Which specialized cardiac conduction tissue cell type is found in the subendocardial layer, and what is a hallmark of its cytoplasm?

• A. Purkinje fibers; large diameter cells with pale, glycogen-rich cytoplasm.
• B. Atrial myocytes; dark-staining cytoplasm.
• C. Ventricular myocytes; abundant mitochondria.
• D. Pacemaker cells; small diameter with clear cytoplasm.

Answer: (A)

Purkinje fibers are the specialized conduction tissue in the subendocardial layer of the ventricles, part of the His–Purkinje system that rapidly distributes impulses to squeeze the ventricles into coordinated contraction. Their cytoplasm is hallmarked by a pale appearance due to abundant glycogen, and the cells are large in diameter with relatively few myofibrils compared with ordinary contractile cardiomyocytes. This combination lets them conduct quickly rather than generate strong contractions.

In contrast, regular atrial and ventricular myocytes are contractile and show dense, dark-staining cytoplasm because of plentiful myofibrils. Pacemaker cells (in the SA/AV nodes) are smaller and have less organized cytoplasm, reflecting their role in initiating impulses rather than rapid conduction through the ventricles.