The bifurcation pads seen at the sites of branchings of the anterior cerebral arteries in rats ranging in age from newborn to 20 months were examined electron microscopically. In young rats, intimai smooth muscle cells were arranged in one layer, and cellular contacts between the intimai cells and endothelial cells, and between the intimai cells were observed.
In adult rats, the pads showed valve-like projections or mound-like invaginations; they were rich ground substances and contained abundant elastic tissue. The pads were composed of intima. Cellular contacts between intimal cells and endothelial cells were observed, but no intimai or medial nerve fibers were seen. Cellular debris, mainly in the form of vesicles, granules and electron dense masses, was scattered within the bifurcations.
In aging rats, cellular debris was found mainly in the margins and roots of the pads. Marked alterations of the intima were present only immediately under severely damaged endothelial cells, but fibrinoid degeneration was not seen even in aging rats with severe medial damage. Basement membranelike substances were increased in bifurcations with advancing age.
The significance of bifurcation pads in the arterial walls was not clear, but it is suggested that they may be mobile physiological structures for regulating blood flow.
Bifurcation Cerebral artery Rat Cellular debris Physiological structure
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