Relationship between vascular development and vascular differentiation during liver organogenesis in humans

Abstract

Backgrounds/Aims: The complex vascular architecture characteristic of the normal adult liver is progressively acquired during the fetal life. In this study, we aimed to evaluate the relationship between angiogenesis and vascular differentiation during liver organogenesis.

Methods: We studied, in 51 fetuses of different gestational ages, the expression of markers of endothelial cell differentiation, integrins, pro- and anti-angiogenic extracellular matrix components, vascular endothelial growth factor (VEGF) and its receptors.

Results: Three main stages in the development of the vascular architecture of the liver were identified: (a) from 5 to 10 gestation weeks (GW), no evidence of de novo angiogenesis was detected; the vessels present in the liver primordium were the precursors of portal veins and sinusoids, deriving from preexisting vessels; (b) from 10 to 25 GW, angiogenesis and vasculogenesis resulted in the development of, respectively, arteries and intra-portal capillaries, while portal veins and hepatic sinusoids followed a differentiation process; (c) after 25 GW, little changes were detected in the various vascular compartments. The maximal expression of VEGF and its receptors was from 5 to 25 GW.

Conclusions: The development of the hepatic vascular architecture is a multistep process combining angiogenesis, vasculogenesis and vascular differentiation, regulated by specific growth and differentiation factors including VEGF.

Introduction

The adult liver contains several distinct vascular compartments [1], [2]. Afferent vessels are represented by the portal vein and the hepatic artery, which supply several distinct intra-parenchymal capillary networks. In portal tracts, capillary vessels originate from the hepatic arteries and include the peribiliary plexus responsible for the vascularization of the intra-hepatic biliary tree. In hepatic lobules, highly specialized capillary vessels, known as hepatic sinusoids, run between the hepatocyte plates and transport a blood of mixed portal and arterial origin. Efferent vessels include centrilobular veins and large collecting veins, which supply the supra-hepatic veins.

The different vascular compartments of the adult liver have different embryological origins [1], [3], [4], [5]. Portal veins and sinusoids differentiate from preexisting vessels. Portal veins derive from vitelline veins [5] while sinusoids derive from the capillary vessels of the septum transversum [6], [7], [8]. Intra-parenchymal hepatic arteries progressively extend from the hilum to the periphery of the liver, from 10 to 20 gestation weeks (GW). Intra-portal capillary vessels likely derive from intra-hepatic mesenchymal precursors through a process of vasculogenesis beginning at 10 GW [9], [10].

A distinctive feature of the vascular compartments of the liver is the high degree of differentiation of the corresponding endothelial cell subpopulations. The most differentiated endothelial cells are those lining the hepatic sinusoids. They present distinctive structural and functional characteristics [11] including the presence of cytoplasmic fenestrations, and are characterized by a unique phenotype, defined by the lack of continuous endothelial cell markers and the expression of specific differentiation markers, such as the CD4 protein [12], [13]. The specific differentiation of sinusoidal endothelial cells is progressively acquired between 10 and 20 GW [6], [7], [8]. The subpopulations of endothelial cells associated with other vascular compartments of the liver also present specific immunophenotypic characters [13], [14].

Embryological and experimental studies have shown that angiogenesis is placed under the dependence of specific factors, among which the VEGF family plays a pivotal role [15], and that vascular growth and differentiation result from an interplay between integrins and extracellular matrix components [16], [17]. Little is known about the relationships between vascular development and vascular differentiation during liver organogenesis. We therefore analyzed, at various stages of liver organogenesis: (a) the chronological sequence of expression of a large panel of specific and non specific markers of endothelial cell differentiation, (b) the changes in integrin expression and in the composition of the subendothelial matrix in the various vascular compartments of the liver, and (c) the sites and patterns of expression of vascular endothelial growth factor (VEGF) and its receptors VEGF-R1 and VEGF-R2.