Postnatal development of the rat portal vein: correlation with occurrence of peptidergic innervation

The portal vein of the rat is immature at birth, and is composed of an endothelium surrounded by undifferentiated cells of mesenchymal origin. Three days after birth, these cells have begun to differentiate and aggregate around the lumen to form two separate layers of perpendicularly oriented myoblasts, while a rich calcitonin gene-related peptide (CGRP) innervation is present around the vessel. In the internal circular muscle layer of the media myofibrils first develop on the endothelial side of the myoblasts, and then progressively reach the other side. In the longitudinal muscular layer of the media, which is separated from the circular layer by a connective lamina as early as 3 days after birth, myofibrils develop randomly in the cells. At the time of the enlargement of the longitudinal layer, long close contacts and intermediate junctions between external myoblasts and adventitial fibroblast-like cells were noted, suggesting that recruitment of this cell type is necessary for the maturation of the vessel wall. At about 28 days, the vein has reached its final structure and the smooth muscle cells are fully differentiated. The dense CGRP perivascular innervation already present at birth persists for the first 14 days of postnatal life when most of the cells have not yet acquired their complete contractile differentiation and are still capable of division. This innervation decreases transiently between 15-17 days, when the vessel acquires its spontaneous contractile activity, then rises to a peak between 20 and 25 days, and falls again. CGRP innervation, which is very scarce at 28 days, slowly increases during the peripubescent stage, by which time the adult structure of the vessel is established. Similar fluctuations in the density of peptidergic innervation were observed for substance P and neuropeptide Y, although these peptides were not yet present at birth and occurred only after 5 days. Vasoactive intestinal polypeptide- and bombesin-immunoreactive fibres were not found at any stage investigated. In addition to a description of the different cell-to-cell contacts which could play a role in the maturation of the vessel wall, we discuss the possible implication of the different peptides in the differentiation, maturation or maintenance of the vessel wall.