothelial cells which line
sinusoids, Kupffer cells or liver macrophages, pit cells
or liver-associated natural killer cells, and the perisinusoidal stellate cells. The stellate cells occuring in thespace of Disse were ®rst described by von Kupffer in
1898.1 He described that the cells were characterized bycytoplasmic inclusion bodies stained with gold chloride
method, and incorrectly suggested that the gold chloride-reactive cells were phagocytic cells. According to
his description, the inclusion bodies were fragments oferythrocytes taken up from circulation by phagocytosis.
In 1951, when the Keio Journal of Medicine started its
history, Professor Toshio Ito precisely described thatthe cells in the perisinusoidal space are characterized
by abundant fat droplets and differ from Kupffer cells,
publishing as a form of abstract2 and as an article in

Reprint requests to: Dr. Makoto Suematsu, Professor and Chair, Department of Biochemistry and Integrative Medical Biology School of
Medicine, Keio University, 35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan, e-mail: msuem@med.keio.ac.jp66Keio J Med 2001; 50 (2): 66±71

Fig. 1 Professor Toshio Ito; A portrait (cited from Cogito 58 MITAHYORON No. 926 June 1991 with permission).

liver homeostasis and highlights their signi®cance in the
pericyte biology.
How Did Professor Ito Meet His Cell?
Professor Ito graduated from Keio University Schoolof Medicine in 1930 and started his research carrierin Department of Anatomy under supervision by Professor Okajima (Fig. 1). He moved to Tokyo Women's
Medical College as Professor of Anatomy on April
1941, and spent most of his time in this position underWorld War II. In 1948, he took up professorship in
Gunma University, where he met his cell; through lightmicroscopic observation, he found interesting cells
characterized with abundant fat droplets that were
obviously different from Kupffer's phagocytic cells andcalled them fat-storing cells. He reported this discovery in the 55th Annual Meeting of Japanese Societyof Anatomy in 1950.2 It should be noted that, upon
his report in the meeting, Professor Tokuyasu Kudo
Department of Anatomy in Niigata University Schoolof Medicine asked an important question based on his
research experiences of microtopography of vitamin A
in the body, ``If your cells are characterized by abundant fat droplets, they should store lipophilic vitamins
such as vitamin A!''. In these days, a majority of
researchers including Professor Kudo believed that
Kupffer cells constitute a major cellular component forstorage of vitamin A in the liver. In 1952, these ®ndings
were published in Okajima Folia Anatomica Japonica
which was extensively cited by foreign researchers over
the world.3 In 1959, his group carried out electron-

67

Fig. 2 A representative picture of Ito cells. Panel A: Microtopographic relation between Kupffer cells and Ito cells in the rat liver.
The cells stained brown exhibit immunoreactivities to the MoAb
KiM2R, a marker of macrophages, while those in purple were stainedwith anti-desmin MoAb as a marker of Ito cells. Note that processesof Ito cells encircle sinusoidal vessels (askterisk). Bar  20 mm. Panel
B: A representative picture of primary cultured Ito cells isolated fromthe rat liver. Note abundant fat droplets in the cell bodies.

microscopic observation of fat-storing cells and demonstrated that the cells are wrapped with collagen ®bers
and located in the outer surface of sinusoidal endothelium. Processes to establish structural characterizationof the fat-storing cells were described in detail in his
review article.4 Symposia for sinusoidal cells wereorganized in the Netherlands in 1977 and 1982 byProfessors E. Wisse and Knook DL. In these meetings,
fat-storing cells were fully recognized as the third
sinusoidal cells besides Kupffer cells and sinusoidal
endothelial cells, as Professor Wisse cited Professor Itoand his group's investigations on the cells. Approximately 30 years had passed for such an internationalrecognition of the work since he ®rst described his cell.
How Can We Assess Cell Function of Pericytes
in vitro?
Despite morphometrical one, functional characterization and its link to molecular events required
another 20 years until now and still remain to becompleted. From a viewpoint of vascular biology, Itocells are considered to be microvascular pericytes which
occur speci®cally in the liver. Pericytes, also known as
Rouget cells or mural cells, are associated abluminallywith true capillaries and post-capillary venules among
all organ microvascular systems.5 Differences in pericyte morphology and distribution among vascular beds
suggest tissue-speci®c functions. Ito cells are characterized by their dendrititic structures with cytoplasm®lled with vitamin A-containing fat-storing droplets(Fig. 2). Based on their complement of muscle cytoskeletal proteins, pericytes have been proposed to play a

68

Suematsu M and Aiso S: Ito cells in pericyte biology

role in the regulation of microvascular blood ¯ow.6,7
In vitro studies demonstrating the contractile abilityof pericytes support this concept.6 Pericytes have also
been suggested to differentiate multipotentially into
varied types of cells such as adipocytes, osteoblasts andmyo®broblasts. The mechanisms involved in vessel
formation have yet to be elucidated but observations
indicate that the primordial endothelium can recruit
undifferentiated mesenchymal cells and direct their differentiation into pericytes in microvessels, and smoothmuscle cells in large vessels. Communication between
endothelial cells and pericytes may take many forms.
Soluble factors such as platelet-derived growth factor
and transforming growth factors-b are likely to be
involved.8 In addition, physical contact mediated by celladhesion molecules,8 integrins and gap junctions9 appear to contribute to the control of vascular growth andfunction. Ito cells share such properties of pericytes.
Development of culture methods has allowed somefunctions of pericytes to be directly examined. Whenprimarily cultured, Ito cells build up intercellular communications by forming gap junctions that greatly
contribute to electrophysiologic coupling between
the adjacent cells (Fig. 3). Co-culture of pericytes withendothelial cells leads to the activation of transforming
growth factor-b, which in turn in¯uences the growthand differentiation of the vascular cells. Furthermore,
contractility of pericytes has been examined directly
through ``wrinkling assay'' using culture dishes coatedwith silicon membrane;6 upon exposure to vasoconstrictive mediators such as angiotensins or endothelins,
the ability of cultured pericytes to produce contractile
force is able to be judged by formation of silicon wrinkles around the cells. Another methodology to examine
the cellular contractile force is collagen lattices contraction assay.10,11 These two methods have widely
been used to study function of Ito cells and thus remind
investigators that such contractile properties of the cells
could play roles in regulation of local blood ¯ow andtissue contraction upon wound healing during ®brosis.
Ito Cells As A Center for Vitamin A Metabolism
The liver is known to store approximately 80% of
vitamin A in the body. Ito cells constitute a major
cellular component executing its storage in the liver.
The cells distributed almost homogeneously throughout
different zones of the liver lobule, however, Ito cells in
pericentral regions tend to store far less vitamin A thanthose in periportal regions.12,13 On the other hand, Itocells are currently regarded as the principal cell typeresponsible for matrix accumulation during repair
reactions of the damaged liver that are altered as afunction of vitamin A contents in the cells.13 A varietyof reagents causing liver damages or ®brosis such as

Fig. 3 Representative pictures of ¯uorescence recovery after photobleaching in a single Ito cell in culture preloaded with carboxy¯uorescein, indicating the presence of gap junctional communications
between the adjacent cells. Asterisks indicate cells undergoing laser
photobleaching. Serial micro¯uorographs were recorded before (A),and 0 min (B), 2 min (C), and 6 min (D) after completing the laser
photobleaching. Sequence of the color bar 8right) denotes the ¯uorescence intensities. Bar  20 mm. Cited from Am J Physiol (reference8) with a permission.

carbon tetrachloride and PCB, one of the major environmental hormones, and chronic ethanol administration have been known to reduce the vitamin A contents
in the cells.13 Structure and function of the cells havethus been revealed with advances of researches on
pathogenesis of liver ®brosis. A basic feature of the
response of Ito cells to hepatic tissue injury is called``activation''. Such activation processes involve cellproliferation, transformation from star-shaped, vitamin
A-rich cells to vitamin A-de®cient cells with a myo®broblast-like appearance displaying increased contractile properties. These results suggest that a shortage ofvitamin A or metabolites of stored retinoids could
cause transformation of the cell types. Furthermore,
activation is characterized by differential gene expression of connective tissue components, matrix-degrading
enzymes, and their inhibitors, resulting in matrix
accumulation colocalized with activated Ito cells.14
Interestingly, this in vivo activation process strongly
resembles the morphological and functional changes
observed in Ito cells during primary culture, and therefore the cells in vitro are commonly used as a model toKeio J Med 2001; 50 (2): 66±71

study the role of them during hepatic tissue repair.
Under quiescent conditions, Ito cells are unlikely to
express voltage-operated Ca 2 or Na channels which
commonly play a role in cell contraction in other cell
types such as myocytes and vascular smooth muscle
cells. With a reduction of vitamin A storage, the cells in
culture increase their expression of a-smooth muscle
cell actin and voltage-dependent calcium channels,
indicating a contractile phenotype.9 Details of metabolism and mobilization of vitamin A in and around Itocells were described in previous review articles.11
Molecular Markers Altering in Parallel with
Ito Cell Activation
Several extracellular stimuli, including, e.g. in¯ammatory cytokines, growth factors, vasoactive peptides,and extracellular matrix components, as well as a number of intracellular signaling pathways, are involved in
the activation process of Ito cells. However, the overall
picture is far from complete, and the molecular mechanisms regulating Ito cell activation, particularly at thetranscriptional level, are still under investigation. To
address this question recent studies used differential
mRNA display technologies and proteomics analyses
in the cells at different stages of in vitro activation
to identify key regulators involved in this activation
process.14±16With the former technique the transcription factor
Ets-1 was detected through its down-regulation during
Ito cell activation. Ets-1 has been known as an important role in cell proliferation, differentiation, development, transformation, angiogenesis, and apoptosis.
The Ets DNA-binding motif, GGA(A/T), has beenfound in numerous genes, including transcription
factors, receptor-type kinases, and proteases. Amongthe proteases, stromelysins, collagenase, and urokinase
plasminogen activator are typical Ets-1-responsive
genes. Interestingly, all of the latter proteins are
expressed by Ito cells in the early phase of primaryculture. Apart from direct DNA binding as monomers,
Ets-1 cooperates with various transcriptional activatorssuch as the AP-1 family in regulating gene activity
and has been shown to activate gene transcription
through a Ras-stimulated signal-transducing pathwaythat includes MAP kinases.15
Ito Cells: A Metabolic Bridge between
Sinusoids and Parenchyma?
Liver utilizes a variety of biologically active signaling
molecules to execute intercellular communications
between different cell types. Recent studies support
a concept that Ito cells serve a metabolic bridge that
constitutes bidirectional signaling pathways between

69

sinusoids and parenchymal cells. A typical example forthe role of Ito cells in alterations of sinusoidal function
through hepatocyte-mediated mechanisms is a reception of carbon monoxide (CO) by soluble guanylate
cyclase in Ito cells that contributes to maintenance ofsinusoidal patency.17 CO is a by-product of the reactionof heme oxygenase (HO) that oxidatively catalyzes
protoheme-IX into biliverdin-IX a.18 Liver constitutes
a major organ responsible for detoxi®cation of thehemoglobin-derived heme and biliary excretion ofbilirubin-IX, a product generated from biliverdin-IX
through biliverdin reductase.19 We examined intrahepatic distribution of two major HO isozymes immunohistochemically, with the ®nding that the two isozymes have distinct topographic patterns; HO-1, theinducible form, is expressed prominently in Kupffer
cells, while the constitutive HO-2 is abundant inhepatocytes.16 We have shown that CO derived from
HO-2 is necessary to keep sinusoids in a relaxing
state through mechanisms involving soluble guanylate
cyclase in Ito cells. In other words, Ito cells utilize
soluble guanylate cyclase to sense CO for sinusoidal
relaxation.7,19 Considering the microanatomical orientation of the liver cells in and around sinusoids, HO-2 inparenchyma stands in the reasonable position for thegas reception by Ito cells where CO released fromhepatocytes can directly access to the cells and thereby
modulate their contractility without being captured byhemoglobin in circulation.17 When exposed to disease
conditions such as endotoxemia and advanced cirrhosis,
liver could upregulate HO-1 in Kupffer cells and hepatocytes as a result of cytokine responses.20,21 In experimental models of endotoxemia, such an inductionof HO-1 expands the ability of liver to degrade heme
and to generate CO. Under these circumstances, COturned out to contribute to maintenance of blood perfusion as well as bile excretion.21
Besides CO-mediated vasorelaxation, a variety ofvasoactive mediators for controlling sinusoidal tone
have been studied extensively. Such substances involve
endothelins, thrombin and ATP as ``contractile'' mediators for sinusoids, and prostaglandin E2 , nitric oxide(NO) as ``relaxing'' mediators. Among these substances, endothelins, NO and prostaglandin (PG) E2
constitute those which transfer information at thesinusoidal side towards hepatocytes.22 In unstimulated
livers, endothelins and NO are generated in sinusoidal
endothelial cells. Ito cells are thought to express both
ETA and ETB receptors and contribute to cell contraction upon the ligand binding.23,24 Since ETB receptor isalso expressed in sinusoidal endothelium and elicits NO
generation, this pathway could contribute to Ito cellrelaxation. Although most of previous observations
suggest that blockade of endothelin-mediated signaling
pathways attenuates liver injury induced by varied dis-

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Suematsu M and Aiso S: Ito cells in pericyte biology

ease conditions, of interest is a recent observation suggesting that blockade of ETB receptor could suppress
endothelial NO generation and thereby aggravate
reoxygenation injury of the liver, raising a possible
protective role of this pathway in maintenance of thephysiological functions.25
PGE2 has recently been shown to be released from
Kupffer cells in response to administration of ethanol
or endotoxin and thereby stimulate oxygen uptake
and induce glycogenolysis in hepatocytes.26 At thesame time, Ito cells are known to exhibit relaxationin response to PGE2 presumably through the cAMPdependent mechanism. Many of such biological effects
of PGE2 are mediated through its interactions withspeci®c receptors including EP2 and EP4 receptors,
which function via Gs-proteins to increase cAMP.27
Although PGE2 -mediated physiologic link between
glycogenolysis and sinusoidal relaxation is largely unknown, the cAMP-mediated relaxation of Ito cells
could help sinusoids to guarantee ample blood supply
for local oxygen demand and to deliver glucose into
circulation. Such roles of Ito cells as a metabolic bridgefor cooperative and integrative interactions between
blood supply and hepatocellular metabolism should
deserve further studies to understand pathophysiologic
mechanisms for varied disease conditions such as decompensatory cirrhosis or primary non-function liver intransplantation.
Future Perspectives of Ito Cell Biology
The previous dogmatic aspect that Ito cells are
simply desmin-positive cells with vitamin A-containing
droplets is changing. Recent studies provided evidence
for the presence of a variety of molecular markers
including RhoN, glial ®brillary acidic protein, nestin,and neurotrophin receptors in a subpopulation of Itocells.14 These ®ndings raised a fundamental question as
to whether all the mesenchymal cells derive from thesame embryonic source in the liver. Even more important in the related ®eld is a concept that cellular plasticity may not be con®ned within the same mesenchymal lineage but rather trans-differentiation from one
cell lineage to another could be possible even in adulttissues. Such a paradigm shift on pluripotentiality of Itocells could lead to development of new therapeutic
approaches on the cells to control liver ®brosis throughcell reconstitution as well as to establishment of tissue
engineering approaches for liver regeneration.
Acknowledgments: This work was supported in part by grants from
Tokyo Biochemical Research Foundation and from Advanced Medical Technology in Health Sciences Research Grants from Ministry of
Health and Welfare in Japan. The authors greatly thank the late Professor Emeritus Masaharu Tsuchiya, who passed away on April 7 thisyear, for leading us to pay attention to Ito cells on occasion of the

4th World Congress for Microcirculation in Tokyo, 1987, which heorganized, enjoying a discussion on roles of the cells in regulation of
microvascular blood ¯ow in the liver.

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