EXPRESSION AND POTENTIAL ROLE OF THE EXTRACELLULAR-MATRIX IN HEPATIC ONTOGENY - A REVIEW

Studies from a number of laboratories have provided information on the temporal and spatial expression of a variety of extracellular matrix (ECM) components in the developing liver and insight into their potent ial roles in hepatogenesis. Collagen type IV and laminin are present i n the basement membranes of the capsular mesothelium, vascular structu res of the portal and hepatic vein branches, and the ductular elements of the developing liver. The mesothelial, vascular, and ductular epit helial cells synthesize laminin and type IV collagen. In contrast, fib ronectin and type I collagen are restricted to the adjacent or surroun ding interstitium of those ductal and vascular elements, but are not w ithin the basement membrane proper. The hepatic perisinusoidal space ( Space of Disse) of the fetal rat develops a delicate extracellular mat rix by 12.5 days of gestation, which is characterized by banded collag en fibrils and bundles associated with filamentous and flocculent mate rial. Fibronectin, laminin, and collagen types I, III, and IV are pres ent in the developing perisinusoidal space by this early gestational d ate, with laminin being the most prevalent component detected. The lam inin chains localized to that region in the fetal/neonatal period are alpha 2, beta 1, beta 2, and gamma 1, whereas the alpha 1 chain of lam inin is absent from the developing Space of Disse. Similar data have b een reported on the laminin phenotype in the perisinusoidal space duri ng hepatic regeneration. Electron microscopy immunohistochemistry stud ies have demonstrated that the sinusoidal lining cells and hepatocytes synthesize these ECM proteins during hepatogenesis. By 6 to 8 weeks o f postnatal life, laminin is not detectable in the perisinusoidal spac e. Both the transient expression of laminin and the similarity of the laminin chain phenotype expressed in the perisinusoidal space in the d eveloping and regenerating liver suggests a role for this protein in t he organization of the hepatic lobule in those forms of hepatic morpho genesis. (C) 1997 Wiley-Liss, Inc.