Immunolocalization of matricial components during the early stages of chick embryonic liver development

In the chick embryo, the first liver primordium is observed at the end of the second day of incubation. At 3 and 4 days, ultrastructural analysis of the primitive vascular spaces showed that the endothelial limiting plate was constituted by one or several cell layers. At the vascular pole of the hepatoblasts, mesenchymal cells and connective matrix, present as fibrillar and non fibrillar components, were closely associated. At 5 days, some vascular spaces were limited by a simple endothelial layer. The limiting plate was fenestrated and the connective matrix was reduced to rare collagen fibrils and fibers. Collagen types I, III, IV, procollagen type III, fibronectin and laminin were visualized in the perivascular spaces using immunoperoxidase labeling methods. These components were also detected in the endoplasmic reticulum of hepatoblastic, endothelial and mesenchymal cells. All these appeared to be involved in connective matrix synthesis. Comparing 4 and 5 days, we demonstrated that the number of cells showing intracellular labelling of matricial components dropped dramatically at 5 days, indicating a possible decrease of connective matrix synthesis. Quantification of parenchymal and vascular surfaces was carried out using a semi-automatic image analyzer on consecutive parasagittal sections chosen in the axial part of the embryonic liver. These measurements were performed in order to quantitate the vascular distribution pattern during early development of the liver. These combined immunomorphological studies and morphometrical analyses suggest that during embryogenesis of the liver the synthesis of connective matrix precedes and possibly initiates the vascular differentiation.