**5. Comparison of the CDE and DDC models with chronic human liver diseases, HCV and PSC, respectively**

In humans, DR is seen in most chronic liver injury, irrespective of the etiology. Historically, DR has been categorized on morphology into "*typical*" and "*atypical*" DR, based on rodent studies [47]. *Typical* DR have a lumen lined by cuboidal cells and are the result of proliferation of preexisting ductules, in analogy with the DR seen after biliary obstruction, while *atypical* describes thin, elongated structures that extend into the lobules and lack discernible lumen as preferentially seen after hepatocytic damage. Therefore, DDC diet best models *typical* DR, while CDE diet replicates pathological pattern of *atypical* DR. However, this dichotomic classification was discouraged some years ago because it could not readily accommodate the range of patterns seen clinically [10]. Another classification schemes attempted to integrate the histologic features, inciting disease and immunophenotyping of DR [48, 49]. However, not all DR fit in this classification, and especially not when biliary obstruction becomes chronic (as in the DDC livers). So far, there is a lack of consensus regarding DR classification in humans, as DRs are diverse, covering a spectrum of features rather than clear subphenotypes [10].

However, based on histological analysis, the DR phenotype in the CDE model resembles the one observed in human chronic Hepatitis C virus (HCV) infection depicting portal fibrosis and in a series of autoimmune hepatitis (AIH) (**Figure 3A**–**D**). HCV- and AIH-associated DRs have only a vague or no lumen, and comprise small elongated cells with little cytoplasm extending in the periportal parenchyma and associated with dense collagen fibers [10, 11, 50]. In HCV, the extension of DR into the parenchyma and DR severity correlate with the severity of fibrosis and the inflammatory activity, supporting that extracellular matrix and inflammatory signals influence DR [50]. DR in AIH has been proposed to represent a regenerative response as DR persists after the inflammatory activity subsided following immunosuppressive treatment [51]. At the early stage of fibrosing, cholestatic variants of coinfection with hepatitis B and C, expanded DR into the hepatic parenchyma also resemble the DR phenotype seen in CDE livers [10, 52]. With regard to LPC fate, the observation of a phenotypic continuum between DR cells and hepatocytes in the livers of patients suffering from HCV supports differentiation of LPC toward hepatocytes [4, 50]. Besides hepatitis, the CDE diet also recapitulates features of the DR associated with lipid accumulation (steatosis) as in chronic alcoholic and nonalcoholic fatty liver diseases [11, 29, 42, 53].

by T-cells and activating its receptor fibroblast growth factor-inducible 14 (Fn14), is suggested to be an exclusive LPC mitogen. After both CDE and DDC treatment on Fn14 knockout mice,

As described above, DR requires a typical niche provided by extracellular matrix-producing and inflammatory cells, which are located in the sinusoids closely adjacent to DR. Additionally, sinusoidal endothelial cells themselves could also have an important role in regulating DR. Signaling molecules specifically expressed within the endothelial compartment of the central vein have been shown to have a crucial role in liver zonation [45]. Moreover, in another model of liver injury, hepatocytes divide along the closest microvessel as order principle to restore liver architecture [46]. Either a signaling or a guiding role of endothelial cells on LPC response could be envisaged. However, so far, no experiments have been done to study endothelial regulation of DR in the CDE or DDC model.

**5. Comparison of the CDE and DDC models with chronic human liver** 

In humans, DR is seen in most chronic liver injury, irrespective of the etiology. Historically, DR has been categorized on morphology into "*typical*" and "*atypical*" DR, based on rodent studies [47]. *Typical* DR have a lumen lined by cuboidal cells and are the result of proliferation of preexisting ductules, in analogy with the DR seen after biliary obstruction, while *atypical* describes thin, elongated structures that extend into the lobules and lack discernible lumen as preferentially seen after hepatocytic damage. Therefore, DDC diet best models *typical* DR, while CDE diet replicates pathological pattern of *atypical* DR. However, this dichotomic classification was discouraged some years ago because it could not readily accommodate the range of patterns seen clinically [10]. Another classification schemes attempted to integrate the histologic features, inciting disease and immunophenotyping of DR [48, 49]. However, not all DR fit in this classification, and especially not when biliary obstruction becomes chronic (as in the DDC livers). So far, there is a lack of consensus regarding DR classification in humans, as DRs

are diverse, covering a spectrum of features rather than clear subphenotypes [10].

However, based on histological analysis, the DR phenotype in the CDE model resembles the one observed in human chronic Hepatitis C virus (HCV) infection depicting portal fibrosis and in a series of autoimmune hepatitis (AIH) (**Figure 3A**–**D**). HCV- and AIH-associated DRs have only a vague or no lumen, and comprise small elongated cells with little cytoplasm extending in the periportal parenchyma and associated with dense collagen fibers [10, 11, 50]. In HCV, the extension of DR into the parenchyma and DR severity correlate with the severity of fibrosis and the inflammatory activity, supporting that extracellular matrix and inflammatory signals influence DR [50]. DR in AIH has been proposed to represent a regenerative response as DR persists after the inflammatory activity subsided following immunosuppressive treatment [51]. At the early stage of fibrosing, cholestatic variants of coinfection with hepatitis B and C, expanded DR into the hepatic parenchyma also resemble the DR phenotype seen in CDE livers [10, 52]. With regard

a significant reduction of the LPC response is observed [31, 44].

152 Experimental Animal Models of Human Diseases - An Effective Therapeutic Strategy

**4.3. Nearby endothelial cells**

**diseases, HCV and PSC, respectively**

The DR pattern seen in DDC livers is more comparable to that of chronic fibrosing cholangitis such as primary sclerosing cholangitis (PSC) and primary biliary cholangitis (PBC) with DR proliferation restricted within the portal area and accompanied by concentric periportal fibrosis (**Figure 3E** and **F**). In these diseases as in DDC, the primary damage is directed toward cholangiocytes. Intrahepatic bile duct destruction and ductopenia seen in advanced PBC and the fibrous obliterative lesions of PSC do not occur in the DDC model, a phenomenon most likely related to the specific immune component of PBC and PSC which is lacking in the DDC model.

**Figure 3.** DR observed in human chronic hepatitis C infection and in primary sclerosing cholangitis. Liver stained with anticytokeratin 7 (CK7) and Masson-trichrome of an HCV case with mild inflammation (A and B), AIH with moderate inflammation (C and D), and PSC with cholangitis, edema, and portal fibrosis (E and F). HCV- and AIH-associated CK7+ DRs (A and C) have only a vague or no lumen and comprise and elongated cells with little cytoplasm extending in the periportal parenchyma and associated with dense collagen fibbers (B and D). While the CK7+ ductular proliferation seen in PSC (E) is enclosed in portal mesenchyma and concentric periductular fibrosis occurs (F).

As mentioned above, the Notch and Wnt signaling pathways are involved in the divergence of DR cells fate toward hepatocyte or biliary cells observed in response to CDE versus DDC. Similarly, in human diseases, prevalence of Notch signaling, driving biliary phenotype, is strong in PSC while the expression of Numb, a negative regulator of Notch, is more elevated in HCV samples compared to PSC [54]. Moreover, β-catenin, a component of the Wnt pathway, is found within the cytoplasm and nucleus of human DR cells of HCV-infected livers, signing enhanced Wnt signaling and promoting hepatocyte regeneration, whereas in PSC, β-catenin is predominantly localized to the cell surface, suggesting low activation of the canonical Wnt signaling pathway promoting biliary regeneration [42].

**References**

1999;**154**(2):537-541

2009;**137**(2):466-481

1994-2002

in Liver Disease. 2003;**23**:385-396

in Liver Disease. 2004;**24**(1):43-48

Hepatology. 2005;**41**:1252-1261

738-750

interface. Hepatology. 2011;**54**(5):1853-1863

tor cells and steatosis. Hepatology. 2005;**41**(4):809-818

PH rat model. Laboratory Investigation. 2012;**92**:72-81

[1] Lowes KN, Brennan BA, Yeoh GC, Olynyk JK. Oval cell numbers in human chronic liver diseases are directly related to disease severity. American Journal of Pathology.

Relevance of the CDE and DDC Mouse Models to Study Ductular Reaction in Chronic Human...

http://dx.doi.org/10.5772/intechopen.69533

155

[2] Roskams TA, Libbrecht L, Desmet VJ. Progenitor cells in diseased human liver. Seminars

[3] Theise N, Saxena R, Portmann BC, Thung SN, Yee H, Chiriboga L, et al. The canals of

[4] Zhang L, Theise N, Chua M, Reid LM. The stem cell niche of human livers: Symmetry

[5] Riehle KJ, Dan YY, Campbell JS, Fausto N. New concepts in liver regeneration. Journal

[6] Duncan AW, Dorrell C, Grompe M. Stem cells and liver regeneration. Gastroenterology.

[7] Kuwahara R, Kofman AV, Landis CS, Swenson ES, Barendswaard E, Theise N. The hepatic stem cell niche: Identification by label-retaining cell assay. Hepatology. 2008;**47**(6):

[8] Saxena R, Theise N. Canals of hering: Recent insights and current knowledge. Seminars

[9] Kaneko K, Kamimoto K, Miyajima A, Itoh T. Adaptive remodeling of the biliary architec-

[10] Gouw ASH, Clouston AD, Theise N. Ductular reactions in human liver: Diversity at the

[11] Clouston AD, Powell EE, Walsh MJ, Richardson MM, Demetris AJ, Jonsson JR. Fibrosis correlates with a ductular reaction in hepatitis C: Roles of impaired replication, progeni-

[12] Dusabineza A, Van Hul KN, Abarca-quinones J, Starkel P, Najimi M, Leclercq IA. Participation of liver progenitor cells in liver regeneration : Lack of evidence in the AAF/

[13] Kofman AV, Morgan G, Kirschenbaum A, Osbeck J, Hussain M, Swenson S, et al. Doseand Time-Dependent oval cell reaction in acetaminophen-induced murine liver injury.

[14] Factor VM, Radaeva SA, Thorgeirssont SS. Origin and fate of oval cells in dipin-induced hepatocarcinogenesis in the mouse. American Journal of Pathology. 1994;**145**(2):409-422

[15] Sell S. Heterogeneity and plasticity of hepatocyte lineage cells. Hepatology. 2001;**33**:

ture underlies liver homeostasis. Hepatology. 2015;**61**:2056-2066

Hering and hepatic stem cells in humans. Hepatology. 1999;**30**:1425-1433

between development and regeneration. Hepatology. 2008;**48**:1598-1607

of Gastroenterology and Hepatology. 2011;**26**:203-212
