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 origi n. Three days after birth, these cells have begun to differentiate and aggregate around the lumen to form two separate layers of perpendicul arly oriented myoblasts, while a rich calcitonin gene-related peptide (CGRP) innervation is present around the vessel. In the internal circu lar muscle layer of the media myofibrils first develop on the endothel ial 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 afte r birth, myofibrils develop randomly in the cells. At the time of the enlargement of the longitudinal layer, long close contacts and interme diate junctions between external myoblasts and adventitial fibroblast- like cells were noted, suggesting that recruitment of this cell type i s necessary for the maturation of the vessel wall. At about 28 days, t he vein has reached its final structure and the smooth muscle cells ar e fully differentiated. The dense CGRP perivascular innervation alread y present at birth persists for the first 14 days of postnatal life wh en most of the cells have not yet acquired their complete contractile differentiation and are still capable of division. This innervation de creases transiently between 15-17 days, when the vessel acquires its s pontaneous contractile activity, then rises to a peak between 20 and 2 5 days, and falls again. CGRP innervation, which is very scarce at 28 days, slowly increases during the peripubescent stage, by which time t he 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 polypeptid e- and bombesin-immunoreactive fibres were not found at any stage inve stigated. In addition to a description of the different cell-to-cell c ontacts which could play a role in the maturation of the vessel wall, we discuss the possible implication of the different peptides in the d ifferentiation, maturation or maintenance of the vessel wall.