The atrioventricular valves of the guinea‐pig. II. An ultrastructural study

Each atrioventricular valve of the guinea‐pig consists of a core of connective tissue and muscle covered by endothelial cells. The muscle forms an annulus at the attached margin of the valve adjacent to the fibrous annulus. Some fibers from the lower part of the muscular annulus turn sharply toward the free margin of the valve and form a complex meshwork. These fibers resemble atrial muscle, but are noteworthy in that each contains myofibrils that are oriented in different directions. Some muscle fibers have features similar to those of conducting fibers, i.e., glycogen rich cytoplasm containing few myofibrils. Unmyelinated nerve fibers are numerous in both the mitral and tricuspid valves, but myelinated fibers were found only in the mitral valve. Typical adrenergic terminals were found adjacent to some cholinergic endings. Mitochondria‐filled terminals (sensory?) and a third type of motor terminal were present also. Bundles of collagenous fibers extend from the chordae tendineae into the valve cusps where they fan out into small bundles. The other formed elements of the stroma are microfibrils and a flocculent material. There are a few elastic fibers beneath the endothelium. Interstitial cells, many of which possess solitary flagella are scattered throughout the matrix.