[Functional and morphologic changes in adrenergic innervation of the portal veins in guinea pigs towards the end of the prenatal period].

Contractions induced by transmural nervous stimulation were studied on isolated preparations of the portal vein of newborn and adult guinea pigs and the walls of the studied vessels were examined by electron microscopy. Transmural nervous stimulation elicited contractions of the portal vein in all developmental types yet with a higher threshold frequency for inducing contractions in fetuses than in newborn animals. Guanethidine and phentolamine reduced while cocaine enhanced the neurogenic contractions, indicating their adrenergic nature. Electron microscopic studies revealed uncompleted differentiation of the tunica media of the fetal portal vein into the inner circular and outer longitudinal layer. The region of the adventitio-medial border locates the external nervous plexus from which already in the fetus nerve fibers penetrate between the longitudinal and circular layer of the tunica media to constitute there inner nervous plexus. The vesicles in the nerve endings displayed the same characteristics in all the developmental types studied. The obtained results indicate a high degree of morphological and functional maturation of the adrenergic regulation of the portal vein already in the fetus at the end of the prenatal period in guinea pigs. (Fig. 9, Ref. 16.)